taxonID	type	description	language	source
03A5566CFFCFFFB1FF1DFED82EC7FC13.taxon	description	Figs 1 – 8, 23; Table 1	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFCFFFB1FF1DFED82EC7FC13.taxon	materials_examined	Material examined. Type material: ZSM 20040176: Australia, Western Australia, Shark Bay, NNE of Heirisson Prong, 11 – 12.5 m, 18 Jun 1905, ethanol-preserved colonies on Schizophrys dama (Herbst, 1804). — ZSM 20041647 – 20041651, 5 microslide preparations with detached polyps. Additional material: MSNMCoe 375: Indonesia, Bali, Amed, dive site known as “ Ghost Bay ”, - 8.332965 °, 115.643044 °, 20 m, 25 Apr 2023, four formalin-fixed crabs (Hyastenus sp.) with polyps on their carapace. — MSNMCoe 376: Indonesia, Bali, Tulamben, dive site known as “ Seraya ”, - 8.295692 °, 115.612838 °, 18 – 22 m, 23 Apr 2023, one formalin-fixed crab (Hyastenus sp.) with polyps on its carapace. — MSNMCoe 377: Indonesia, Bali, Amed, dive site known as “ Melasti ”, - 8.330810 °, 115.639825 °, 18 – 20 m, 20 Apr 2023, one formalin-fixed and one ethanol-fixed crabs (Hyastenus sp.) with polyps on their carapaces, the latter specimen used exclusively for DNA extraction, GenBank: OR 872049 – OR 872051 (16 S), OR 872005 – OR 872007 (18 S), OR 872029 – OR 872031 (28 S) (the sequences correspond to 3 individual hydroid polyps); OR 876279 is the COI sequence obtained from the Hyastenus crab.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFCFFFB1FF1DFED82EC7FC13.taxon	description	Description. Colonies invariably associated with a decapod host of the genus Hyastenus (Fig. 1); composed of numerous polyps fixed exclusively to the dorsal side of the carapace, not only restricted to the cephalothorax, but also to the eight walking legs; a stolonal web connecting the polyps to one another is obviously absent, and each of them is attached individually to the crab carapace by simple adhesion, as demonstrated by the agglutination of silt particles around the bases of hydrocauli; the latter, depending on the age of the polyps and their state of contraction, are 795 – 1790 µm long in the menthol-relaxed, formalin-fixed specimens examined herein; cauli (Fig. 2) gradually tapering distally, divided into two distinct parts (Fig. 3 B): a proximal one, usually accounting for about 1 / 3 of their total length, and the remaining 2 / 3 distal portion, both delimited by a transverse, 205 – 380 µm wide furrow (Fig. 3 D); a dozen peripheral, rather broad, unbranched, longitudinal canals (not forming lateral diverticuli) in the endoderm, spanning from one end to the other of the caulus (Fig. 3 F); core of the latter composed of large, polygonal, parenchyme cells, leaving centrally a narrow lumen (Fig. 3 F); proximal part of cauli covered by flimsy, highly transparent, closely appressed perisarc layer, epidermis not provided with nematocysts, and coenosarc not forming papillae; distal part of cauli naked, epidermis with scattered, ovoid, transparent patches comprising 1 – 2 large nematocysts in their center (Fig. 3 C); distally, a 175 – 310 µm wide constriction at junction between the caulus and hydranth base (Fig. 3 A, B). Hydranths flask-shaped (Fig. 3 A), 515 – 900 µm high and 440 – 675 µm wide; an internal, transverse septum divides them into a lower, relatively shallow, non-digestive part, filled with parenchyme cells, and an upper, comparatively taller, hollow, digestive part; a whorl of 10 – 15 aboral tentacles near their bases, and 8 – 11 oral tentacles, equally in a whorl, surrounding distally a dome-shaped hypostome (Figs 2, 3 A, 4 A); aboral tentacles 755 – 1990 µm long, gracefully arching upwards, slightly flattened laterally, gradually tapering distally, core solid, of pseudofiliform type; oral tentacles 230 – 440 µm long, basal 1 / 3 flattened laterally and adnate to the hypostome, distal 2 / 3 being free and arching outwards, with circular cross section, tapering only distally, surface rough due to the presence of irregular, vaguely annular clusters of nematocysts. Up to five short, conical blastostyles in a whorl, a short distance above the aboral tentacles (Fig. 4 A); each blastostyle giving rise to a reduced number of medusa buds at various stages of development (Fig. 4 B); buds before liberation (Fig. 5 B) with a 450 – 470 µm high and 360 – 400 µm wide umbrella, circular in cross-section; exumbrella provided with numerous, scattered, small, spherical nematocysts; mesoglea thin; manubrium club-shaped, extending almost to the closely-stretched velum, ending in pore-shaped mouth; four narrow radial canals ending basally in circular canal; four solid, perradial tentacle bulbs of inequal size: one bearing a club-shaped, 345 – 450 µm long, capitate tentacle, a conspicuouslyswollen, 305 – 325 µm long bulb opposite to it, as well as two comparatively less developed, 180 – 200 µm long, lateral bulbs; capitate tentacle with moderately-long stem ending distally in conspicuous, ovoid nematocyst knob, sometimes with an additional, intermediate, circular cluster, suggesting a moniliform structure in mature medusae; sense organs absent, and no gonads produced around the manubrium at this stage. Cnidome: its composition is summarized in Table 1 and the various capsules are illustrated in Fig. 6. Except for the unmistakable stenoteles and desmonemes, all the remaining capsules were seen undischarged, and their assignation to a given type was not attempted, until additional observations on living specimens are done. Colors in life: basal part of caulus and endodermal canals milky-white, remainder of caulus, as well as the tentacles, translucent; hydranth base yellow-orange, hypostome white; manubrium and tentacle bulbs of the medusa white to yellow.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFCFFFB1FF1DFED82EC7FC13.taxon	discussion	Remarks. Multi-locus and COI phylogenetic reconstructions revealed that Balinese C. balssi sequences are different from any other sequenced Corymorpha species (Fig. 23 A). It is unclear how the hydroid colony is established on the carapace of its decapod host. Given that the dispersive stage is a free-swimming medusa produced in small numbers, the settlement of planula larvae seems statistically unlikely. No apparent stolonal web connects the polyps between them, and all individuals carried by a crab could not be regarded as a colonial animal stricto sensu. Some polyps show large, formless, basal bulges, suggesting that they may reproduce asexually by budding, thus forming clonal populations. It is possible that some polyps are either picked up by uncolonized crabs from individuals already bearing hydroids on their carapaces, or that crabs promote asexual spreading of the associated hydroid, as observed in the association between Lybia leptochelis crabs and Alicia sea anemones (Schnytzer et al. 2017). The short rooting filaments illustrated by Stechow in his unpublished plate [Ruthensteiner et al. (2008: fig. 7 B) and Fig. 5 A herein] are likely artefacts (as demonstrated by the examination of many polyps, including those mounted on the five microslides belonging to the type), nor could they be found in our specimens, in which the polyps simply adhere basally to the crab carapaces. Syntype material, including alcohol-preserved specimens and microslide preparations (Fig. 7) were reexamined. The polyps 1 proved virtually indistinguishable from the new material at hand, except for their proportionally slightly bigger size (2 – 3.5 mm high, as reported in the original account). It should be stated that significant size variations were encountered in our specimens from Bali, depending on the age of the polyps and the size of their crab host; consequently, we consider the differences noted with the type to be intraspecific. The cnidome composition of the type specimens (Fig. 8) is the same as that of the material at hand (compare to Fig. 6 A – E). Stechow (1932: 82) stated 2 that his colonies were mostly composed of infertile hydranths, and only one proved fertile and had very young, spherical gonophores, “ apparently ” provided with “ one large and two lateral, poorly-developed tentacles ” (Fig. 5 A, enlargement); that polyp could not be located. Although Stechow could not be certain in his observations, the medusa buds in our material possess a main, relatively long tentacle with a terminal capitation, a large tentacle bulb opposite to it, and a pair of globular tentacle bulbs laterally (Fig. 5 B). Some hydroid colonies photographed in the field show medusa buds with a relatively distinct, though small, intermediate ring of nematocysts, suggesting that the main tentacle in free-swimming, well-developed specimens is likely of moniliform type. This leads us question the identity of the mature medusa among the congeners described so far based on their planktonic stage. In an earlier paper, one of us (Galea 2023: Appendix 1) provided a tabulated list of species of Corymorpha considered as valid. Among those occurring in the tropical Indo-Pacific, C. bitungensis (Xu, Huang & Guo, 2013) (Fig. 5 C), described from the Lembeh Strait, Indonesia, an area where the presence of the C. balssi is clearly documented in a series of underwater photographs available online (e. g. Levantovsky 2012), is a likely candidate. According to the original account, the medusa of C. bitungensis reaches up to 3 mm in bell height by 1.8 mm in width; its club-shaped manubrium projects for about 2 / 3 into the subumbrella; the main tentacle is provided with 3 – 10 intermediate rings of nematocysts besides the comparatively larger, terminal knob; the opposite tentacle bulb is quite well-developed and club-shaped, while the two laterals are similar, though shorter and slenderer (Lin et al. 2013: 246, 248 – 250). While providing an account on their supposedly new species, no comparison was made by these authors with the previously-described C. juliephillipsi (Gershwin, Zeidler & Davie, 2010) (see original account) from Moreton Bay, QLD, Australia, a nominal species with a very similar, if