Ocnus rowei, Thandar, 2008

Thandar, Ahmed S., 2008, Additions to the holothuroid fauna of the southern African temperate faunistic provinces, with descriptions of new species, Zootaxa 1697 (1), pp. 1-57 : 19-21

Ocnus rowei sp. nov.

Figure 8

Diagnosis

A somewhat U-shaped species, up to 20 mm in length. Tentacles 10, ventral-most two reduced. Podia usually rigid, in double rows, interambulacra mostly naked. Body wall rigid, thin (<1 mm). Body wall spicules comprise baskets, knobbed buttons/plates and large lenticular plates/spheres. Baskets spinose, of one type, minute, 11-21 µm, flat, usually quadrilocular. Buttons 40-76 µm, usually with large (> 15 µm) knobs but some smaller ones with smaller knobs (<15 µm). Lenticular plates/spheres large, 190-370 µm, knobbed, made up of several layers of calcareous material. Podial deposits include baskets, knobbed plates, rods and end-plates. Tentacle stalks with perforated rods, branches with multilocular plates. Introvert with large-knobbed plates and rods.

Etymology

This species is named in honour of Dr Francis W. Rowe in recognition of his excellent contributions to the taxonomy of holothuroids.

Material examined

Holotype: SAM-A 27991, UCT Ecol. Surv., False Bay , St. FAL 493 , 34 o 14' S, 18 o 30' E, 27.x. 1961, 42 m; paratypes, SAM-A27992, same data as holotype, 14 spec.

Description of holotype

Body somewhat U-shaped, length along ventral surface 20 mm, width in mid-body 3 mm, posterior end narrower, directed upwards, ventral surface not sole-like. Tentacles retracted, eight large, ventral-most two reduced. Podia well extended, rigid, restricted to ambulacra in double rows, larger ventrally, one or two also scattered in interambulacra but these are mostly naked. Anus encircled by papillae, teeth not detected. Colour, in alcohol, including tentacles and podia, uniform off-white. Body wall rigid, thin (<1 mm), ambulacra and interambulacra pitted, pits not indicative of presence of papillae/papulae which are both absent.

Radial plates of calcareous ring ( Figure 8K ) truncate, slightly longer than interradial plates, with an anterior depression for retractor muscle, posterior surface slightly concave; interradial plates triangular, anteriorly bifid with medial depression, posterior surface deeply concave. Polian vesicle single, elongate; stone canal short, lodged in dorsal mesentery, madreporite bean-shaped ( Figure 8L ). Gonad well developed, almost mature, comprising elongate, unbranched tubules, filling much of body cavity. Respiratory trees well branched, short, right one reaching only mid-body, left one much shorter. Longitudinal muscles faint, retractors originating in anterior third of body, more anteriorly in ventral ambulacra.

Spicules of body wall comprise baskets, knobbed buttons/plates and large, knobbed, lenticular plates/ spheres. Baskets ( Figure 8C ) of one type - minute, 11-21 µm (mean 16 µm), rugose, flat, quadrilocular but occasionally with 2-5 holes; rim and base spinose. Buttons ( Figure 8B ) of moderate size, 40-76 µm (mean 61 µm), with large (> 15 µm) knobs but a few smaller ones ( Figure 8D ), 39-56 µm (mean 47 µm), with smaller knobs (<15 µm) also present, holes usually four, sometimes more or less. Lenticular plates/spheres large ( Figure 8A ), 190-370 µm (mean 306 µm), of several layers of calcareous material. Podial deposits include baskets, perforated plates ( Figure 8F ) and rods ( Figure 8E ) and end-plates. Baskets numerous, similar to those of body wall; plates and rods, 32-209 µm (mean 120 µm), with marginal knobs, the latter also perforated either throughout length or perforations restricted to ends; end-plates reduced, 151 µm, with small marginal and larger central holes. Tentacle stalk deposits as smooth, perforated rods ( Figure 8H ), 106-179 µm (mean 134 µm); primary branches with thickened, curved, multilocular plates ( Figure 8I ), 40-96 µm (mean 63 µm); tips with delicate, curved, multilocular plates ( Figure 8J ), 40-63 µm (mean 51 µm). Introvert with mostly largeknobbed plates, 47-111 µm (mean 72 µm), with four or more holes and a few large rods (ca. 187 µm) with marginal knobs and few perforations ( Figure 8G ).

Paratypes

Paratypes 10-20 mm in size and all, except two, correspond well with the holotype but in one the anteroventral surface is somewhat sole-like (perhaps a preservation artefact) and has baskets that are slightly larger than those of the holotype whereas, in the other, small-knobbed buttons are not as rare as in the holotype; in a few paratypes the podia are mostly retracted.

Distribution False Bay, WCP, 42 m.

Remarks

The specimens at hand come very close to Pentacta doliolum in their complement of spicules, so much so that they may be confounded for juvenile of this species. Natasen Moodley (2000), who initially studied this material, highlighted the differences, but still referred her specimens to P. doliolum , despite the fact that she did observe mature gonadal tubules in some specimens. The new species differs from P. doliolum in its body form (cylindrical, somewhat U-shaped), unequal tentacles, thinness of the body wall (<1 mm), usually nonrectractile podia, distinct anal papillae in most specimens, minuteness and flatness of baskets, and the dominance of large-knobbed buttons. The possibility that the current material may represent juvenile of P. doliolum is here discounted because of the near maturity of the gonad and the fact that juvenile of P. doliolum , as stated by Thandar (1991) and here confirmed from a specimen of about the same size as the holotype of the new species, have the same complement of spicules as that of adults. In fact, because of the unequal size of the tentacles the new species is not referred to Pentacta but to Ocnus following the advice of Rowe (pers. comm.), differing from O. capensis in the greater complexity and more rugose nature of the baskets and the greater complexity of the large plates.

Figures

FIGURE 8. Ocnus rowei sp. nov. Holotype, SAM-A27991. A. Scale from lateral body wall; B. buttons from lateral body wall; C. baskets from lateral body wall; D. developing buttons from lateral body wall; E. rods from podia; F. plates from podia; G. plates and rod from introvert; H. rods from tentacle stalk and branches; I. thickend plates from tentacle branches; J delicate plates from tentacle tips; K. calcareous ring; L. madreporite. A scale a; B-J scale b; K & L scale c.