Placostegus leslieharrisae, Kupriyanova & Flaxman, 2024

Kupriyanova, Elena K. & Flaxman, Beth, 2024, Serpulidae (Annelida) of the Australian Indian Ocean Territories, Records of the Australian Museum (Rec. Aust. Mus.) 76 (4), pp. 211-242 : 235-236

Placostegus leslieharrisae n. sp.

urn:lsid:zoobank.org:act:798078AA-27D5-4880-969B-48E9235440F5

Fig. 13A–E

Material examined. Holotype: W.54407 (LK311), Cocos (Keeling) Islands Territory , Muirfield Seamount (13°10'06"S, 96°11'14"E), depth 271–311 m, 22/10/2022 (photo, DNA and SEM) . Paratype: W.54410, Cocos (Keeling) Islands Territory , Muirfield Seamount (13°11'13"S, 96°08'51"E), depth 367 m, 23/10/2022 (1 empty tube) .

Description. Tubes: triangular in cross section, thick-walled, nearly transparent ( Fig. 13 , A, a; smaller empty white tubes unidentifiable). Tube attached to substrate for most of its length, only distal-most part free, tube mouth rounded, lacking any denticles.

Radiolar crown: 22 radioles not connected by inter-radiolar membrane at base.

Peduncle: smooth cylindrical, only slightly wider than normal radioles ( Fig. 13A, a, B ), inserted medio-dorsally at base of radiolar lobe.

Collar and thoracic membranes: collar trilobed, long, covering radiolar lobes and proximal part of radioles. Lobes approximately of same length, dorsal lobe wider than lateral lobes. Collar margin smooth. Collar chaetae absent.

Operculum: elongated inverted cone covered with a slightly concave brown chitinous endplate ( Fig. 13A, a, B ). Constriction between operculum and peduncle present ( Fig. 13A, a, B ).

Thorax: short, compact, thoracic notopodia positioned close to each other, along mid-lateral line of thorax, triangular depression absent. Collar segment with girdle of redpigmented ocelli ( Fig. 13b ). Thoracic membranes continuing to end of thorax forming short apron ventrally. Thoracic chaetae ( Fig. 13C ) simple limbate of two sizes, Apomatus chaetae absent. Thoracic uncini ( Fig. 13D ) rasp-shaped, with over 200 small teeth in profile view and up to 10 teeth per row, anterior peg wide with rounded edges.

Abdomen: chaetae with long shafts ( Fig. 13E ) and distal tips with 2–3 rows of small denticles ( Fig. 13e ). Chaetae of posterior abdominal segments slightly longer than more anterior ones. Abdominal uncini similar to thoracic uncini. Achaetous anterior abdominal zone present. Long posterior capillary chaetae present. Posterior glandular pad absent.

Size: total body length of holotype 16.9 mm, including up to 5.2 mm long radioles, up to 1.6 mm long thorax, 10.1 mm long abdomen, width of thorax 0.7 mm. External tube diameter in holotype 1.3 mm, corresponding lumen diameter 1.1 mm.

Diagnostic remarks. According to WoRMS ( Read & Fauchald, 2024), six species are currently considered valid in the genus Placostegus . The best known species Placostegus tridentatus originally described from Northern Europe is recognisable by its transparent triangular tube with keels extended into three denticles at the mouth and radially symmetrical operculum. The species has been widely reported not only from the Mediterranean and the Atlantic Ocean, but also from the Indo-West Pacific, an unlikely distribution for a shallow-water species. However, the only record from Japan was reported by Imajima (1978), who stressed that he referred his material to P. tridentatus only tentatively and that a revision of the genus is needed.

Placostegus assimilis McIntosh, 1885 fromtheBermudas has a radially symmetrical operculum and tube with a denticulate mouth, according to the original illustration, but the tube mouth of P. assimilis has more than three main denticles unlike that of P. tridentatus . Both Placostegus langerhansi Marenzeller, 1893 from Madeira and Placostegus californicus Hartman, 1969 from California have distinctly zygomorphic (bilaterally symmetrical) opercula and tubes with tri-denticulate mouths, but apparently these species differ by the collar having seven lobes in the former and three lobes in the latter. Placostegus crystallinus (non-Scacchi, 1837) sensu Zibrowius, 1968 (likely an undescribed species) has a recognisable tube with numerous rounded transverse ridges and somewhat similar ridges in the posterior part of the tube have been reported for Placostegus incomptus Ehlers, 1887 .

The new species is distinct from all described species of the genus because of the combination of radially symmetrical operculum and the tube with a rounded mouth lacking any denticles. In this respect the new species resembles Placostegus sp. from Lizard Island, Qld, Australia (Kupriyanova et al., 2015, fig. 14B, C). Also, abdominal chaetae in P. leslieharrisae n. sp. are similar to the abdominal chaetae found in some Hyalopomatus spp. (see above) in having distal tips with 2–3 rows of small denticles. The tips ( Fig. 13E ) are not hollow and are very different from the almost rectangular angle of the true trumpet-shaped chaetae as illustrated in ten Hove and Kupriyanova (2009, fig. 34C) for P. tridentatus . This is the first newly described species of the genus in which a detailed illustrated description is accompanied by the molecular sequence data from the type specimen.

Etymology. The species was named in honour of Leslie Harris (Senior Collection Manager, Natural History Museum of Los Angeles County, CA, USA) for her remarkable dedication to studies of polychaetes and her important contributions to the field of polychaete taxonomy.

Distribution. Only known from Muirfield Seamount, Cocos (Keeling) Islands, Indian Ocean, 271– 367 m.

Figures

Figure 13. Placostegus leslieharrisae n. sp. W.54407, A – photo, holotype in tube, a – close-up view of anterior end of the tube showing operculum; B – SEM of anterior end of the same specimen, b – close-up view thorax showing a band of eyes (arrow), C – thoracic chaetae, D – thoracic uncini, E – long abdominal chaetae, e – close-up view the tip of abdominal chaeta. Scale: A, a, B – 1 mm; b – 100 µm; C – 50 µm; D, E – 10 µm; e – 2 µm.