not identical, morphology and size (Gershwin et al. 2010: 59, 62). The hydroid of C. balssi was essentially described based on material from Shark Bay, WA, Australia (Stechow 1932), a locality that is located at nearly the same latitude as Moreton Bay, but on the opposite (western) coast of Australia, where no records of C. balssi from QLD came to our knowledge. The colonies of C. balssi studied by Stechow (1932) reportedly occurred on Schizophrys dama (Herbst, 1804) (Crustacea: Decapoda: Majoidea: Majidae). Indeed, one of his crabs [Fig. 7 A, but see also Ruthensteiner et al. (2008: fig. 3 E)] corresponds to the syntype illustrated by Lee et al. (2018: fig. 3). However, the Indonesian host is a yet to be identified, possibly new species of Hyastenus belonging to the H. inermis group (Crustacea: Decapoda: Majoidea: Epialtidae) (P. K. L. Ng, pers. comm.), which is known to associate with cnidarians (Lee & Ng 2019). Stechow’s (1932: 82) material was collected from Shark Bay in June, August and September 1905, and from Onslow (> 500 km to the N) in July of the same year. According to SeaTemperature. org, the average temperatures for Shark Bay are 20.97 ° C (18.83 ‒ 23.72 ° C) in June, 20.03 ° C (18.51 ‒ 22.72 ° C) in August, and 20.50 ° C (18.98 ‒ 22.57 ° C) in September, and for Onslow 22.59 ° C (20.51 ‒ 25.26 ° C) in July. Conversely, the annual minimum temperature recorded for Bali is 26.8 ° C (25.3 ‒ 28.4 ° C) in September, but the temperature rises to as much as 29 ° C (27.9 ‒ 30.1 ° C) in April, when our material was collected, indicating that C. balssi can live under different ecological conditions.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFCFFFB1FF1DFED82EC7FC13.taxon	distribution	Distribution. Australia: Shark Bay and Onslow, WA (Stechow 1932); Indonesia: Bali (present study), Borneo, Sumbawa, Komodo Archipelago, Lembeh Strait, Ambon (based on photographs available on Flickr, iNaturalist, Instagram and Facebook).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFC7FFB6FF1DFCDF2E3BFF36.taxon	description	Figs 9 ‒ 11, 29	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFC7FFB6FF1DFCDF2E3BFF36.taxon	materials_examined	Material examined. Holotype: MSNMCoe 361, Indonesia, Bali, Pemuteran, vicinity of the pontoon, - 8.142954 °, 114.660906 °, 01 Apr 2023, female colony growing on floating, ripped Turbinaria sp., GenBank: OR 872056 (16 S), OR 866286 (COI). Paratypes: MSNMCoe 362, same collecting data as for the holotype, female colony. — MSNMCoe 363, same collecting data as for the holotype, male colony. — MSNMCoe 364, same collecting data as for the holotype, male colony. Additional material: MSNMCoe 365, same collecting data as for the holotype, sterile colony. — MSNMCoe 366, same collecting data as for the holotype, sterile colony.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFC7FFB6FF1DFCDF2E3BFF36.taxon	description	Description. Colonies, often profuse, are formed on both the thalli and blade stalks of Turbinaria spp., never occurring on the distally-expanded, externally-exposed part of the blades, thus finding shelter from predation and / or direct mechanical injury (Fig. 9 A, B); arising from dense, branching and anastomosing hydrorhizal web creeping on the surface of its algal substrate (Fig. 9 C, D); stems (Fig. 10 A ‒ H) exclusively monosiphonic, borne on minute apophyses of the hydrorhiza from which they are delimited by a deeply-incised, transverse constriction; a distinct basal bulge on first internode immediately above constriction; usually short (175 ‒ 375 µm), slightly inverted-conical cauli, ending distally into a hydrotheca, are formed, although small, sympodially-grown stems, composed of up to six hydrothecate internodes arising irregularly in more than one plane, co-occur seldomly; branches, given off singly (Fig. 10 D ‒ H) or in opposite pairs (Fig. 10 C), are borne on short (60 ‒ 70 µm), upwardly-curved apophyses arising from below a hydrotheca, and are provided basally with a bulge, similarly to that of the first internode. Hydrothecae (Fig. 10 I) shallow (25 ‒ 30 µm), walls slightly flared (140 ‒ 155 µm wide at base and 155 ‒ 175 µm at aperture), rim not everted; renovations occasional (Fig. 10 C ‒ F), up to two observed; a band of ovoid desmocytes above a transversely-set, thin diaphragm (Fig. 10 I); hydranths (Fig. 10 A) composed proximally of a tubular column capable of great extension, and of a distally-swollen, digestive region comprising the hypostome on top, surrounded by a whorl of 17 ‒ 21 filiform, unicoronate tentacles; endoderm of polyps densely filled with zooxanthellae (Fig. 9 E, F, J), conferring them in life a distinctive, brown color. Colonies dioecious; gonothecae arising laterally, either singly (Fig. 10 K, L) or in pairs (Fig. 10 J), from below the primary or secondary hydrothecae; sexually dimorphic; male (Fig. 10 J) elongate-ovoid (615 ‒ 725 µm long, 245 ‒ 270 µm wide), thin- and smooth-walled, enclosing an unbranched, tubular, central blastostyle surrounded by a homogenous mass of sperm cells; female gonothecae (Fig. 10 K, L) sac-shaped (865 ‒ 1050 µm long, 355 ‒ 430 µm wide), laterally-flattened, with an “ anterior ”, almost straight outline, and a “ dorsal ”, arched wall, ending distally in an almost conical projection; commonly 2 ‒ 3 (occasionally only one), large (240 ‒ 270 µm wide) spherical oocytes are formed; spadix trifid, with two parallel-running, “ anterior ” branches, ending distally into two functional hydranths (Fig. 11 A), and an unpaired, “ dorsal ” branch, curving over the oocytes (Fig. 10 K, L); aperture of female gonotheca provided with a twin hydrotheca (Fig. 10 M, O), their respective lumina, deeply immersed into the gonothecal wall, being separated anteriorly by a distinct, longitudinal, perisarcal septum (Figs 10 M, 11 C), and posteriorly by a thick perisarc sheet (Fig. 10 O) projecting into the gonotheca for a certain distance, forming an “ anterior ” compartment for the hydranths, separated from the “ dorsal ” one restricted to the oocytes (Figs 10 N, 11 B). Zooxanthellae also present in the gonophores, except for the gametes proper. Long, slender, distally bifid propagules, given off laterally from below some hydrothecae, have been observed infrequently (not illustrated). Cnidome (Figs 9 G ‒ I, 10 P): large, ovoid, microbasic euryteles [(8.1 ‒ 9.3) × (3.6 ‒ 4.1) µm, a single capsule seen discharged], banana-shaped microbasic mastigophores [(6.0 ‒ 6.5) × (1.5 ‒ 1.7) µm, none seen discharged], and sausage-shaped microbasic heteronemes [(5.7 ‒ 6.4) × (1.4 ‒ 1.5) µm, none seen discharged]. Color in life: brown.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFC7FFB6FF1DFCDF2E3BFF36.taxon	discussion	Remarks. 16 S rRNA data supports the divergence of Halecium turbinariae sp. nov. from all other sequenced congeners (Fig. 29). The new species is primarily distinguished from its congeners through its substrate preference and the occurrence of zooxanthellae in its coenosarc. The hydroid from the SW Indian Ocean whose frustulation was documented by Gravier-Bonnet & Bourmaud (2005, as Halecium sp.) is evidently the same. Despite the presence of female gonothecae in their material (Gravier-Bonnet & Bourmaud 2005: 65), the authors refrained from assigning it a specific name, stating that “ more taxonomic work has to be done to identify it with confidence ” (Gravier-Bonnet & Bourmaud 2005: 68). The fertile material from Bali allows us to provide, herein, a comprehensive description and to assign it conclusively to a new species. Only a few congeners share with H. turbinariae sp. nov. the presence of zooxanthellae in their coenosarc, namely H. nanum Alder, 1859 (Calder 1991), H. lankesterii (Bourne, 1890) (Peña Cantero & García Carrascosa 2002) and H. xanthellatum Galea, 2013 (original account). Its mode of branching and the structure of its trophosome have much in common with those of both H. nanum and H. xanthellatum; however, unlike in the new species, the female gonothecae of H. nanum have transversely-corrugated hydranth tubes (Congdon 1907: figs 21 ‒ 22, as H. marki; Fraser 1912: fig. 29 C; Calder 1991: fig. 12 C), while those of H. xanthellatum are dorsally-ringed and their aperture surpasses the top of the theca (Galea 2013: fig. 5 G). On the other hand, the female gonotheca of H. lankesterii has a broad, dome-shaped top and its aperture is situated much lower than in H. turbinariae (Peña Cantero & García Carrascosa 2002: fig. 13 C). The female gonotheca of the new species is very similar to that of: 1) H. labiatum Billard, 1933 [original account: fig. 8 L ‒ M; Galea & Ferry (2015: fig. 3 J ‒ K); Calder et al. (2019: fig. 4 B)], but in that species the internodes are long and geniculate (Billard 1933: fig. 8 K); 2) H. praeparvum Calder, 2017, but here it produces as much as 4 ‒ 6 oocytes, and the species can also be distinguished through its strongly flaring hydrothecae, and an occurrence in cold waters (Bay of Fundy, Canada).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFC7FFB6FF1DFCDF2E3BFF36.taxon	etymology	Etymology. Named after its occurrence on thalli of Turbinaria algae (Fucales: Sargassaceae).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFC7FFB6FF1DFCDF2E3BFF36.taxon	distribution	Distribution. Glorioso Islands (French Southern and Antarctic Lands) (Gravier-Bonnet & Bourmaud 2005, as Halecium sp.), Bali (Indonesia) (present study). Incertae sedis 3	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFC3FFA9FF1DFED82A63FBD0.taxon	description	Figs 12, 13 A ‒ H & M ‒ U, 14, 30	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFC3FFA9FF1DFED82A63FBD0.taxon	materials_examined	Material examined. Holotype: MSNMCoe 367, Indonesia, Bali, Amed, Lipah Bay, - 8.350152 °, 115.683861 °, 2 ‒ 4 m, 26 Jan 2020, GenBank: OR 872061 – OR 872064 (16 S), OR 872010 (18 S). — Paratype: MSNMCoe 368, same collecting data as for the holotype, GenBank: OR 872057 – OR 872060 (16 S), OR 872009 (18 S). — Additional material: MSNMCoe 369, Nemalecium sp., same collecting data as for the holotype, GenBank: OR 872065 – OR 872068 (16 S), OR 872011 (18 S).