Tylosigma ostreicola Van, 2017, sp. nov.

Tylosigma ostreicola sp. nov.

Figures 79 a–e

Material examined. Holotype RMNH Por. 9955, Suriname, ‘ Snellius O.C.P.S. ’ Guyana Shelf Expedition, station G7, 7.28°N 56.7933°W, depth 64 m, bottom sand, 7 May 1966.

Description. (Fig. 79 a) Encrusting eroded rims and holes of a dead oyster. Surface hispid-conulose. Colour in alcohol beige. Size several mm2. Consistency soft.

Skeleton. (Fig. 79 b) Single tylostyles penetrating the surface, heads embedded in the substratum. Low spicular density. Tissue crowded with sigmas.

Spicules. (Figs 79 c–e) Tylostyles, sigmas.

Tylostyles (Figs 79 c–d), curved, thin, with prominent round tyles, in a large size range, but not clearly divisible in categories, 342– 779 – 1092 x 6 – 9.1 –13 µm (diameter of tyles 8.5–15 µm).

Sigmas (Fig. 79 e), with incurved apices, one of which may be faintly rugose, 16– 21.8 –27 µm.

Distribution and ecology. Guyana Shelf, on a dead shell at 64 m depth.

Etymology. The name ‘ostrea’ (L.) means oyster, and suffix cola (Gr.) means ‘living on’ or ‘in’, together the compound name refers to its habitat.

Remarks. The new species was compared to a slide of Tylosigma campechianum (originally Hymedesmia campechiana, see Topsent 1889, p. 14, fig. 8c) in the Paris Museum, registered as MNHN D.T. 1844. The new species differs from it by the lack of a separate small category of tylostyles three/four times as short as the larger tylostyles, and a separate category of very small thick sigmas. The short tylostyles are also provided with a few microspines on the head, which is lacking in the present material. These differences appear too great to assume variability and thus I propose here a new species. Alcolado & Gotera (1986) report T. campechianum from Cuba (as Desmacella).

The two species are of uncertain affiliation, as the tylostyles remind of the genera Eurypon (Raspailiidae) or Prosuberites (Hymerhabdiidae), but these genera do not have sigmas or other sigmiform microscleres. A combination of tylostyles and sigmas is found in the family Desmacellidae and thus I propose to reassign Tylosigma to that family, until molecular analysis will have been done.