Celsinotum macronyx (Daday, 1898)

Daday, 1898: 35–37, Fig. 15 ( Alona); Shen, Tai & Chiang, 1966: 36–37, 42, Figs 24–26 ( Kurzia yunnanensis); Smirnov, 1971: 505, Fig. 646 ( Indialona); Chiang & Du, 1979: 205–206, Fig. 47 ( Kurzia yunnanensis); Idris, 1983: 102, Fig. 47; Rane, 1983: 194–195, Figs. 1–4 ( Indialona jabalpurensis); Rajapaksa & Fernando, 1985: 970–975, Figs 1 –26 ( Alona); Rajapaksa & Fernando, 1987: 216–218, Figs 1–5 ( Alona); Sharma & Sharma, 1990: 106, Figs 8–10 ( Alona); Sanoamuang, 1998: Figs 25–26 ( Alona).

Material studied. 8 parthenogenetic females and 1 male from a paddy field II, near Phitsanulok, foodplain of the River Nan, Phitsanulok Province Thailand, 27.09.1996, coll. L. Sanoamuang, AAK 2004-050; 4 females from littoral of Bau Sau Lake, Cat Tien National park, Dong Nai Province, Vietnam, N 11°45.908’ E 107°.34.475’, 8.05.2009, coll. A.Y. Sinev; 2 females from small forest pool near Bau Sau lake, Cat Tien National park, Dong Nai Province, Vietnam, N 11° 27.178, E 107°20.328, 8.05.2009, coll. A.Y. Sinev.

Redescription. Parthenogenetic female. Body (Fig. 8 A–C, 9A–D) high and rounded in lateral view, strongly compressed laterally, with expressed dorsal keel. Maximum height before the middle of the body, height/length ratio 0.85–0.9 in adults. Dorsal margin evenly arched, postero-dorsal angle not expressed, posterior margin almost straight. Postero-ventral angle (Fig. 8 F) broadly rounded, with about 20 short setules of variable thickness, forming weakly separated groups. A row of about 50–70 long, very thin setules along posterior margin on inner side of carapace, with smaller setules between. Ventral margin convex, with a distinctive outer flange in anterior part (Fig. 8 E), such flange was never recorded for any other Aloninae . About 40 ventral setae (Fig. 8 D), ten ateriormost setae long, with no recognisable setules, next five setae short, about 25 setae in central portion of margin of moderate length, densely setulated bilaterally, last 10–12 setae very short, similar in size to spines of postero-ventral angle. Antero-ventral angle rounded. Carapace ornamentation as well-developed non-anostomosing lines.

Head (Fig. 8 G) elongated and narrow in lateral view, rostrum narrow, obliquely truncated, protruding forward and downwards. Ocellus small, eye two-three times larger than ocellus. Distance from tip of rostrum to ocellus 1.5–2 times larger than that between ocellus and eye. Head shield with ridge going along the midline, maximum width behind mandibular articulation, very wide if flattened (Fig. 8 H). Rostrum narrow, truncated (Fig. 9 F). Posterior margin straight in middle portion. Three major head pores with narrow connection between them, located on a flattened area of head shield (Fig. 8 I, 9E). Middle pore smaller than two others. PP about 0.3–0.5 IP in adults. Lateral head pores at about 0.7 IP distance from midline, at level of central major head pore.

Labrum of moderate size (Fig. 10 A,B). Labral keel oval, with rounded or blunt apex, height/width ratio about 2.5. Anterior margin of keel evenly convex, posterior margin convex without clusters of setae.

Thorax three times longer than abdomen. Dorsal surface of abdominal segments not saddle-shaped. No abdominal projections.

