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 : 368-369

? Thyone sp. indet.

Figure 4 (B & D)

Material examined. 1967.10.4.5, St. Mary’s, Scilly Island; Sub-Aqua Expedition.

Description. Only calcareous ring and introvert with tentacles preserved. Tentacles 10, clearly dendritic, dark brown, ventral two reduced, others of unequal length, longest about 10 mm. Small portion of the creamish body wall with some, apparently scattered tube feet, still persists, attached to part of introvert. Calcareous ring elongate, complex, radial and interradial plates fused, of more or less equal length, radial plates anteriorly bifid, bifurcate posterior processes of radial plates almost the length of ring, both tube and processes highly fragmented. Polian vesicle single, elongate, about 40 mm in length; stone canals three, highly convoluted, tendril-like, longest 5 mm, madreporic bodies horse-shoe shaped. No ossicles in tentacles, introvert, tube feet, or in the remaining minute fragment of body wall.

Remarks. Because of the presence of only the aquapharyngeal bulb and the absence of ossicles in the small, retained part of the body wall, tentacles, introvert and tube feet, both the generic and specific identity of this material is in doubt. Judging from the structure of the calcareous ring and the assumed distribution of podia, it perhaps belongs to Thyone . Of the four European species of Thyone listed by McKenzie (1991) as the last reviser, Thyone fusus Müller, 1776 and T. wahrbergi Madsen, 1941 , always contain ossicles, while T. roscovita Hérouard, 1889 and T. inermis Heller, 1869 lack ossicles in the main body wall except the anal region, tentacles and introvert. Mortensen (1927) also showed that T. inermis has ossicles in the posterior region of the body wall and tentacles. T. elegans Norman, 1869 , described from the Shetlands as being totally devoid of body wall ossicles, was suspected by McKenzie (1991) as a possible synonym of either T. inermis or T. roscovita . The type specimen of T. elegans which is still extant in the NHMUK, is completely devoid of ossicles as noted by McKenzie (1991) and confirmed by the writer. However, its tentacle ossicles were described as a dense layer of plates (?rosettes) and end plates were stated to be present. There are no tentacle ossicles in the current specimen for any comparison with Norman’s (1869) species. Although some aspects of the calcareous ring of the current material with multiple stone canals may resemble those of some species of Stolus , the lack of body-wall ossicles (abundant in all Stolus spp.) and the absence of this genus in the North-East Atlantic waters, lend no support. The writer is of the opinion that the multiplicity of the stone canals may be a significant feature for the future description of another species when complete specimens become available. Because of the incompleteness of the specimen it is here left unassigned but its locality and presence of a calcareous ring in which the radials are prolonged before bifurcation and not deeply cleft perhaps indicate that it may be either a T. roscovita or T. elegans , but not T. inermis which has deeply cleft radial plates. Mckenzie (1991) suspects the synonymy of T. roscovita and T. elegans and did not consider Pawson & Miller’s (1981) T. inermis as identical to the species he described as T. inermis but gives no reasons for this. Subsequently Hendler et al. (1995), in their analysis of the Carribean and Florida echinoderms, accepted Madsen’s (1941) viewpoint and described the Carribean species as T. deichmannae which Madsen (suggested long before Deichmann’s revision (1947). Despite all the arguments presented by McKenzie (1991), WoRMS [edited by Paulay in 2015 (accessed July 2019) and based on several works] lists both T. inermis and T. roscovita but regards T. elegans as a synonym of the former. This may be true as the type localities of both are Shetland Islands or its vicinity. However, the deeply cleft radials in T. inermis and posteriorly prolonged radials in T.roscovita and clinal variations of a single species along the north-west Atlantic, might speak against this.

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.