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFC3FFA9FF1DFED82A63FBD0.taxon	description	Description. Colonies living in anfractuosities of scleractinian corals (Fig. 12 A), where they form a continuous belt along the lateral walls (but not the bottom of the cavities), partly embedded among sand grains, detritus, filamentous algae and diatoms (Fig. 12 B); stems erect, monosiphonic, either unbranched (Fig. 13 M) or sympodially-branched (Fig. 13 B ‒ H, N ‒ R), up to 5 mm high, borne on short apophyses of a creeping, branching hydrorhiza firmly adhering to the substrate; composed of fairly long hydrophores, often with 1 ‒ 3 intervening, proximal athecate internodes with slight bulges at both ends (Fig. 13 B ‒ H, M ‒ R), nodes straight; a hydrotheca on distal part of the hydrophore; one (Fig. 13 C ‒ F, H, N, O) or a pair (Fig. 13 B, G, P ‒ R) of 75 ‒ 125 µm long apophyses are given off laterally from below the hydrothecal base, each bearing a hydrophore of varied length, ending distally into a hydrotheca; an intervening athecate internode may be occasionally present between the apophysis and the hydrophore (Fig. 13 D, N, P, Q); subsequent branching pattern similar throughout the colony, with up to 5 th order branches formed, giving rise to cymose colonies (e. g. Fig. 13 F, G, P, Q), composed of up to a dozen polyps; hydrophores 155 ‒ 425 µm long, intervening athecate internodes 230 ‒ 380 µm long, diameter at nodes 70 ‒ 90 µm. Hydrothecae (Fig. 13 S ‒ U) moderately deep (55 ‒ 65 µm high), with circular, 170 ‒ 200 µm wide apertures, walls slightly diverging distally from a 125 ‒ 140 µm wide, transversely-set, thin diaphragm (topped by a belt of large, ovoid desmocytes), rim smooth, sometimes inwardly-curved; rare renovations could be noted, with the secondary hydrothecae borne on fairy long hydrophores (Fig. 13 F, O). Hydranths (Fig. 13 A) highly extensible, up to 4 mm long, composed proximally of an exceedingly long, slender, tubular, non-digestive portion (containing large, vacuolated cells), and distally of a short, globular, digestive part (filled with gland cells) bearing apically a conical hypostome, encircled by a whorl of 22 ‒ 25 filiform, amphicoronate tentacles, among which occur 1 ‒ 2 scorpioid nematodactyls; tentacle bases with adjacent clusters of gland cells, appearing visually as refringent patches; when a couple of nematodactyls is present, they are set at an angle of 80 ‒ 120 ° between each other with respect to the perimeter of the hypostome; non-digestive and digestive parts of the hydranth separated by a narrow constriction, allowing the head to move freely in all directions. Gonothecae not seen. Cnidome (Fig. 14): pseudostenoteles [(23.8 ‒ 28.9) × (10.0 ‒ 11.0) µm], banana-shaped microbasic mastigophores [(5.8 ‒ 6.2) × (1.5 ‒ 1.7) µm], large, ovoid microbasic heteronemes [(7.6 ‒ 7.9) × (2.7 ‒ 3.1) µm], and small, ovoid microbasic heteronemes [(5.2 ‒ 5.5) × (2.4 ‒ 2.6) µm]. Color in life (Fig. 12): the hydranth column is, for most of its length, translucent, except for its lumen and distalmost part that are white; the apical, digestive region is translucent, the tentacles have a distinctive fluorescent bluish tinge, while the hypostome and nematodactyls are milky white.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFC3FFA9FF1DFED82A63FBD0.taxon	discussion	Remarks. The new species is immediately distinguished from its two congeners [viz. N. lighti (Hargitt, 1924) and N. gracile Galea et al., 2012] both ecologically and morphologically. Unlike them, it lives exclusively in coral anfractuosities, forming minute, sympodial stems, able to find shelter in such small cavities. Additionally, it forms a well-supported monophyletic clade, well separated from specimens of N. lighti from multiple localities (Fig. 30). However, genetic divergence exists also between N. caeruleus sp. nov. and other morphologically indistinguishable specimens identified as Nemalecium sp. in this study and in Boissin et al. (2018). The present material matches the typical ecology and distinctive “ white blue opalescent coloration of the top of the hydranths ”, documented by Gravier-Bonnet & Mioche (1996: 169) in specimens from Reunion Island. Gravier-Bonnet & Migotto (2000, as N. lighti) discussed its reproduction, although based on mixed specimens of this species with Brazilian counterparts of N. lighti. The authors appear not to have undertaken a morphological comparison of the colonies from the two localities 4, although a thorough description of N. lighti from Brazil was published earlier (Migotto 1996: 36, fig. 7 H – I). Differences in the “ shape and size of the corpuscles ” of the medusoid were noted, and it was concluded that “ we are not dealing with one widespread or nearly circumtropical species, but rather with two or more sibling species ” (Gravier-Bonnet & Migotto 2000: 213). Moreover, there could be additional, though unstated, differences between their material and specimens of N. lighti described by Bouillon (1986) and Calder (1991) 5, and the authors decided not to name the species “ until thoroughly morphological comparisons are not [sic] undertaken ” (Gravier-Bonnet & Migotto 2000: 208). The present material from Bali allowed us to characterize the hydroid morphologically and genetically, and to assign to it a specific name. Fertile colonies were found during the warm season (from November to February) of the southern hemisphere (Gravier-Bonnet & Mioche 1996: 174), and they were either mono- or dioecious (Gravier-Bonnet & Migotto 2000: 208). The gonothecae are reportedly borne on both the hydrorhiza and stem internodes, and are broadly conical (with smooth to slightly undulated walls), tapering basally, distally being wide and truncate 6. The gonophore is a medusoid; the males are ovoid, while the females are almost rounded; the origin of the manubrium is slightly eccentrically-placed, and it does not open distally into a mouth, being encircled by a compact mass of gametes occupying the whole subumbrellar cavity; there are no canal system and sense organs, except for a belt of refringent corpuscles around the aperture, the latter being closed by a velum. The gonophores are released at dawn, or when the colonies are left in the dark. Unfortunately, Gravier-Bonnet & Migotto (2000) did not state whether the illustrations provided in their fig. 2 represent French or Brazilian specimens, or both. In colonies from Reunion Island, the hydroid coexisted with unidentified species of Rhizogeton and Sarsia “ either at the same time or alternately ” (Gravier-Bonnet & Mioche 1996), although only a few sterile polyps of a corynid were noted to occur among the colonies from Bali. No propagulae were observed in situ so far, although they were produced in vitro (Gravier-Bonnet & Migotto 2000).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFC3FFA9FF1DFED82A63FBD0.taxon	etymology	Etymology. From the Latin caerŭlĕus, - a, - um, meaning (sky) blue, to describe its color in life.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFC3FFA9FF1DFED82A63FBD0.taxon	distribution	Distribution. Based on ecological preferences and visual appearance, the hydroid is scientifically documented from Reunion Island (Gravier-Bonnet & Mioche 1996; Gravier-Bonnet & Migotto 2000; both as N. lighti), Maldives (Gravier-Bonnet & Bourmaud 2012, as Nemalecium sp. 1) and Indonesia (Di Camillo et al. 2008, as Nemalecium sp.; present study). However, at present, only the material from Bali can be unambiguously assigned to N. caeruleus and other distributional records should be confirmed through molecular analyses. Additionally, photographic evidence from iNaturalist. org shows similar specimens (mainly identified as Nemalecium sp.) from India, Christmas Islands, Taiwan, southern Japan, New Caledonia, Fiji and Hawaii, whose specific identities need to be assessed.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFDCFFA1FF1DFADB2BC4FA23.taxon	description	Figs. 15, 18 A – C, 28	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFDCFFA1FF1DFADB2BC4FA23.taxon	materials_examined	Material examined. Holotype: MNHN H. L. 709, Indonesia, Borneo Bank, Siboga Stn. 80, - 2.4166 °, 117.7166 °, 34 m, 13 Jun 1899, microslide preparation belonging to the syntype series, comprising 3 sterile stems growing on Macrorhynchia phoenicea; this is one of the two slides 8 housed in MNHN of Paris that was examined, and contains one stem arising from a portion of hydrorhiza, the proximal portion of a second stem detached immediately above its origin from hydrorhiza, and the distal portion of a third stem. Additional material: MSNMCoe 370, Indonesia, Bali, Pemuteran, - 8.144146 °, 114.657994 °, 0 ‒ 0.5 m, 01 Apr 2023, a profuse, fertile colony (with stems reaching 9 mm in height) partly growing on ship hull, and partly on stems on M. philippina growing themselves on the same hull, GenBank: OR 872071 (16 S). Comparative material: NHM 23.2.15.46 (designated here as lectotype) and NHM 23.2.15.47 (designated here as paralectotype), Republic of Maldives, Huvadhu Atoll, 57 m, two microslides as parts of the syntype of Thuiaria 9 maldivensis Borradaile, 1905, each showcasing a single stem (Fig. 16 A herein). — MNHN-IK- 2012 - 16518, French Polynesia, Marquesas, MUSORSTOM 9, Stn. CP 1265, colony of Dynamena heterodonta (Jarvis, 1922), with sterile stems, up to 2.6 cm high (Galea 2016: 6). — ZSM 20041646, locality unknown [“ possibly Indian Ocean ” (Stechow 1926: 105)], holotype of Tridentata occulta Stechow, 1926, microslide with a “ tiny piece ” of colony (Ruthensteiner et al., 2008: 24) (Fig. 16 C herein). — MHNG-INVE- 91120, French Lesser Antilles, Martinique, Le Vauclin, Pointe Faula, 23 Feb 2014, fertile colony of Amphisbetia distans (Lamouroux, 1816) on floating Sargassum sp. [see Galea & Ferry (2015: 234)]. — ZSM 20041550, Kingom of Tonga, leg. Kirchenpauer, holotype of Sertularia tongensis Stechow, 1919 b, microslide with a “ small piece ” of colony (Ruthensteiner et al., 2008: 24) (Fig. 16 D herein). — ZSM 20050705 & 20050706, French Lesser Antilles, Martinique, Ste. Anne, 03 Apr 1898, leg. F. Doflein, holotype of Tridentata westindica Stechow, 1919 a, two microslides with “ colony pieces ” (Ruthensteiner et al., 2008: 24) (Figs 16 F, 17 D herein). — HRG- 0342, French Lesser Antilles, Les Saintes, Terrede-Haut, 27 Mar 2008, colony of Tridentata turbinata (Lamouroux, 1816) composed of several sterile stem, up to 4 mm high, on Dictyota sp. (Galea 2008: 37).