Postabdomen (Fig. 9 G, 10C–D) of moderate length and width, strongly narrowing distally, length about 3.0–3.2 heights in adult. Ventral margin straight. Basis of claws separated from distal margin by deep incision. Distal margin convex, distal angle broadly rounded. Dorsal margin straight in both postanal and anal portions, with distal part about 4 times longer than preanal one, with postanal portion 2 times longer than anal one. Preanal angle weakly protruding, postanal angle well-defined. Preanal margin almost straight. Setae natatoriae about 2.5 length of preanal portion of postabdomen. Postanal margin with 11–14 clusters of 2–5 small, sharp, narrow denticles in each, size of denticles increasing distally, number of denticles per cluster decreasing distally, in some specimens several distalmost denticles are single. Length of longest denticles slightly smaller than width of base of postabdominal claw. Anal margin with 4–5 groups of marginal setules. Postanal portion with 8–10 moderately broad lateral fascicles, longest posteriormost setae of each fascicle, longer than nearby marginal denticles. Anal portion with several smaller fascicles, spaced irregularly in two rows. Postabdominal claw long, weakly curved, 1.7–2 times longer than preanal portion of postabdomen (Fig. 9 H). Basal spine very long, straight and slender, about 0.4–0.5 times claw length.

Antennule (Fig. 10 E) long and narrow, not reaching tip of rostrum. Antennular seta thin, of about half length of antennule, arising at 2/3 distance from the base. Nine terminal aesthetascs, two longest longer than antennule itself. All aesthetascs projecting beyond anterior margin of head shield.

Antenna relatively short (Fig. 10 F). Antennal formula setae 0-0-3/1-1-3, spines 1-0-1/0-0-1. Basal segment robust, branches relatively short, with basal segments 1.5 times longer than middle and apical segments. Setae arising from basal and middle segment of endopodite, 1.5 times longer than endopodite itself. Spine on basal segment of exopodite significantly shorter than middle segment, about 2/3 of its length. Spines on apical segments short, about 1/3 length of the segment.

Maxillule (Fig. 11 A) with three setae.

Thoracic limbs: five pairs.

Limb I large, trunk clearly separated from its base (Fig. 11 B–C). Epipodite small, rounded, with long fingerlike projection 3 times longer than epipodite. Accessory seta short, about 1/4 length of ODL seta. ODL seta with thin setules in distal portion. IDL with three setae and two large clusters of stiff setules. IDL seta 1 very small, rudimentary; setae 2 and 3 without clearly defined annulus, armed with thick stiff setules in distal part, of similar length, slightly shorter than ODL seta. Endite 3 with four setae subequal in length. Endite 2 with three setae, longest of them (e) longer than ODL seta. Endite 1 with two 2-segmented setae, and a stub-like rudiment of flat seta (i). Endites 1 and 2 both without naked seta or sensillum on anterior face. Seven-eight rows of thin setules on ventral face of limb. Two ejector hooks, one little shorter than other. Maxillar process elongated, with short setulated in distal part seta.

Limb II subtriangular (Fig. 11 D–E). Exopodite elongated, setulated distally, without seta. Eight scraping spines, spines 1–2 long, spines 3–5 of moderate length, spines 6–8 short. Size of denticles on basal part of spines evenly decreasing from basal to distal spines. Distal armature of gnathobase (Fig. 9 G) with four elements: two lateral are minute, pointed; one of the middle elements geniculated, setulated in distal part; other broad, with two rows of denticles. Filter plate II with seven setae, two posteriormost members 3–4 times shorter than others.

Limb III (Fig. 11 F–H). Epipodite very small, oval, with finger-like projection two times longer than exopodite itself. Exopodite very small in comparison with limb I, subquadrangular, with six setae. Seta 3 longest, seta 4 and 5 about 1/2 and 1/3 length of seta 3, respectively, other setae short. Setae 1–4 plumose, setae 5 with with short thick setules in distal part, seta 6 naked. Distal endite with 3 setae, two distalmost members scraping, slender, sharp, with spinules in distal part, short bottle-shaped sensillum located between their bases; basalmost seta two times shorter, with long setules on basal side. Basal endite with 4 plumose setae increasing in size basally. Four pointed soft setae increasing in size basally, a small sensillum near the distalmost seta. Distal armature of gnathobase with four elements: first one an elongated, narrowing distally sensillum; second strongly geniculated seta; third and fourthspines. Filter plate III with seven setae of equal length.

