Ocnus placominutus, Thandar, Ahmed S., Zettler, Michael L. & Arumugam, Preyan, 2010

Thandar, Ahmed S., Zettler, Michael L. & Arumugam, Preyan, 2010, Additions to the sea cucumber fauna of Namibia and Angola, with descriptions of new taxa (Echinodermata: Holothuroidea), Zootaxa 2655, pp. 1-24 : 2-4

Ocnus placominutus n. sp.

Figure 1

Diagnosis. A small cucumariid, up to 10 mm, with slightly U-shaped, angulated body. Mouth and anus surrounded by valves. Tentacles 10, ventral two reduced. Tube feet restricted to radii in single rows, ± 10 per ambulacrum; interambulacra naked. Skin rigid. Radial and interradial plates of calcareous ring anteriorly cleft. Polian vesicle single. Ossicles of body wall of three types: external layer of minute, flat baskets with spiky margin and a spinulated base; a layer of large, multilayered, knobbed, imbricating plates with minute holes; and a layer of minute, complex plates, similar in structure to large plates. Tube feet deposits table-like rods, frequently knobbed; end-plates stellate. Tentacle deposits include rods of the pedicel type and oblong perforated plates, both spinulated or feebly-knobbed.

Etymology. The species is so named because of its characteristic minute plates beneath the large, imbricating plates.

Material examined. Holotype (female), SAM A28092, AHAB-9, stn. 88, dredge, Angola, 7.235° S, 12.684° E, 28.v.2004, 41 m, Forster & Zettler; Paratype, SAM A28093, same data as holotype.

Description of holotype. Specimen small, slightly U-shaped, length along ventral surface 9 mm, breadth in mid-body 1.5 mm ( Figure 1 I). Form slightly angular in the radii; preserved colouration off-white, uniform. Mouth anterior, terminal, covered by valves; tentacles retracted; anus also terminal, surrounded by valves; anal teeth absent. Tentacles 10, well-branched, ventral two slightly reduced. Tube feet restricted to radii in single rows, scanty, ± 10 per ambulacrum, longer in the ventral radii, shorter dorsally, most extending rigidly from body wall, longest one ≈ 0.83 mm, suckers present, of same diameter as tube foot. Interambulacra naked, papillae/warts/tubercles absent. Skin rough, slightly rigid, packed with ossicles of which the large plates are clearly visible through stereoscope as imbricating scales.

Calcareous ring simple ( Figure 1 H), well calcified, easily fragmented. Radial plates slightly longer than interradial plates, anteriorly bifid, without posterior prolongations; interradial plates also anteriorly bifid, all plates with a posterior notch or concavity, that of the radial plate deeper. Polian vesicle single, short, saccular, slightly to left of ventral mesentery; stone canal thin, straight, remarkably short; madreporite well calcified, exact shape not discernable. Longitudinal muscles as thin single strands, retractors also thin, originating anteriorly. Respiratory trees confined to posterior quarter of body, both poorly branched, especially the left one, right one also much reduced. Gonad as a simple tuft of short tubules attached anteriorly, full of eggs in various stages of development.

Ossicles of body wall characteristically of three types: external layer of minute (14–22 µm, mean 17.25 µm, ± 2.57, n = 20) flat baskets with a rather spiky margin and a spinulated base perforated by four or more holes ( Figure 1 C); followed by a layer of large (224–382 µm, mean 273.95 µm, ± 44.28, n = 10) multilayered, knobbed plates with minute holes ( Figure 1 A); and a layer of minute (65–141 µm, mean 96.45 µm, ± 20.3, n = 10) complex plates, similar in structure and form to the large plates, but extremely diminutive ( Figure 1 B). Tube feet deposits mostly in the form of table-like rods (82–135 µm, mean 114.83 µm, ± 15.01, n = 20) with a curved base, perforated at ends and in the middle which appears in a spire-like extension ( Figure 1 F), most rods finely knobbed, rarely smooth but sometimes irregular, plate-like with several holes. End-plates ( Figure 1 G) very characteristic, of stellate form, with large medial perforations and smaller ones in the arms, all arms with jagged margins. Tentacle deposits include rods (73–130 µm, mean 95.25 µm, ± 16.12, n = 10) and plates (24–41 µm, mean 36 µm, ± 5.23, n = 10), the former ( Figure 1 E) of the pedicel type and the latter ( Figure 1 D) as oblong straight or slightly curved and/or branched perforated deposits, the medial perforations being the larger.

Description of paratype. Length 4 mm. Ossicles of body wall similar to those of holotype but those of the tube feet frequently without knobs as they are not fully developed.

Distribution. Type locality only, 41 m.

Habitat. Sandy substrate.

Remarks. The specimens in hand satisfy the diagnosis of Ocnus as amended by Thandar (2008). In the structure of the calcareous ring and the presence of three types of body wall ossicles it comes close to most species currently classified in Ocnus , notably O. brunneus Forbes, 1841 , O. lacteus Forbes, 1841 , O. planci (Brandt, 1835) , O. calcareus Dendy, 1897 , O. capensis (Théel, 1886) , O. corbulus (Cherbonnier, 1953) and O. rowei Thandar, 2008 , but not O. vicarius (Bell, 1883) , which is characterised by cup-like baskets. As far as satisfying the characters of the type species depends on which species is really the type species of the genus. Forbes (1841) chose O. brunneus as the type species of his genus but according to Panning (1971) brunneus is a synonym of planci (Brandt, 1835) , while Rowe (1970), following Mortensen (1927), suggested that brunneus is a synonym of lacteus and hence lacteus should replace brunneus as the type species of Ocnus . McKenzie (1984 & 1991) as the last reviser, attempted to separate the above three species and his suggestion should be accepted that brunneus must be retained as the type species of this genus.

In the presence of a single row of tube feet, multilocular plates in the body wall together with other small plates/buttons, the new species agrees with both O. brunneus and O. lacteus . However, its large plates are always knobbed and never smooth and its other plates are minute and, unlike those of O. lacteus , never fourholed but complex and multilocular, resembling in form the large plates. The tube feet ossicles are also different from those of the other species of Ocnus but the end-plates resemble those of both brunneus and lacteus .

Figures

FIGURE 1. Ocnus placominutus n. sp. Holotype, SAM A 28092. A. Large knobbed body wall plates; B. minute body wall plates; C. flat baskets from the body wall; D. oblong rods from base of the tentacles; E. plates from base of tentacles; F. rods from tube feet; G. endplate; H. calcareous ring; I. specimen entire.