Psolidium acorbulum, Thandar, Ahmed S., 2006

Thandar, Ahmed S., 2006, New species and new records of dendrochirotid and dactylochirotid holothuroids (Echinodermata: Holothuroidea) from off the east coast of South Africa, Zootaxa 1245, pp. 1-51 : 38-40

Psolidium acorbulum sp. nov.

Figure 13

Diagnosis

Minute, sub­globose species, holotype (almost mature) 4 mm long, with distinct ventral sole; oral valves absent, scales completely concealing mouth and anus. About 10–12 scales between mouth and anus, about 12 scales laterally. Tentacles eight, fingershaped, without lateral branches. Dorsal podia distinct, emitting from between scales. Sole bordered by a single row of podia laterally, mid­ventral ambulacrum without podia except a couple both anteriorly and posteriorly. Dorsal scales imbricating, up to 342 µm long, simple, consisting of a single layer of calcareous material, multilocular, smooth or slightly knobbed; knobs, when present, restricted to one side of plate. Sole deposits exclusively knobbed plates or buttons, up to 133 µm long, with 4–5 (sometimes 2–10) holes. Baskets and rosettes absent.

Material examined

Holotype SAM A­A 27911; Paratype SAM­A27912.

Type locality

Between Port St. Johns and East London, 32 28.6’ S, 28 58.8’ E, R.V. ‘Meiring Naude’, St. SM 226, 24.vi.1979, 710– 775 m.

Description

Both specimens minute, sub­globose, but distinctly psolid­like ( Figure 13 M) with well­arched dorsal surface and a flattened ventral surface forming a distinct sole. Oral cone not distinct, no oral valves. Scales completely conceal both oral and anal apertures. Largest specimen (holotype) 4 mm long; the other (paratype) 3.5 mm, width of mid­body of both approximately half body length. Colour, in alcohol, yellowish­white. Dorsal and lateral surfaces covered by simple, imbricating scales, about 10–12 scales between oral and anal apertures, decreasing in size posteriorly. Only eight tentacles detected, of unequal length, finger­shaped, without lateral branches. Dorsal podia distinct, non­retractile, about 50, scattered, emitting from between scales and appearing as spire­like projections from body wall. About 12 scales laterally. Sole covered by thin, translucent membrane extending from just behind the anterior end to extreme posterior end, boarded by a single row of about 10 podia on each side and two anteriorly. Mid­ventral ambulacrum without podia, except for the two anterior ones and those of the lateral series forming an arc posteriorly. Ventral podia better developed than dorsal and with well­formed suckers.

Calcareous ring ( Figure 13 I, J) simple but well developed, delicate. Radial plates with a triangular anterior projection, a depression for attachment of retractor muscle and a slightly concave posterior margin; interradial plates of much the same form. Polian vesicle single slightly elongated; stone canal short, madreporite funnel­like ( Figure 13 L), attached to dorsal mesentery. Gonad developed as three unbranched, slightly coiled tubules, attached to dorsal mesentery at anterior end, in process of maturity. Respiratory trees simple, short with few terminal branches.

Spicules of dorsal body wall only comprise simple, single­layered, imbricating scales ( Figure 13 A, B), up to 342 µm, perforated by numerous holes and with or without a slightly­knobbed surface; knobs, when present, restricted to one end of plate. Spicules of sole exclusively knobbed plates or buttons ( Figure 13 C) 71–133 µm long (mean 98 µm), with usually 4–5 (sometimes 2–10) holes, with an uneven, spinous margin; knobs when present minute and rather faint. Ventral podia with well­developed endplates (ca. 97 µm) ( Figure 13 F) and other densely packed plates ( Figure 13 E), 73–123 µm long (mean 89 µm) with up to eight holes and a spinose or slightly knobbed margin; endplates may also bear one or two tiny knobs on surface. Dorsal podia with similar deposits ( Figure 13 D). Tentacle stalk supported by rods and plates ( Figure 13 G), 90–198 µm long (mean 134 µm), the former more numerous, frequently curved and with several holes, usually restricted to the ends but sometimes also in the middle, which may bear a short arm, with or without perforations; plates few, restricted to the terminal end, with a spinose or slightly­knobbed margin and pierced by up to 10 holes. Tentacle branches supported by rods of varying shapes ( Figure 13 H).

Distribution

Type locality only.

Remarks

In its size, simplicity of the tentacles and dorsal plates, presence of a single row of marginal podia on each side of the sole, and the absence of mid­ventral tube feet in combination, this species differs from all nominal species of the genus. In the nature of its spicules it comes quite close to Psolidium vitreum Ohshima from the north­east Pacific and to the Psolidium sp. ( Theel, 1886b) from the Mexican Gulf. It, however, differs from both these species by the simplicity of its tentacles and the presence of a single, instead of a double row of marginal podia. Thandar (1999) hesitantly identified a single specimen from the south­west coast of South Africa as P. vitreum despite the fact that it had only a single row of marginal podia and no mid­ventral podia, and also referred Thėel’ s (1886b) Psolidium sp. to the synonymy of P. v i t re u m, thus supporting the suspicions of Ohshima (1915). This view can no longer be upheld as the writer is now of the opinion that the distribution of podia is an important taxonomic feature in Psolidium species. It is therefore possible that Thandar’ s P. v i t re u m is perhaps another species similar but not identical to the new species here described as it possesses bushy tentacles, complex multi­layered dorsal scales and more pronounced knobs on the ventral buttons.

Rowe (pers. comm.) thinks that the new species, because of its simple, finger­like tentacles, should be referred to the order Dactylochirotida, perhaps to Psolidothuria , or it may represent a young Ypsilothuria . However, this is inadmissible as the form of the body and the presence of a distinct sole with marginal podia are psolid characters. Further, the gonad appears too well developed for the specimen to be a young Ypsilothuria . Incidentally, Pawson (1971) also described digitate rather than dendritic tentacles in the psolid Ekkentropelma brychia and Pawson & Valentine (1981) described weakly dendritic to unbranched tentacles in juvenile of Psolidium prostratum . Simple, weakly developed dendritic tentacles are also present in the new Ekkentropelma species described below. E vidently then the psolids may bridge the gap between dendro­ and dactylochirotid holothuroids.

Figures

FIGURE 13. Psolidium acorbulum sp. nov. Holotype. SAM­A 27911. A. scale from dorso­lateral body wall; B. scale from dorsal body wall; C. plates from ventral body wall; D. plates from dorsal podia; E. rods / plates from ventro­lateral podia; F. end­plate from the ventro­lateral podium; G. rods from tentacle stalk; H. rods from tentacle branches and tips; I. calcareous ring showing mid­ventral radial plate; J. dorsal radial plate; K. single tentacle; L. polian vesicle and stone canal; M. Paratype. SAM­A 27912, entire. A & B scale a; C – H scale b; I – K scale c.