Limb IV (Fig. 11 I–J). Pre-epipodite setulated. Epipodite oval with finger-like projection 1.5 times longer than epipodite itself. Exopodite very small in comparison with limb I, with six short setae. Setae 1 and 3 being longest, setae 2 and 5 little shorter about 2/3 length of seta 3, setae 4 and 6 about 1/3 length of seta 3. Inner portion of limb IV with four setae and small cylindrical sensillum. Scraping seta slender, without spinules in distal part, first flaming-torch seta large, with broad base, two other slender, with narrow bases. Small sensillum between the bases of middle and basal flaming-torch setae. Three soft setae slightly increasing in size basally. Gnathobase with short two-segmented seta and small hillock distally. Filter plate IV with five setae of equal length.

Limb V (Fig. 11 K). Epipodite oval, with finger-like projection 1.5 times longer than epipodite itself. Exopodite very small in comparison with limb I, subrectangular, not divided into two lobes, with four short plumose setae, with length gradually decreasing basally. Inner limb portion as oval lobe, with setulated inner margin. At inner face, two setae densely setulated in distal part, of same length as exopodite seta 2. Filter plate consisting of single seta, small sensillum-like structure located between filter plate and gnathobase seta.

Male. Similar in shape to juvenile female of instar II but slightly larger (Fig. 8 J). Dorsal keel developed. Maximum height before the midline, height/length ratio about 0.77. Eye and ocellus of same shape as in female.

Postabdomen (Fig. 10 G) long, irregularly narrowing distally, maximum height at postanal angle. Length about 4 heights. Ventral margin wavy. Postabdominal claws situated on large protrusion. Sperm ducts open laterally, at some distance from the end of postabdomen. posteroventral and posterodorsal rounded. Dorsal margin weakly concave in both postanal and anal portions. Preanal angle well-defined, postanal angle weakly defined. Preanal margin almost straight. Clusters of thin setules in place of female marginal denticles. Lateral fascicles of setules same as in female. Postabdominal claw curved, much shorter than in female, length about 1.5 of that of postanal margin. Basal spine shorter than in female, straight, about 1/3f length of claw itself.

Antennule was not studied.

Limb I (Fig. 11 L–M) with U-shaped copulatory hook. IDL with three setae: seta 1 absent; setae 2 and 3 similar to that of female. Male seta curved, about 2/3 length of seta 2. Copulatory brush seta about 1/2 length of IDL seta 2. Ventral face of the limb under copulatory brush with two rows of 20–25 short stiff setules. Unlike in female, endite 1 with three setae, flat seta (i) not reduced to a stub.

Size. According to Rajapaksa & Fernando (1985), length of adult female is 0.41–0.52 mm, length of all studied adult specimens was within this range. Length of single studied adult male was 0.36 mm.

Distribution. According to Rajpaksa & Fernando (1997), С. macronyx is widely distributed in the Oriental zone, inhabiting India and Sri Lanka, Indochina, Indonesia, Philippines, and South China. Here it is reported from Vietnam for the first time.

Taxonomic notes. Our data on C. macronyx fully agrees with the redescription of the type material conducted by Rajapaksa & Fernando (1985), there is no doubt of the species identity. At the moment, genus Celsinotum includes three West Australian ( C. hypsophilum Frey, 1991, C parooensis Frey, 1991, and C. platamoides Frey, 1991) and two Brazilian species ( C. laticaudatum Smirnov & Santos-Silva, 1995, and C. candango Sinev & Elmoor-Loureiro, 2010). C. macronyx is the only species distributed in South-East Asia. Celsinotum is closely related to genus Leberis Smirnov, 1989, sharing numerous similarities with the latter (Sinev et al. 2005). Celsinotum macronyx shares distinctive features of trunk limb morphology of the Leberis - Celsinotum clade: (1) truncated rostrum, (2) reduced IDL setae 1, (3) exopodite II without setae, (4) filter plate II with two very short posteriormost setae, (5) six setae on exopodite III, (6) truncated postabdomen with numerous small marginal denticles, and others.