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFDCFFA1FF1DFADB2BC4FA23.taxon	discussion	Remarks. For contemporary descriptions of Tr. borneensis, refer to Gibbons & Ryland (1989) and Schuchert (2003) (both as Sertularia). The species appears genetically divergent from all its congeners, with which it nevertheless forms a monophyletic clade, exclusive of Tr. loculosa (Fig. 28). Morphologically, it should be added that its hydrothecal walls are distinctively chiseled, especially when observed laterally (Figs 15 I, 18 C) or apically (Figs 15 J, 18 B), with a broad, transverse constriction in middle part and an arching ridge running parallel to the adnate adaxial wall (Figs 15 K – L, 18 A). These features are easily overlooked in specimens retaining their coenosarc, unless the stems are carefully examined in lateral view (and at a high magnification of the stereomicroscope). Ideal observations are made on material cleared with a 1 ‒ 2 % solution of domestic bleach. Sole Pennycuik (1959: 197, pl. 6 fig. 5, left-hand side drawing) was able to notice “ [o] ne detail which [Billard] failed to observe ”, namely “ the well developed tooth to which the abcauline caecum of the hydranth is attached ” (see Figs 15 K, 18 A herein). The best illustration of a fully-formed gonotheca so far is that of Gibbons & Ryland (1989: fig. 34 D). Calder et al. (2022: fig. 6 I) also provided a good figure, and a new illustration is now available in Fig. 15 F herein. Mammen (1965: fig. 71, as S. west-indica) and Schuchert (2003: fig. 43 C, as S. borneensis) obviously had specimens with immature gonothecae whose distal horns were not yet formed. It should be noted that the latter author misinterpreted the outer ornamentation of the gonothecal wall (“ wall with sharp, projecting spiral structure in 7 ‒ 8 loops ”), the ridges being irrefutably transverse in this species. The specific name became available in 1925, not 1924, as mentioned in several earlier accounts (e. g. Billard 1925 b; Van Soest 1976; Vervoort & Vasseur 1977; Van Praët 1979; Gibbons & Ryland 1989; Preker 2001; Preker & Lawn 2010). Indeed, Billard’s report was read at the December 23 th, 1924 meeting of the French Zoological Society, and published subsequently on April 10 th, 1925 in numbers 8 ‒ 10 of its Bulletin. As noted by Calder (1991: 110), “ [t] he hydroid identified by Vannucci (1949) as Sertularia borneensis f. parvula appears to have had an intrathecal septum, and it is referred […] to T [ridentata] turbinata (Lamouroux, 1816) ”, and we agree. We also concur with Gibbons & Ryland’s (1989: 419), Calder’s (1991: 110, 2020: 221) and Schuchert’s (2003: 190) arguments that Vervoort & Vasseur’s (1977) specimens assigned to Tr. turbinata (Lamouroux, 1816) belong to the present species instead. The scanning electron microscope images provided by Migotto & Vervoort (1998: figs 2 ‒ 3) of their S. vervoorti leave no doubt about its conspecificity with the present hydroid. Their material obviously bore immature gonothecae. The Guam record by Kirkendale & Calder (2003: 176) is unreliable in the absence of a formal description and / or illustrations. Their colonies are reportedly said “ [r] igid, linear […], with upright cormoids perpendicular to the substratum and usually not more than ⁓ 3 cm in height ”, a size not reported so far in Tr. borneensis [literature data indicate stems ranging commonly from “ about 1 cm ” (Billard 1925 b: 55; present study), to 1.3 cm (Gibbons & Ryland 1989: 418), to 1.5 cm (Preker & Lawn 2010: 131), to maximum 2 cm (Schuchert 2003: 189)]. In addition, for the synonymy of this species, the authors referred to Calder (1991), who considered it as a junior synonym of Tr. tumida (Allman, 1877), which further complicates the taxonomic assessment of their material. Calder et al. (2022: 592), however, listed this record in the reported distribution of Tr. borneensis. The material from Guadeloupe assigned to S. tumida by Galea (2008: 36) may belong to the present species, as well. Unfortunately, the specimen used to draw fig. 7 A in that report is now lost, but photomicrographs of it are still available, though of limited value, as the hydrothecae are only seen in frontal view. Calder et al. (2022: 591) noted that the “ report of S [ertularia] longa from the Galápagos Islands by Calder et al. (2003, as T [r]. longa), based on sterile specimens, is believed […] to have been based on T [r]. borneensis instead ”, and we concur. One of the two slides deposited at the MNHN of Paris, belonging to the syntype of Tr. borneensis, was reexamined. Although the mounted stems could only be seen frontally, the outer, arched hydrothecal ridge (running parallel to the adnate portion of the adaxial wall) is well visible in some hydrothecae (Fig. 15 G, K), as is the internal perisarc projection for the attachment of the blind sac (Figs 15 K, 18 A). There is no doubt that the type material of Tr. borneensis exhibits the same morphological features as those met with in the new material from Bali. The taxonomic status of Tr. borneensis was a matter of debate for nearly a century. Many authors [e. g. Billard (1925 b: 172 ‒ 173); Mammen (1965: 41); Cooke (1975: 100); Calder (1991: 109 ‒ 110; 2020: 220 ‒ 221); Schuchert (2003: 189 ‒ 190); Calder et al. (2022: 591)] noted morphological similarities with one or several nominal species, namely: S. tumida Allman, 1877, Thuiaria maldivensis Borradaile, 1905, S. tongensis Stechow, 1919 b, Tr. westindica Stechow, 1919 a, S. malayensis var. sorongensis Leloup, 1930. According to Calder (1991: 110), type material of S. tumida could not be located, but he subjectively assigned a Bermudan hydroid to that species on the account of some apparent morphological similarities 10. The original account on S. tumida is, unfortunately, little informative (succinct description, sketchy illustrations) to allow a reliable identification, and Allman’s nominal species may possibly have been based on other hydroids known to occur in the (tropical) western Atlantic, e. g. Amphisbetia distans (Lamouroux, 1816) [Calder (1991), as Tr. distans; Migotto (1996), as S. distans; Galea & Ferry (2015), as S. distans], Tr. turbinata (Lamouroux, 1816) 11 [Migotto (1996), as S. turbinata; Galea & Ferry (2015), as S. turbinata], and Dynamena dalmasi (Versluys, 1899) [Vervoort (1959), as S. dalmasi; Calder (2013); Galea et al. (2021)]. The illustration provided by Allman (1877: pl. 16 fig. 4) suggests, at least, that S. tumida has longer and slenderer internodes compared to Tr. borneensis, and it is unlikely to be a senior synonym of it. Type material of Th. maldivensis was reexamined (Figs 16 A – B, 17 A, 18 D – E) and, as originally reported, both stems mounted on slides are unfortunately sterile. Specimen NHM 23.2.15.46 is complete (Fig. 16 A, left), and it is here designated as lectotype; it originates from a branched stolon and is composed proximally of a smooth, 390 µm long, athecate part, ending in an oblique hinge joint, and a 5.15 mm long, distal part divided into a regular sequence of thecate internodes; the nodes are faint and transverse, 70 ‒ 85 µm wide, and the internodes 540 ‒ 675 µm long, rather slender, though gradually expanding from their proximal to distal ends, and there merging smoothly into a pair of contiguous hydrothecae; the latter are slightly more than half adnate (adnate part 180 ‒ 235 µm long, free part 150 ‒ 180 µm long), almost tubular (maximum width 135 ‒ 150 µm), though curving sharply away from the internode, more markedly on their abcauline side (proximally on stem, the angle is of 70 °, but decreases distally to 50 °; the abcauline wall is 195 ‒ 240 µm long). The aperture, whose diameter is 80 ‒ 90 µm, comprises 2 large, lateroabcauline, triangular cusps with rounded tips, in addition to a minute adcauline cusp. The hydranths (all retracted into their corresponding thecae) are not satisfactorily preserved, and do not seem to possess an abcauline caecum. Though difficult to observe, the opercular apparatus is composed of a large abcauline flap, and what appears now to be 2 latero-adcauline, triangular flaps, the latter possibly resulting from the shredding of an original, pleated flap (upon the chemical treatment used to prepare the mounts). Gravier-Bonnet & Bourmaud (2012: 110, as S. maldivensis) included a putative new record of this species from the Maldives but did not provide a description and / or illustrations of this poorly-documented species, simply stating that “ [f] ertile specimen (sic) is needed to go further in identification ”. The record from Hawaii by Calder (2020: 220, fig. 6 C – E, as Tr. maldivensis), is clearly a misidentification, as the hydrothecae do not curve rapidly away from their internodes. The available, scant information on Th. maldivensis appears, at first, potentially problematic. However, the apparent lack of an abcauline caecum would place it into the genus Dynamena Lamouroux, 1812, where it comes close to D. heterodonta (Jarvis, 1922). Type material of the latter (Fig. 18 F, G) was reexamined by Vervoort & Vasseur (1977: 39) 12, and the specimens from the Marquesas studied by Galea (2016: 6, fig. 1 L, N) appear also very similar (Fig. 18 H). Hydrothecae of Th. maldivensis, through their distinctive abcauline bent, recall from afar those of Tr. malayensis (Billard, 1925 a), but their free part in that species is comparatively longer, and the marginal cusps are much sharper [Vervoort & Vasseur (1977: fig. 25 C ‒ D), type material]. The quite unusual specimens of Amphisbetia distans (Lamouroux, 1816) depicted by Calder (1991: fig. 55 A ‒ B, as Tr. distans) have similar hydrothecae, but the division of the stem into internodes is typical of Lamouroux’ hydroid [“ Occasionally two thecate internodes separated by a short athecate internode marked by an oblique node at proximal end and an oblique hinge-joint at distal end ” (Calder 1991: 106); see also below the note on the taxonomic status of Tr. occulta Stechow, 1926]. No additional nominal species of sertulariids are known to have hydrothecae resembling those of Th. maldivensis. Therefore, Th. maldivensis and D. heterodonta are very likely coterminous. Consequently, Borradaile’ (1905) species has priority over that created by Jarvis (1922), and it is here referred to as D. maldivensis (Borradaile, 1905), comb. nov., due to the absence of an abcauline caecum in the polyps. It should also be noted that the type localities of the two nominal species are both situated in the western part of the Indian Ocean [Huvadhu Atoll, the Republic of Maldives for Th. maldivensis, and Saint Brandon (Cargados Carajos Shoals), Mauritius, for D. heterodonta], being distant of 2400 km. Type materials of three nominal species created by Stechow, namely S. tongensis (Fig. 16 D), Tr. westindica (Fig. 16 F, 17 D), as well as the rather enigmatic taxon (not illustrated so far) Tr. occulta (Fig. 16 C), were also examined. The hydrothecae of S. tongensis show the same shape, submarginal intrathecal perisarc projections (two latero-adcauline, one abcauline), and horseshoe-shaped internal ridge (Figs 16 E, 17 C, 18 K) as S. orthogonalis Gibbons & Ryland, 1989, the latter evidently becoming a junior synonym of Stechow’ species (N. B.: the type locality of the former, the Kingdom of Tonga, is distant of only about 800 km from that of the latter, the Fiji Islands). Although originally described based on infertile stems, a description of its gonothecae should be based on Gibbons & Ryland’s account. The paired condition of its hydrothecae, their three-cusped margin and the transversely-ridged gonothecae justify the assignment of Stechow’s hydroid to the genus Tridentata Stechow, 1919 a, under the binomen Tr. tongensis (Stechow, 1919 b), a combination introduced earlier by Calder (1991: 110). In an earlier paper, one of us (Galea 2010: 18 – 20) considered Sertularella tongensis Stechow, 1919 as belonging to the genus Sertularia Linnaeus, 1758, on the account of the morphology of its closing apparatus. In doing so, it became a secondary homonym of the present hydroid, for which the replacement name S. ephemera Galea, 2010 was introduced. However, molecular evidence led Song et al. (2019) create a new genus, Bicaularia, for it. Consequently, S. ephemera should be now dropped. The geographical distribution of S. tongensis is restricted, to date, to the Kingdom of Tonga (Stechow 1919 b, as S. tongensis; Gibbons & Ryland 1989, as S. orthogonalis) and the Hawaiian Islands (Calder & Faucci 2021, as Tr. orthogonalis). Tridentata westindica has hydrothecae provided with a transverse, horseshoe-shaped, internal ridge in their middle part (Figs 16 G, 17 E, 18 L), and are indistinguishable from those of the type of Tr. turbinata (Lamouroux, 1816) depicted by Billard (1925 b: fig. 34 M, as S. turbinata). Tridentata westindica is thus a junior synonym of Lamouroux’ species. The occurrence of Tr. turbinata in Martinique, its type locality, is confidently-documented (Galea & Ferry 2015, as S. turbinata). Based on specimens from Les Saintes (HRG- 0342), whose hydrothecae have been perfectly cleared with domestic bleach, it is realized that there are many attachment sites of the hydranths to the inner walls of their corresponding hydrothecae, as illustrated in Fig. 18 M. The shape of the hydrothecae of Tr. occulta, together with the characteristic presence of internal, perisarcal projections (Figs 17 B, 18 I), as well as the division of the stem into moderately-long, hydrothecate internodes occasionally alternating with comparatively-shorter, athecate internodes (the latter with a proximal, transverse node and a distal, oblique node; Figs 16 C, 18 I), are features of Amphisbetia distans (Lamouroux, 1816) (Billard 1925 c: fig. 1, as S. distans; N. B.: his figs 1 F ‒ G illustrate type material). Consequently, T. occulta becomes a junior synonym of Lamouroux’ species. In conclusion, Tr. borneensis has only two junior synonyms: an objective one, namely S. vervoorti, and a subjective one, S. malayensis var. sorongensis (specimens of the latter should be examined to confirm the synonymy). As demonstrated above, none of the nominal species whose basionyms are S. tumida, Th. maldivensis, S. tongensis and Tr. westindica belong to its synonymy.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFDCFFA1FF1DFADB2BC4FA23.taxon	distribution	Distribution. Indonesia (Billard 1925 a, b, as S. borneensis; Leloup 1930, as S. malayensis sorongensis; Schuchert 2003, as S. borneensis; present study), India (Mammen 1965, as S. west-indica), Spratly Islands (Tang 1991, as S. westindica), Philippines (Gibbons & Ryland 1989, as S. borneensis), Australia (Pennycuik 1959, as S. borneensis; Preker 2001, as S. borneensis; Preker & Lawn 2010, as S. borneensis), possibly Guam (Kirkendale & Calder 2003), Marshall Islands (Cooke 1975, as S. westindica), Fiji (Gibbons & Ryland 1989, as S. borneensis), French Polynesia (Vervoort & Vasseur 1977, as S. turbinata), Galápagos (Calder et al. 2003, as S. longa), Cocos Island (Calder et al. 2022), Brazil (Migotto & Calder 1998, as S. vervoorti), Caribbean coast of Panama (Calder & Kirkendale 2005, as Tr. vervoorti), possibly French Lesser Antilles (Galea 2008, as S. tumida).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFD4FFA3FF1DF96D2D94FDF3.taxon	description	Figs 19, 20, 32; Table 2	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFD4FFA3FF1DF96D2D94FDF3.taxon	materials_examined	Material examined. Holotype: MSNMCoe 371, Indonesia, Bali, Tulamben, dive site known as “ Seraya ”, - 8.295692 °, 115.612838 °, 15 – 20 m, 23 Apr 2023, fertile colony on Halimeda sp. with stems up to 1 cm high, many bearing gonothecae of both sexes. — Paratype: MSNMCoe 372, Indonesia, Bali, Tulamben, dive site known as “ Seraya ”, - 8.295692 °, 115.612838 °, 20 m, 16 Apr 2023, colony on Halimeda sp. with stems up to 1.1 cm high, some bearing very incipient gonothecae, GenBank: OR 872074 (16 S), OR 872013 (18 S), OR 872034 (28 S). Comparative material [Antennella secundaria (Gmelin, 1791)]: HRG- 0124, France, La Ciotat, Mugel creek, 43.164097 °, 5.607644 °, 0.5 m, 05 Aug 2009, fully fertile colony on alga. — HRG- 0153, France, Le Pradet, 43.194122 °, 6.023140 °, 1 m, 27 Aug 2008, colony with incipient gonothecae on alga.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFD4FFA3FF1DF96D2D94FDF3.taxon	description	Description. Colonies arising from branched, anastomosed stolonal fibers creeping on Halimeda sp. (Chlorophyta: Ulvophyceae: Bryopsidales: Halimedaceae); no nematothecae on hydrorhiza; stems borne on short apophyses given off from hydrorhiza; simple (unramified), up to 1.1 cm high, composed of a proximal, ahydrothecate portion, and a much longer, distal, hydrothecate part, the latter sometimes showing apical stolonization; proximal portion comprising 1 – 3 internodes of varied length, each bearing a few frontal nematothecae in a row, internodes separated by transverse nodes, except for the distalmost node that is an oblique hinge joint, allowing the stem to move freely in the current; distal cauline part heteromerously divided into a regular succession of hydrothecate and ahydrothecate internodes, proximal most internode always hydrothecate; the latter, with an oblique node proximally and a transverse counterpart distally; ahydrothecate internodes with nodes in the reverse position; oblique nodes always deeply-incised, transverse nodes distinctly marked in front of the stem, fading away backwards; internodes relatively short, ahydrothecate ones as long or slightly longer than the hydrothecate ones; the latter, bearing a hydrotheca (and its associated nematothecae) in their middle portion, leaving distally a free portion of varied length; ahydrothecate internodes with commonly two (exceptionally three) frontal nematothecae, each placed towards one end. Hydrothecae cup-shaped, relatively deep, adnate for about half their length to the internode behind, wall cylindrical in lateral view; abcauline side relatively thickened, of a slightly sigmoid appearance, being concave proximally and convex distally, giving the hydrothecal aperture a slightly flared appearance abaxially; adnate adcauline wall and hydrothecal floor continuous and of a concave appearance, with relatively thickened perisarc; free part of the adaxial wall almost straight; hydrothecal aperture set at a right angle to the long axis of the theca, circular in apical view, rim even; hydropore eccentrically-placed, adjacent to the abcauline wall, where it occasionally forms a small, inner perisarc thickening; four nematothecae associated to each hydrotheca: a mesial, a pair of laterals, and an axillar one; all nematothecae bithalamic and movable, mesial and axillar sessile, laterals borne on well-developed, cylindrical apophyses flanking the hydrotheca; basal chamber taller than the upper chamber; adaxial wall of the latter distinctly lowered, notably in the axillar nematothecae, where it reaches the inner annular thickening separating the two chambers; lateral nematothecae reaching the hydrothecal rim, adaxial wall sinusoid, with a central, rounded cusp separated laterally from the abaxial side by 2 rounded embayments; axillar nematothecae smaller than all their counterparts; cauline nematothecae sessile, similar to the laterals of the hydrotheca, but upper chambers with walls simply lowered on adaxial side. All hydranths retracted within their corresponding hydrothecae, their