Thyasira obsoleta (Verrill & Bush, 1898)

(Fig. 26)

Material examined. All from Gulf of Cadiz; two live-collected specimens, MSM01-03, stn 145 GKG3, Porto mud volcano, 35º33.699'N, 09º30.437'W, 3860 m, 21 April 2006 (DBUA 00831.01); one live-collected specimen MSM01-03, stn 190 MUC8, Captain Arutyunov mud volcano, 35º39.665'N, 07º19.970'W, 1322 m, 28 April 2006 (DBUA 00831.02); one live-collected specimen, MSM01-03, stn 241 GKG12, Mercator mud volcano, 35º17.918'N, 06º38.717'W, 353 m, 6 May 2006 (DBUA 00831.03); one live-collected specimen MSM01-03, stn 169 MUC7, Carlos Ribeiro mud volcano, 35º47.256'N, 08º25.361'W, 2199 m, 25 April 2006 (DBUA 00831.04).

Remarks. All specimens are very small (not exceeding 2.0 mm) but exhibit the characteristically weakly sulcate posterior and angular outline. This is a common and widespread species recorded from across the Atlantic Ocean in depths from 24 to 2900 m (Payne & Allen 1991). It has not been associated with any particular reducing environment. Dufour (2005) classified this as having Type 2 gills with both demibranchs but showing no evidence of bacterial symbiosis.