There are four main characters suggesting the position of “ Alona ” macronyx in the genus Celsinotum: (1) A well-developed keel on the valves, characteristic of Celsinotum . Species of Leberis lack a keel, having only weekly developed dorsal ridge. (2) Lateral position of sperm ducts on male postabdomen. Such position is observed in the Australian species of Celsinotum (Frey 1991), while in all Leberis species sperm duct openings are located on the ventral margin of postabdomen (Frey 1998; Sinev et al. 2005; Elías-Guttiérez & Valdes-Moreno 2008). (3) Reduction of the third seta of endite 3 of limb I. This seta is present in Australian species of Celsinotum (Frey 1991), but in south American C. candango it is reduced to a small stub, similarly with C. macronyx . In contrast, in all species of Leberis this seta is always well-developed, long, reaching almost to the end of maxillar process (Frey 1998; Sinev et al. 2005; Elias-Guttierez & Valdes-Moreno 2008). (4) Posterior margin of the head shield straight, perpendicular to midline. In Leberis, posterior portion of the head shield is broadly rounded.

Relationships between species within Celsinotum are not obvious yet. Apparently, Australian species are closely related to each others, having similar outer morphology and shape of postabdomen. But morphology of limbs was studied only for one species, C. parooensis, and some important features, including inner portions of limbs III–IV were not described (see Frey 1991). C. macronyx shares some affinities with South American C. candango, including similar shape of the postabdomen, a large spine on basal segment of antenna exopodite, reduction of a seta on endite III of limb I, and a transformed spine-like posteriormost ventral setae of valves. The description of C. laticaudatum (Smirnov & Santos-Silva 1999) is incomplete, and no information about the trunk limbs was provided, so relationships of this species are completely unclear.

Genera Leberis and Celsinotum form a monophyletic clade within the subfamily Aloninae . According to the opinion of Van Damme & Dumont (2008a), Leberis belongs to the Coronatella -branch of Alona -like Aloninae, which include the genera Coronatella, Karualona, Anthalona, and the dentifera -group of Alona s. lato. In our opinion, such placement of Leberis (and, therefore, more advanced Celsinotum) is justified. Common features between Leberis and other genera of the Coronatella -branch are numerous, but most of them are reductions of limb setae, which happened independently in different branches of Aloninae (see Kotov 2000). On the other hand, the main evolutionary tendency within Coronatella is miniaturization: most species of these genera are less than 0.5 mm, and some are as small as 0.25 mm, while most species of Leberis and Celsinotum are quite large for Aloninae . Other important tendency of the Coronatella -group is specialization of the antenna for the pushing, which includes elongation of antennal spines and appearance of clusters of long hard setules on middle and basal of exopod.

In our opinion, the closest group to the Leberis-Celsinotum clade is the elegans -group of Alona s. lato. It includes five species with limited distribution. A. elegans distributed in West and Central Europe and North Africa; A. orellanai Alonso, 1996 and A. salina Alonso, 1996 – on Iberian peninsula; A. irinae Sinev, Alonso & Sheveleva 2009 and A. floessneri Sinev, Alonso & Sheveleva 2009 – in South-East Siberia and Mongolia (see Alonso 1996; Sinev et al. 2009). Comparison between these groups reveals no significant differences in limb morphology. An important common feature of these groups is presence of lateral aesthtascs on the male antennula, absent in the Coronatella -branch. Unlike species of Coronatella -branch, species of the elegans -group are of moderate size for the subfamily: about 0.55–0.6 mm in length.