Hemiocnus rubrobrunneus, Mjobo, Sifiso & Thandar, Ahmed S., 2016

Mjobo, Sifiso & Thandar, Ahmed S., 2016, A new genus and a new species in the sea cucumber subfamily Colochirinae (Echinodermata: Holothuroidea: Dendrochirotida: Cucumariidae) in the Mediterranean Sea, Zootaxa 4189 (1), pp. 156-164 : 160-162

Hemiocnus rubrobrunneus n.sp.

( Figures 3–6 )

Diagnosis. Species soft, flaccid; body subcylindrical to barrel-shaped; colour in alcohol dark reddish-brown in bright light. Length up to 32 mm. Mouth and anus terminal. Anal teeth/papillae absent. Tube feet short, mostly non-retractile, two rows per ambulacrum, few also scattered in dorsal interambulacra. Body wall ossicles comprise a layer of complete and incomplete baskets, accompanied by rosettes, in addition to small, rounded, knobbed plates. Large smooth to knobbed plates sometimes present in anal region. Tube feet deposits comprise baskets, rods and rosettes. Tentacles and introvert with rods and rosettes; knobbed, rounded plates also occur in introvert.

Etymology. The species is named for its reddish-brown colouration.

Material examined. Holotype: USNM (Smithsonian Institution), E22590 , Jerba Island , between Houmt Sauq and Bordj Djillidj, Tunisia, 1 m ., Jones, M.L. 1973; Paratypes, same data as holotype, 2 spec. (all specimens formerly determined as Pseudocnella syracusana by C. Gust (C. Ahearn), 1981).

Description of holotype. Form subcylindrical to barrel-shaped ( Figure 4 D), 29 mm in length, 13 mm in breadth in mid-body (paratypes 29 x 12 mm and 32 x 12 mm). Skin soft, colour in alcohol, a dark reddish brown in bright light. Mouth and anus terminal. Tube feet short, distributed in two distinct rows ventrally, and in somewhat zigzag rows dorsally with few also in dorsal interrambulacra. Tentacles retracted, bushy, 10, ventral-most two reduced. Calcareous ring ( Figure 6 G) typically cucumariid, simple, without posterior prolongations, radial plates bifid anteriorly, all plates deeply notched posteriorly. Madreporite bean-shaped, stone canal short; Polian vesicle single. Retractor muscles short, arising from longitudinal bands at about half body length from anterior end. Longitudinal muscles not paired. Respiratory trees well branched, about ¾ of body length from posterior end. Gonad in single tuft, tubules unbranched, partially full of developing eggs.

Body wall deposits consist of baskets and knobbed, perforated buttons/plates and rosettes. Buttons (70–100 µm) with mostly 4(–6) holes, rarely more, most about 100 µm, with 1–3 central knobs, rarely more ( Figure 3 A). Baskets numerous, both complete and incomplete ( Figures 3 B & D), incomplete baskets appearing as X- or Yshaped simple, smooth deposits with dichotomous branchings, whereas complete baskets appear as shallow, flat cups with blunt spines around the rim. Rosettes appear as both open and closed types. Anal region with multilocular, smooth and/or knobbed plates ( Figure 5). Tube feet deposits include rods, rosettes and baskets. Rods simple, mostly table-like, perforated at ends and with a spire-like medial extension ( Figure 6 C). Rosettes simple ( Figure 3 E), or developing into complex, opened ones ( Figure 3 F), and eventually into closed knobbed ones ( Figure 3 G). Baskets of tube feet complete and incomplete, similar to those of body wall. End-plates present, better developed ventrally. Tentacle and introvert ossicles include rosettes in addition to rods ( Figures 4 A, B&C; 6A,B,D & E); some simple knobbed rounded plates ( Figure 6 F) also observed in introvert.

Distribution. At present known only from Tunisia.

Habitat. Unknown.

Remarks. The 3 specimens from the USNM, labelled as Pseudocnella syracusana , are small (max. length 32 mm) and very distinct from the single specimen from the Hamburg Museum and those examined at NHMUK by AST, which are much larger (maximum length 56 mm) and identified as Pseudocnus syracusanus . The colour of the preserved USNM material, collected in 1973, is a dark reddish brown ( Figure 2 D), while that of the Hamburg specimen, collected in 1982, is light brown ( Figure 6 F) similar to those at NHMUK . Both materials also differ in their size and tube feet distribution. The maximum length recorded for a true P. syracusana is 58 mm (by Panning 1962). While the Hamburg specimen is hard and has, in addition to scattered tube feet, distinct interradial ‘papulaelike’ extensions, reminiscent of those of Pseudocnella spp., the USNM specimens are soft, with tube feet restricted in two distinct rows per ambulacrum, especially ventrally, and lack ‘papulae-like’ structures of any sort. As far as ossicles are concerned the new species has in its body wall knobbed rounded plates and both complete and incomplete baskets whereas the Hamburg and NHMUK specimen have large fir-cone shaped plates in addition to round plates and a few cross-shaped incomplete baskets but the latter were absent in most specimens from NHMUK. The new species has also rosettes in the body wall and rods, rosettes and baskets in the tube feet. Hence, there is no doubt that both materials are not conspecific.

At first we were of the opinion that the locality of the species and form of the ossicles, except for the presence of quadrilocular baskets, clearly places the new species in the genus Ocnus and we described it as such in the manuscript. However, Dr Rowe who is most familiar with the Mediterranean holothuroids and especially the genus Ocnus , first reviewed the manuscript and is of the opinion that the baskets are not reminiscent of species of Ocnus and the incomplete ones suggest that the specimens perhaps represent juvenile of the preceding species or at best a new species within Hemiocnus . We contest the former viewpoint as the gonadal tubules of at least one specimen is in the process of maturity, the specimens lack the complex plates of H. syracusanus and those of the anal region of at least one specimen are rather simple, there are rosettes present together with complete baskets, and the colour and texture of the body wall are quite different. We therefore agree with Dr Rowe’s second point and here describe the new species in Hemiocnus as no other genus exists to accommodate it. It is possible that the Tunisian fauna is quite different from that of the western Mediterranean as quite recently, Stohr (2015) (in Echinoderm-l) commented, with reference to the ophiuroids, that Tunisia is at the western border of the Levant and thus it has a somewhat different fauna from the northern and western Med. Hence this may also apply to other echinoderm groups.

Figures

FIGURE 2. Hemiocnus syracusanus. A. Fir-cone-shaped plates from body wall; B. Large round plates from body wall; C. Buttons from body wall; D. Small rods from tentacles; E. Large rods from tentacles; F. Specimen entire.

FIGURE 3. Hemiocnus rubrobrunneus n. sp. ossicles. A. Buttons from body wall; B. Incomplete baskets from body wall and tube feet; C. Basket on knobbed button; D. Complete baskets from tube feet; E. Simple open rosettes from body wall; F. Complex open rosettes from body wall; G Closed rosette from body wall. (B, D, E, F & G = scale b; A & C = scale a).

FIGURE 4. Hemiocnus rubrobrunneus n. sp. A. Rosettes from tentacles; B. Small rods from tentacles; C. Large rods from tentacles. D. Paratype (entire).

FIGURE 6. Hemiocnus rubrobrunneus. A. Rods from tentacles; B. Rosettes from tentacles; C. Rods from tube feet; D. Rosettes from introvert; E. Rods from introvert; F. Round knobbed button from introvert; G. Part of calcareous ring. A, D, E & F = scale a.