Ocnus paracorbulus Thandar, Zettler & Arumugam, 2010, n. sp.
Creators: name: Thandar, Ahmed S.
name: Zettler, Michael L.
name: Arumugam, Preyan
Publisher: name: Zenodo
Year: 2010
Subjects: Biodiversity
Taxonomy
Animalia
Echinodermata
Holothuroidea
Dendrochirotida
Cucumariidae
Ocnus
Ocnus paracorbulus

description: Ocnus paracorbulus n. sp.
Figures 2 & 3
Diagnosis. Minute (4 mm), barrel-shaped yellowish-white species, currently represented by only juvenile. Mouth covered by scales; tentacles 10, ventral-most two reduced; anal teeth present. Tube feet restricted to ambulacra in single, straight or zigzag rows. Calcareous ring simple. Polian vesicle single. Ossicles of body wall an external layer of baskets and an inner layer of plates. Baskets deep, cup-like, rims variable in form, with pronounced spines, best developed at corners, if rim squarish, or all round; base cross-shaped, spiny. Plates well developed, usually multilocular. Tube feet deposits include curved or straight, smooth perforated rods and plates, with slightly “spined”/knobbed margins. End-plates stellate. Tentacles with mostly smooth, perforated rods and plates.
Etymology. The species is so named because of its obvious resemblance to Ocnus corbulus Cherbonnier, 1953.
Material examined. Holotype (? juvenile), SAM A28094, Kunene, stn. BE17, Hole 1, Namibia 17. 316º S, 11.723º E, 5.iii.2008, 26 m, Zettler; Paratypes, SAM A28095, same data as holotype, 16 specimens (?juveniles).
Description of holotype. Specimen minute, 4 mm long, 2 mm wide in mid-body. Colour yellowish white; body barrel-shaped (Figure 2 A). Mouth and anus terminal; the former covered by scales; tentacles 10, retracted, well-branched, ventral-most two reduced; anal teeth minute. Tube feet restricted to the ambulacra in single rows, better developed ventrally where they appear in a single zigzag row on the mid-ventral ambulacrum and in single straight rows in the other two ventral ambulacra, reduced dorsally in two single rows. Suckers well developed, diameter slightly larger than that of the tube foot. Body wall opaque, thin, soft, filled with ossicles.
Calcareous ring (Figure 2 B) delicate, simple, typically cucumariid, radial plates anteriorly bifid with a medial depression for attachment of retractor muscle; interradial plates triangular; posterior margin of ring undulating. Polian vesicle single, large, sac-like; stone canal short, straight, madreporite apparently beanshaped. Longitudinal muscles as single strands, retractors arise anteriorly. Gonad absent. Respiratory trees branched, right one longer, left one more branched, but restricted to posterior half of body. Cloaca elongate.
Ossicles of the body wall comprise an external layer of baskets and an internal layer of plates. Baskets (Figure 2 C) well developed, deep, cup-like with an oval, circular or squarish rim with well developed spines, mostly at corners of the rim if rim squarish, in others equally well developed but all around the rim (24.5–39 µm, mean 31.35µm, ± 4.46, n = 10). Base of basket typically cross-shaped with spines distributed medially and at ends of cross. Plates (Figure 2 D) exceptionally well developed (98.5–207.5 µm, mean 155.1 µm, ± 30.67, n = 10), multilocular, finely knobbed around margins and on surface, margins uneven, some plates with fewer holes (four or more), but these are perhaps developing large plates. Tube feet deposits (Figure 3 A) include curved or straight, smooth perforated rods and plates, with uneven slightly “spined” margins, the latter more prominent in the wider part of the ossicle (83–118.5 µm, mean 93.8 µm, ± 9.95, n = 10). Suckers supported by stellate end-plates (Figure 3 B) bearing relatively few perforations. Mouth surrounded by larger, elongated plates (up to 320 µm) with knobs usually developed at one end which is also narrowed and slightly spinous (Figure 2 E). Anal region with smooth, perforated plates with marginal knobs or spines. Tentacles with mostly smooth, perforated rods and plates (up to 64 µm) (Figure 3 C). Introvert without deposits.
Description of paratypes. 1–4 mm long, barrel-shaped, smaller specimens almost rounded. Tube feet as in holotype but in some well extended and fairly elongated, reaching about 1 mm in length. Deposits as in holotype.
Distribution. Type locality only, 26 m.
Habitat. Shells of brachiopod Discinisca tenuis (Sowerby, 1847).
Remarks. Since the holotype, the largest specimen in the series, lacks a gonad, it is obviously a juvenile. Hence, it is concluded that the entire material is juvenile. Despite this, a full complement of ossicles appears to be present and therefore the entire material is here treated as representing a new species. Since baskets of the form here described, in combination with the knobbed plates and the absence of button-like deposits, do not occur in any nominal species of Ocnus, it is highly likely that the material may be referable to a new genus, but lack of adults prevents this action. It is noteworthy that the pedicel end-plates are typically of the Ocnus type and the baskets bear a close resemblance to those described for Ocnus corbulus (Cherbonnier, 1953) from False Bay, South Africa. However, the latter species, described from a 10 mm adult, has two rows of tube feet per ambulacrum, end-plates are absent and in addition to the huge plates which resemble those of the current material, there are other knobbed plates with one end spinous/denticulate, with the spines borne on a short handle. Such plates are of the ‘ Pseudocnella insolens ’ type and are absent in the new species, except for a few very large ones at the oral end. The possibility therefore exists that such plates and the multiplication of tube feet may be a later development and the new species may turn out to be juvenile of O. corbulus but the lack of any intermediates prevents this conclusion. A critical look at all related genera of Ocnus excludes the possibility of referring the current material to any of them.
description: Published as part of 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), pp. 1-24 in Zootaxa 2655 on pages 4-7, DOI: 10.5281/zenodo.198925