Thyone, Oken, 1815

Thandar, Ahmed S., 2019, On some lesser known sea cucumbers in the Natural History Museum, UK (Echinodermata: Holothuroidea), Zootaxa 4688 (3), pp. 361-381 : 369

Thyone ? elegans Norman, 1869 or T.? roscovita Hérouard, 1889

Figure 4 (A, C & E)

? Thyone elegans Norman, 1869: 317 .

? Thyone roscovita Hérouard, 1889: 682 ; McKenzie, 1991 (syn. complete).

Material examined. Isle of Muck, Rudha leam na Caraiche (perhaps Lubh’ Leam na Lá Rudha ceam na Caraich), littoral, 27.VII.38, A.D.H. Collection, 1 spec.

Description. Body form cylindrical, vermiform, with posterior end slightly curved. Length 55 mm, breadth in mid-body 10 mm. Tentacles 10, ventral two reduced. Skin and tentacles off-white in alcohol. Tentacles very bushy, including the two reduced ones. Calcareous ring and introvert eviscerated but still attached to body by retractor muscles. Mouth and anus terminal, non-calcified minute anal papillae evident. Hardly any distinction between dorsal and ventral surfaces. Tube feet short, scattered, but more numerous in ambulacra than elsewhere, some serial arrangement evident. Calcareous ring typically tubular, elongated, complex, somewhat twisted. Both radial and interradial plates fragmented, fused, anteriorly triangular, radials with a depression for attachment of retractor muscle. Radial plates prolonged posteriorly before bifurcating into fragmented processes which infringe on the anterior part of the gut (stomach), processes approximately one-third the length of ring. Polian vesicle single, elongated. Stone canals two, delicate, proximally straight, distally coiled, ending in a bean-shaped madreporite. Gonad immature, tubules unbranched, long, thin, restricted to mid-body. Longitudinal muscles as single strands. Retractor muscles thick and tough, the three ventral ones arising more anteriorly, proximally thin but widening distally before insertion onto radial plates. Except for a few fragmented multilocular, smooth periproctal plates, no ossicles detected in any part of the body, except for scattered fragmented/reduced/corroded end plates in some tube feet.

Remarks. This specimen is identified as Thyone roscovita Hérouard by A.D. Hobson perhaps based on McK- enzie’s (1991) suspicion. However, the label inside the original jar states Hebron’s Thyone but the writer was unable to ascertain what was implied by this label. Because of the presence of some anal plates and tube feet end plates, the writer with some hesitation, agrees with Hobson’s identification that the specimen represents T. roscovita , which according to McKenzie (1991) is perhaps a junior synonym of T. elegans . This is discussed in some detail under remarks above. I have no reason not to support this synonymisation. However, the presence of a calcareous ring with the radials prolonged before bifurcation and two stone canals suggest that this specimen may be a juvenile and identical to the Thyone sp. described above.

Figures

FIGURE 4. A. Thyone?elegans / T.?roscovita (ex Isle of Muck), specimen entire; B. Aquapharyngeal bulb of?Thyone sp. (ex St Mary’s Scilly Island); C. Calcareous ring of Thyone?elegans / T.?roscovita (ex Isle of Muck); D. Calcareous ring of Thyone sp.; E. Tube feet end-plates of Thyone?elegans / T.?roscovita (ex Isle of Muck). MDIR = mid-dorsal interradial plate.