tentacle number cannot be ascertained. Colonies monoecious, female gonothecae occurring proximally on stems, the male ones more distally; gonothecae sexually dimorphic, though not differing fundamentally in shape; borne laterally, usually singly, from below the hydrothecal bases, on short, cylindrical stem apophyses; a short, intermediate internode (delimited at both ends by transverse nodes) at the base of the gonotheca proper; gonothecae piriform, tapering proximally and there with a distinct bent; distally rounded; female with an articulate, glass-watch-shaped operculum closing a wide, circular aperture; males with comparatively narrower apertures closed by thin perisarc layer; a basal pair of nematothecae in both sexes, thecae similar to the cauline ones, but proportionally bigger. Almost all female gonothecae spent, the structure of the gonophore could not be ascertained. Coenosarc of the colony with numerous, spherical zooxanthellae (with large nuclei surrounded by numerous small chloroplasts filling the whole cytoplasm) and unidentified, formless, vacuolated bodies of unknown nature and function. Cnidome (Fig. 20 I): large, ovoid pseudostenoteles [(16.6 – 17.4) × (6.2 – 6.8) µm, some seen discharged], banana-shaped microbasic mastigophores [(5.9 – 6.2) × (2.1 – 2.2) µm, none seen discharged], and small, eggshaped microbasic heteronemes with a distal, eccentric, small prominence [(4.1 – 4.7) × (2.8 – 3.1) µm, none seen discharged]. Color in life (Fig. 19): yellow, due to the presence of zooxanthellae in the coenosarc.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFD4FFA3FF1DF96D2D94FDF3.taxon	discussion	Remarks. The new species comes close to A. secundaria (Gmelin, 1791), but the intense, unmistakable yellow color of its stems is distinctive, while Mediterranean (type locality) specimens of the latter are translucent-white in life (Galea, pers. obs.). Antennella flava sp. nov. is also genetically divergent from all other halopteridid species (including A. secundaria, with which it forms a sister-group relationship) and other A. secundaria - like specimens (Fig. 32). Morphologically, however, there are almost no salient features allowing a reliable distinction between A. flava and A. secundaria. However, the examination of two samples of A. secundaria from southern France (HRG- 0124 and - 0153) revealed that the lateral nematothecae have the adaxial walls of their upper chamber simply lowered, not adopting a marked sinusoid shape as in the new species. Schuchert (1997: 15) also noted that his NE Atlantic specimens of A. secundaria had the “ inner side of the wall of upper chamber lowered almost to bottom ”, Peña Cantero & García Carrascosa (2002: fig. 18 F) depicted lateral nematothecae with adaxially lowered walls in their Mediterranean material, and Medel & Vervoort (1995: 35, fig. 14 B) described and illustrated nematothecae “ with shallow adcauline embayment ” in specimens from around the Strait of Gibraltar. When fixed material of A. flava is examined, the abundant presence of zooxanthellae in the coenosarc indicates that living stems must show some coloration. On the other hand, the examination of the Mediterranean material at hand, however, demonstrated that A. secundaria does have zooxanthellae, as well, but their number is comparatively lower, and the pigment they produce does not confer the stems a notable coloration in life. There are two Australian records of short-stemmed A. secundaria - like congeners exhibiting either a “ golden yellow ” (Watson 2002: 347) or a “ golden brown ” (Watson 2005: 537) coloration when alive. Their relationship to the new species could not be established strictly based on morphological grounds.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFD4FFA3FF1DF96D2D94FDF3.taxon	etymology	Etymology. From the Latin flavus, - a, - um, meaning yellow, to describe its color in life.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFD4FFA3FF1DF96D2D94FDF3.taxon	distribution	Distribution. Only known from Bali, Indonesia.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFD1FFA5FF1DF9152C1CFE3F.taxon	description	urn: lsid: zoobank. org: act: AD 646 F 86 - 77 CA- 43 F 0 - 8 F 5 F- 4046057 ED 127 Figs 21, 22, 34	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFD1FFA5FF1DF9152C1CFE3F.taxon	materials_examined	Material examined. Holotype: MSNMCoe 373, Indonesia, Bali, Amed, dive site known as “ Ghost Bay ”, - 8.332965 °, 115.643044, 15 – 20 m, 18 Apr 2023, sterile colony composed of several branched stems, tallest 7 cm high, GenBank: OR 872020 (18 S), OR 872040 (28 S), OR 866288 (COI). — Paratype: MSNMCoe 374, same collecting data as for the holotype, 10 – 15 m, 04 Oct 2022, sterile colony composed of several branched and unbranched stems, tallest reaching 10.5 cm high, GenBank: OR 872019 (18 S), OR 872039 (28 S), OR 866287 (COI). Additional material: HRG- 1783, same data as for the holotype, 15 m, 25 Apr 2023, a 4 cm high, sterile colony. — HRG- 1784: same data as for the holotype, 10 – 20 m, 07 Oct 2022, 2 sterile colonies ca. 11 cm high. — HRG- 1785: same data as for the holotype, 10 – 15 m, 04 Oct 2022, a 10.5 cm high, sterile colony.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFD1FFA5FF1DF9152C1CFE3F.taxon	description	Description. Colonies large (up to 11 cm high in the field), of bushy appearance, with fascicled stems and branches, usually fixed in sediment by means of a dense, ramified, stolonal fiber network attached not only to the proximal tip of the caulus, but also laterally to a whole proximal portion of it, ensuring an effective anchoring among sand grains and small pebbles. Branching occurring at irregular places, with a tendency to alternate, to ensure a good balance of the colonies in an environment dominated by currents; up to 3 rd order branches, given off laterally and slightly in front of the stem or higher order branches. Main tubes of stem and branches reinforced dorsally by bundle of undivided auxiliary tubes (not provided with nematothecae), but communicating with one another and with their main counterparts through pores in the perisarc; main tubes of side branches not arising from the main tube of the stem, but from different cauline auxiliary tubes; all main tubes in a colony regularly divided into moderately-long (460 – 590 µm), rather slender internodes, delimited by 130 – 175 µm wide, transverse nodes; each internode provided with a 110 – 135 µm long, lateral, subterminal, slightly anteriorly-shifted apophysis supporting a cladium, and two nematothecae, one towards each end; apophyses biseriate, alternate, relatively short, with a basal, conical mamelon (45 – 50 µm high, 15 – 20 µm wide at aperture) and an oblique node distally; nematothecae sac-shaped, elongate (120 – 140 µm long, 60 – 75 µm wide), monothalamic, adnate for more than half their length to the internode behind, aperture either small (10 – 15 µm wide), circular (mounted atop a conical projection provided with a second, axillar aperture behind) or gutter-shaped (the apical aperture having fused with the axillar one); proximal nematotheca a short distance above the proximal node; distal nematotheca adjacent to the cladial apophysis. Proximal parts of the main tubes of branches of varied length, undivided, and provided with a succession of frontally-placed, widely-spaced nematothecae in a row, ending distally in oblique hinge joint, the latter allowing the much longer, distal, cladiate part to move freely in the current. Cladia borne on both stem and branches, distant of ca. 1 mm from one another (on the same side), long (up to 8 mm), given off at an angle of 50 – 55 ° with the main tubes, composed of a regular succession of up to 18 cormidia delimited by slightly oblique, 85 – 90 µm wide nodes (in lateral view); each cormidium relatively short (400 – 440 µm), accommodating a hydrotheca and its associated nematothecae; hydrothecae adnate for most of their length (leaving distally but a reduced, 46 – 60 µm long portion free from the internode), elongate, concave along their longitudinal axis; adnate adaxial wall 305 – 315 µm long, thin, concave, in proximal most hydrothecae projecting basally an incomplete annular ridge into the internode behind; mesial nematotheca adnate for about 2 / 3 of its length (distal 80 – 100 µm are free), leaving only a short portion of the abaxial hydrothecal wall free, the latter being fully occupied by a prominent, triangular septum projecting halfway into the thecal lumen; hydrothecal aperture 165 – 175 µm wide, tilted abaxially, margin provided laterally with a pair of board, very low, triangular cusps, in addition to the central, abaxial, linguiform cusp formed by the upper tip of the intrathecal ridge; mesial nematotheca arching adaxially, length not surpassing the level of the hydrothecal rim, provided distally with a small (ca. 25 µm wide), rounded aperture, and basally (in the axil formed with the abaxial hydrothecal wall) with an additional, rounded foramen, nematotheca separated from the hydrothecal lumen by a thin, concave, perisarcal septum; lateral nematothecae 125 – 150 µm long, horn-shaped, adnate to the hydrothecal wall for most of their length, apically provided with a small (ca. 15 µm wide), circular aperture, and basally with a large, ovoid foramen on their adaxial side. Hydranths bearing a whorl of 14 – 16 filiform, unicoronate tentacles surrounding a small, dome-shape hypostome. Gonosome absent. Color in life (Fig. 21): stems and branches with a greenish to pale brown tinge, cladia white.