Ocnus rowei Thandar, 2008

Thandar, Ahmed S., 2018, On some miscellaneous sea cucumbers (Echinodermata: Holothuroidea) in the collections of the South African Museum with three new species, Zootaxa 4532 (1), pp. 57-85 : 60-62

Ocnus rowei Thandar, 2008

( Figure 3 )

Ocnus rowei Thandar, 2008:19 –22, fig. 8. Diagnosis (see Thandar 2008).

Material examined SAMC-A 090912 , SM 185, off Port Alfred , 33°39.03’ S, 27°11.06’ E, heavy dredge, 90 m, 31.V.1978, 1 spec .

Description. Specimen small, slightly U-shaped. Mouth and anus directed upwards ( Figure 3A & B ). Form sub-cylindrical, dorsal surface arched, ventral surface flattened, sole-like. Length about 12 mm along ventral surface; mid-body width about 2.8 mm, much broader anteriorly and narrower posteriorly. Colour off-white in alcohol. Tentacles retracted, eight large and of unequal length; ventral two hardly discernible. Anal teeth and/or special anal papillae absent. Tube feet in ambulacra, in single zig-zag rows situated on longitudinal ridges (indicating position of longitudinal muscles, both dorsally and ventrally), decreasing in size in both directions. Ventral tube feet also in single rows, with most of the anterior ones of the mid-ventral ambulacrum extending from about quarter body length from anterior end to about quarter body length from posterior end. Most tube feet nonretractile but better developed dorsally, all supported by well-developed end-plates; 1 or 2 tube feet also in interambulacra. Large, non-imbricating, knobbed plates clearly visible especially ventrally, situated more or less equidistant from each other. Calcareous ring as in holotype ( Figure 3L ; see also Thandar 2008). Polian vesicle single, ventral, saccular. Gonad immature. Body wall ossicles characteristically of three types: minute rugose baskets ( Figure 3H ); mostly four-holed buttons, some with large, some with small knobs, or a mixture of two ( Figure 3E & G ); and large multilocular knobbed and smooth plates/scales ( Figure 3D & I ). Tube feet ossicles include simple, curved, perforated rods with a varying number of holes ( Figure 3C ) and small end-plates ( Figure 3F ). Tentacle and introvert ossicles as in holotype, the former comprising knobbed plates/buttons, smooth perforated plates, and small to large perforated rods ( Figure 3J & K ); the latter (introvert) comprising buttons/plates and rods similar to those of tentacles, but fewer.

Distribution. From False Bay, South Africa, as far east as Port Alfred in Eastern Cape Province, 42– 90 m.

Remarks. This obviously juvenile specimen corresponds well with the description of the holotype by Thandar (2008), who compared it with the well-known Pentacta doliolum ( Pallas, 1766) which it strongly resembles but differs in the unequal size of the tentacles (the mid-ventral two being reduced), softer body wall and usually nonrectractile tube feet. Its placement in Ocnus is questionable as Rowe (in Rowe & Gates, 1995) implied restriction of the genus to include only the Mediterranean and the North-West African forms.

Figures

FIGURE 3. Ocnus rowei Thandar, 2008. SAMC-A090912. A. Specimen (entire) dorsal; B. Specimen (entire) ventral; C. Rods from tube feet; D. Large knobbed plates from body wall; E. Buttons from body wall; F. Tube feet end-plate; G. Small knobbed buttons from body wall; H. Baskets from body wall; I. Large smooth plates from body wall; J–K. Tentacle ossicles. (C–I same scale; J–K same scale).