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFD1FFA5FF1DF9152C1CFE3F.taxon	discussion	Remarks. The new species resembles M. philippina Kirchenpauer, 1872, but it is readily distinguished in situ through the absence of a striking irisation of its cladia, and the rather greenish to light brown tinge of its stems and branches (instead of being dark brown to black). Macroscopically, the examination of several colonies revealed that it has longer (6.5 – 8 mm) and more widely-spaced cladia (10 – 12 per side / cm), compared to specimens of M. philippina from various localities (Bali, Martinique, Madeira, Madagascar and United Arab Emirates), in which their length is 3.5 – 5 mm and their density of 13 – 17 per side / cm. Cladia not reaching the length met with in the new species were also reported in Kirchenpauer’s hydroid by Migotto (1996, up to 4.8 mm), Calder (1997, up to 5 mm), Pictet (1893, up to 6 mm), Watson (2000, up to 6 mm). Genetically, M. fallax is closely-related to M. philippina and M. filamentosa (Lamarck, 1816), being nevertheless divergent from them (Fig. 34). Microscopically, the hydrothecae of M. fallax sp. nov. are comparatively deeper and slenderer, their abaxial wall is longer (and, implicitly, the intrathecal septum is broader), and the mesial nematothecae less adjacent to their corresponding hydrothecae; additionally, the internodes of the main tubes are longer than in M. philippina, and the nematothecae they bear are comparatively more elongate than the relatively compact and broader ones in Kirchenpauer’s hydroid. Additionally, unlike in M. philippina, internal perisarcal ridges are absent from the cladial internodes of the new species, and this is also true for the oldest parts of the colonies. Some literature records assigned to M. philippina may have been based on specimens belonging to the present species instead, as demonstrated by their long cladia and more elongate and slenderer hydrothecae compared to Kirchenpauer’s hydroid, e. g. Schuchert (2003), whose colonies had ca. 10 cladia per side / cm (deduced from his fig. 67 A) and up to 18 cormidia / per cladium. Macrorhynchia ambigua Watson, 2000, a closer congener from northern Australia, builds more denselypinnate colonies and its hydrothecae come closer in shape to those of M. philippina (Watson 2000: fig. 55 B and D, respectively). In our opinion, the absence of the gonosome in the specimens examined does not represent an impediment in describing them as belonging to a new species. Indeed, the phylactocarps of Macrorhynchia have a rather uniform structure, and emphasis is primarily placed on the trophosome, especially the hydrothecal features.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFD1FFA5FF1DF9152C1CFE3F.taxon	etymology	Etymology. From the Latin fallax, - acis, meaning misleading, to emphasize its deceptive resemblance to its closest congener, M. philippina.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFD1FFA5FF1DF9152C1CFE3F.taxon	distribution	Distribution. Known with certainty only from Bali.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFD0FFA5FCECFE6D2EB3FDF3.taxon	materials_examined	HRG- 1768, Indonesia, Bali, Pemuteran, - 8.144146 °, 114.657994 °, 0 – 0.5 m, 01 Apr 2023, a fully fertile colony. — HRG- 0817, France, Martinique, Le Prêcheur, Ponton des Abîmes, 14.807505 °, - 61.226567 °, 5 – 18 m, 25 Feb 2012, sterile colony. — HRG- 0191, United Arab Emirates, near Al Badiyah, 25.429167 °, 56.368055 °, 1 m, 23 Aug 2006, sterile colony. — HRG- 1480, Portugal, Madeira, Ponta da Cruz (western side of Funchal), 12 m, 25 Sep 2000, sterile colony.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE9FF9CFF1DFA902CFBF9CF.taxon	discussion	Ectopleura viridis (Pictet, 1893) is sister to all other Ectopleura species included in the analyses, but the nodal support of the genus based on 16 S rRNA data is low (Fig. 24 A). Of note, sequences obtained from specimens of E. dumortierii (Van Beneden, 1844) from Chile (Galea 2007: 31) and France (Atlantic Ocean) are here demonstrated to belong to different species, not forming a monophyletic clade and showing a high genetic distance of 12.7 % between them.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE9FF9CFF1DFDA02E07FAB7.taxon	description	A phylogenetic hypothesis based on the COI gene was also produced to assess the phylogenetic affinities of the recently-described C. tropica, given the presence of multiple COI sequences in GenBank, and its sister-group relationship with C. bigelowi was confirmed (Fig. 23 B). Notably, sequences obtained from Chinese samples identified as C. bigelowi and C. verrucosa (Bouillon, 1978), respectively, clustered together, showing minimal genetic distance (intra-group distance: 0.7 ± 0.2 %), indicating that they likely belong to the same species, viz. C. verrucosa. As noted by Galea (2023: 457), the present concept of C. bigelowi likely comprises a species complex. The Japanese material from which the sequences EU 448099 (16 S), EU 272618 (18 S), EU 272563 (28 S) and JX 121581 (COI), used to build the trees illustrated in Fig. 23 herein, bears the catalogue number KUNHM 2829 of the Kansas University Biodiversity Institute and Natural History Museum, and is illustrated under that species in the World Hydrozoa Database.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE9FF9CFF1DF9582B63F8BB.taxon	discussion	The 16 S rRNA sequence of C. haddoni Kirkpatrick, 1890 from Bali is sister to conspecific Maldivian sequences, even though it shows a relatively high genetic distance of 6.8 ± 1.1 % (Fig. 24 B).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE9FF9CFF1DF88C2AF3F867.taxon	discussion	Sphaerocoryne bedoti Pictet, 1893 is a well-established species with a wide distribution and limited intra-specific divergence, as confirmed by the inclusion of the Balinese sequence in the phylogenetic reconstruction (Fig. 24 C).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE8FF9DFF1DFE732E80FDC9.taxon	discussion	Myrionema amboinense Pictet, 1893 is closely-related to a conspecific sequence obtained from aquarium material from the University of Innsbruck, Austria (P. Schuchert, pers. comm.), whereas the two identical 16 S rRNA sequences of Eudendrium sp. (lacking mature gonophores and, hence, unidentifiable) did not match any known sequenced congener (Fig. 25 B).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE8FF9DFF1DFF682CA9FEA9.taxon	discussion	Sequences of the Balinese Corydendrium sp. cluster with counterparts from other tropical localities of the Indo-Pacific and Caribbean Sea, and diverge from sequences obtained from samples collected in Norway and Sweden, with a genetic distance of 12.3 ± 1.4 % between the two clades (Fig. 25 A).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFEAFF9FFF1DF9DF2A3AF97C.taxon	discussion	Genetic data for Hebella laterocaudata Billard, 1942 are produced for the first time, and so are the two sequences of a hydroid recalling H. furax Millard, 1957. The latter, however, are quite divergent from one another (genetic distance of 8.2 %), and one of them is very similar to the sequence JN 714647 provisionally assigned to Anthohebella parasitica (Ciamician, 1880) from the Azores, this actually appearing not to belong to that nominal species (Fig. 26 A).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFEAFF9FFF1DF8BF2FAAF878.taxon	discussion	Synthecium flabellum Hargitt, 1924, here sequenced for the first time, forms a well-supported monophyletic group (BPP = 0.99, MLBS = 95) with both S. evansi (Ellis & Solander, 1786) and S. tubithecum (Allman, 1877) (Fig. 26 B).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE5FF91FF1DFBAA2D26FE67.taxon	discussion	Colonies of Thyroscyphus fruticosus Esper, 1793 from Bali comprise unbranched, up to 7 cm high cauli, sparinglybranched stems, with 1 – 3 irregularly-placed side branches not exceeding 9 cm in height, and more robust, pinnatelybranched stems reaching (or even exceeding) 12 cm in height. The first two display the same phenotype as the material assigned by Pictet (1893: 37, pl. 2 fig. 32) to Lytoscyphus junceus (Allman, 1876), and subsequently considered by Splettstösser (1929, as T. bedoti) to represent a species distinct from the widely-spread T. fruticosus 13. A careful microscopical examination off all morphotypes, supported by a comparison of their corresponding line drawings, revealed no difference, suggesting that T. bedoti is an artificially-created nominal species. Schuchert’s (2003: 194, fig. 48) account, based on the reexamination of Pictet’s material, confirms, in our view, this assumption. Our specimens from Bali, clearly do not belong to another congener, e. g. T. torresii (Busk, 1852), also known to occur in Indonesia (Schuchert 2003: 196), and conform to the concept of the species highlighted by Watson (2000: 39), i. e. more straggling colonies vs. a tidier aspect, and the absence of internodes from the stems and branches vs. a regular division by distinct nodes, respectively. Colonies of T. fruticosus from Bali display many colors: yellow, orange, pink and purple. The 16 S sequences obtained from Balinese samples are almost identical, despite their respective colonies showing a different coloration: MG 811641 corresponds to a yellow hydroid, while MG 811642 to a purple one. The material from India assigned by Arun et al. (2020) to the western Atlantic T. ramosus Allman, 1877 is obviously based on a misidentification (Fig. 27 A). Indeed, their sequence MH 392732 clustered with our sequences and is clearly divergent from the well-established Atlantic T. ramosus (Fig. 27 A). Boissin et al. (2018) identified two main clades of Indo-Pacific Thyroscyphus that they assigned to T. fruticosus and T. bedoti. However, they did not provide any morphological information on their samples. According to our data, their clade called T. bedoti, with which our Balinese sequences clusters (Fig. 27 A), appears to be T. fruticosus, whereas their other clade, called T. fruticosus, is likely another species (indicated in our tree as Thyroscyphus sp.). Of note, T. aequalis Warren, 1908, a species known to occur in the tropical East Africa and Madagascar (Millard 1975), builds similar colonies, and specific differences are mainly noted in the morphology of their hydrothecae; this species could have been erroneously taken for T. fruticosus.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE4FF91FF1DFDB82BE5FD63.taxon	discussion	Genetic data (16 S rRNA region) for Sertularella decipiens Billard, 1919 and S. quadridens (Bale, 1884) are provided for the first time, whereas the sequence of S. diaphana (Allman, 1885) appears to be slightly divergent from that of a specimen from the Philippines (Fig. 27 B), with an intra-specific genetic distance of 2.7 %.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE4FF91FF1DFCBC2B96FAC7.taxon	discussion	The 16 S rRNA sequence of Tridentata borneensis Billard, 1925 a from Bali did not match any other sequenced sertulariids, but clustered with other Tridentata species (Fig. 28) in a well-supported clade (BPP = 1, MLBS = 89). Tridentata loculosa (Busk, 1852) has hydrothecae lacking the prominent, lateral, marginal cusps characteristic of the genus Tridentata, its hydranths are strikingly provided with a ligula, and its gonothecae (although transverselyringed) are closed by a watch-glass-shaped operculum (Migotto 1996: 72), making it somehow distinct from members assigned to that genus so far. Additionally, the 16 S rRNA sequence obtained from a Balinese specimen does not cluster with other Tridentata sequences, the latter forming a well-supported monophyletic group (Fig. 28) also including a sequence of T. trigonostoma (Busk, 1852) from Bali, which is identical to a conspecific sequence from Thailand. Two other sertulariids, namely Idiellana pristis (Lamouroux, 1816) and Diphasia mutulata (Busk, 1852), were also sequenced. The first clusters with a sequence of the same species from Thailand (with an intra-specific genetic distance of 0.4 ± 0.2 %), whereas the second, here sequenced for the first time, is sister to all other Diphasia sequences included in the analyses, even though the support values for the genus are low (Fig. 28).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE4FF91FF1DFA582A20F82B.taxon	discussion	According to the 16 S rRNA phylogenetic reconstruction, Halecium turbinariae sp. nov. is divergent to all other sequenced congeners, thus supporting its establishment (Fig. 29). Halecium halecinum var. minor Pictet, 1893, originally described based on sterile material, reminds Linnaeus’ (1758) species through the regularly pinnate structure of its colonies. The species was rediscovered in Bali, although the available specimens are equally devoid of gonothecae. However, upon comparison with a profuse, fully fertile, female colony of H. halecinum (Linnaeus, 1758) from Britanny, France, it is realized that Pictet’s hydroid builds less robust colonies, with comparatively slenderer internodes, and the regular occurrence of a moderately-long, tubular, athecate internode at the origin of its cladia is distinctive. The two 16 S rRNA sequences obtained from Balinese specimens are divergent from Atlantic H. halecinum sequences (Fig. 29), also showing a genetic distance of 11.6 ± 1.4 %. Based on both morphological and genetic evidence, the so-called variety is raised to species, as H. minor, nov. status. Additionally, one newly-obtained and two unpublished 16 S rRNA sequences of Halecium sibogae Billard, 1919 form a fully-supported monophyletic group with minimal intra-specific distance (0.1 ± 0.1 %) (Fig. 29).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE1FF96FF1DFF682C5CFD17.taxon	discussion	Boissin et al. (2018) assessed the genetic diversity of Nemalecium from the Indo-Pacific, including specimens belonging to N. lighti (Hargitt, 1924) and an unnamed congener (identified as Nemalecium sp.), and found a high genetic diversity in the latter, hypothesizing the presence of multiple cryptic species. We provide an updated 16 S rRNA phylogenetic hypothesis for the genus, including sequences of N. caeruleus sp. nov., Nemalecium sp. and N. lighti from Bali (Fig. 30). Nemalecium caeruleus clusters within a clade comprising sequences of Nemalecium sp. 1 sensu Boissin et al. (2018), even though that clade shows low statistical support. The overall genetic distance within the clade composed of N. caeruleus and Nemalecium sp. 1 sensu Boissin et al. (2018) is relatively high (2.5 ± 0.4 %), and so is the genetic distance between N. caerulues and Nemalecium sp. 1 from all western Indian Ocean localities (3.5 ± 1.0 %). These relatively high genetic distance values, together with the absence of any morphological and ecological information on Nemalecium sp. 1, leave open the question whether they belong to the same species or not. Specimens morphologically indistinguishable from N. caeruleus surprisingly clusters with sequences of Nemalecium sp. 2 sensu Boissin et al. (2018) in a well-supported clade (BPP = 1, MLBS = 98), showing a genetic distance of 7.3 ± 1.0 % from the clade composed of N. caeruleus and Nemalecium sp. 1, and an intra-clade distance of 2.0 ± 0.4 %. Finally, the sequence of the Balinese N. lighti clusters in a fully supported clade together with other conspecific sequences from the Indian Ocean and Caribbean Sea. It is evident that a great genetic diversity, with genetic distance values much higher than typical intra-specific values for most hydrozoan species (i. e., pairwise genetic distances between the supposedly cryptic species ranging 7.1 – 7.6 %), occurs among Indo-Pacific Nemalecium specimens studied herein and by Boissin et al. (2018), even between the morphologically indistinguishablesamples analyzed in this study. However, the absence of taxonomical accounts for the specimens dealt with by Boissin et al. (2018), together with the lack of any information about their habitat, hamper a clear understanding of the diversity of this group and, similarly, a correlation with the hydroid inhabiting coral crevices (Gravier-Bonnet & Mioche 1996) could not be established with certainty for all their clades.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE3FF96FF1DFD2C2D24FB9F.taxon	discussion	The sequenced plumulariid species from Bali include: Dentitheca elongata (Billard, 1913), Cladacanthella scabra (Lamarck, 1816), Plumularia procumbens Spencer, 1891 and Sibogella flabellata Di Camillo & Galea, 2020, for which genetic data are provided for the first time, and P. badia Kirchenpauer, 1876, P. strictocarpa Pictet, 1893, Sciurella cylindrica (Kirchenpauer, 1876) and S. erecta Billard, 1911 a (Fig. 31 A), with available sequences so far. A multi-locus phylogenetic hypothesis based on 16 S, 18 S and 28 S rRNA was also produced to assess the phylogenetic position of C. scabra, which results to be closely-related to P. badia, P. spiralis Billard, 1911 b and D. bidentata (Jäderholm, 1905) (Fig. 31 B).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE3FF96FF1DFBA02B25F916.taxon	description	As equally shown in Fig. 32, genetic data (16 S rRNA) on the family Halopterididae are provided for the first time for A. sibogae (Billard, 1911 b), as well as for Balinese specimens of Halopteris vervoorti Galea, 2008 and Polyplumaria cornuta (Bale, 1884), supplementing those obtained earlier (P. Schuchert, unpublished results; Galea et al. 2018; Galea & Maggioni 2020) for A. billardi Galea (in Galea et al.), 2021, H. diaphana (Heller, 1868), H. longibrachia Calder & Faucci, 2021, H. plagiocampa (Pictet, 1893), H. platygonotheca Schuchert, 1997 and H. polymorpha (Billard, 1913). 16 S rRNA sequences of H. diaphana, H. plagiocampa, H. platygonotheca, H. vervoorti and P. cornuta cluster with conspecific sequences from other localities (Fig. 32). Finally, several Antennella secundaria - like colonies, white- to pale-green-colored and not morphologically separable from Gmelin’s (1791) hydroid were collected from the island, showing an overall high genetic divergence from each other (sequences OR 872075, OR 872076, OR 872077, OR 872078, OR 872079, MF 784533) and suggesting that the diversity of A. secundaria - like hydroids is currently underestimated (Fig. 32).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFE3FF97FF1DF9292B10FE67.taxon	description	The 16 S rRNA region of M. fallax could not be amplified, despite several attempts, while other regions were successfully amplified and sequenced (COI, 18 S rRNA, and 28 S rRNA). Specifically, a phylogenetic hypothesis based on the COI region was obtained to assess the relationships between M. fallax and other aglaopheniid species (Fig. 34). The two COI sequences obtained for M. fallax are identical to each other and closely related to sequences obtained from M. philippina Kirchenpauer, 1872 and M. filamentosa (Lamarck, 1816). However, they show high genetic distances from these two species, viz. 9.9 ± 1.2 % and 11.1 ± 1.2 %, respectively.	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
03A5566CFFFCFF89FF1DFB302AEFF9E4.taxon	discussion	Campanularia spinulosa Bale, 1888, originally described without its gonosome and also sterile in the material at hand, recalls Obelia bidentata Clark, 1875 but, unlike that species, it builds flabellate instead of cypress-shaped colonies “ with lateral branches tending to be in right-angled pairs, successively on opposite sides of stem ” (Cornelius 1995: 292, fig. 68 A). Superposition of line drawings of specimens from Bali with Bale’s (1888) pl. 12 fig. 5, Thornely’s (1900, as Gonothyraea longicyatha) pl. 44 fig. 4, and Schuchert’s (2003, as O. bidentata) fig. 24 (right hand side drawing), revealed the same shape and proportions, suggesting that all are very likely conspecific. According to the present molecular evidence based on the combined 16 S, 18 S, and 28 S rRNA (Fig. 35 A), the species obviously belongs to the genus Obelia Péron & Lesueur, 1810, and it should be confidently referred to as O. spinulosa (Bale, 1888) 14. Two 16 S rRNA sequences of Clytia linearis (Thornely, 1900) were also obtained from Balinese samples, both clustering with other available sequences of C. linearis from the Atlantic Ocean and Mediterranean Sea (Fig. 35 B).	en	Galea, Horia R., Maggioni, Davide (2024): On some tropical hydroids (Cnidaria: Hydrozoa), with descriptions of four new species. Zootaxa 5428 (1): 1-57, DOI: 10.11646/zootaxa.5428.1.1, URL: http://dx.doi.org/10.11646/zootaxa.5428.1.1
