Thyasira flexuosa Montagu 1803

Thyasira flexuosa (Montagu, 1803)

Fig. 7 d–f

Tellina flexuosa Montagu, 1803 (p. 72).

? Ptychina biplicata Philippi, 1836 (p. 15, pl. 2, fig. 4).

Axinus flexuosus Montagu—Hidalgo 1917 (p. 160).

Thyasira flexuosa (Montagu) — Tebble 1966 (p. 79, text-figs. 5a–b); Di Geronimo & Panetta, 1973 (p. 103, pl. 2, fig. 13).

Thyasira flexuosa flexuosa (Montagu, 1803) — Nordsieck 1969 (p. 78, pl. 13, fig. 48.00).

Thyasira flexuosa (Montagu, 1803) — Barash & Danin 1992 (p. 265, fig. 284); Cossignani et al. 1992 (fig. 312); Poppe & Goto 1993 (p. 83, pl. 14, fig. 1); Petersen 2004 (p. 74, fig. 63); De Frias Martins et al. 2009 (p. 89, fig. 327).

? Thyasira biplicata (Philippi, 1836) — Giribet & Peñas 1997 (p. 86, fig. 98); Repetto et al. 2005 (p. 308, mid right fig.); Oliver et al. 2016 (online resource).

Thyasira (Thyasira) flexuosa (Montagu, 1803) — Oliver et al. 2002 (pp. 24, 65; pl. 2, fig. A; pl. 4; pl. 6, fig. A; pls. 7–8; pl. 9, figs. C–D; pl. 10, fig. B; pl. 13, figs. A–B, H; text-fig. 6); Oliver et al. 2016 (online resource).

Diagnostic characters. Pentagonal shell; two shallow posterior radial depressions, the dorsal one shorter and narrower; typically two-folded posterior margin. Prodissoconch: shell type ST-2D; length about 170 µm; inequilateral (more expanded posteriorly), roundish D-shaped outline; convex profile; surface smooth; rim-like P-2 with few weak commarginals, barely separated from P-1; transition to the nepioconch well marked.

Remarks. The relationship of T. flexuosa with T. biplicata (Philippi, 1836) is still unclear, because some authors (e.g. Nordsieck 1969; Poppe & Goto 1993) regard this latter as a junior synonym of Montagu’s species, while some others (e.g. Giribet & Peñas 1997) consider it as a valid taxon with a different distribution.

Occurrence. Box-corer samples BC67 (1 specimen), BC71 (5), BC72 (18); core BC72 (6). Maximum height: 6 mm.

Distribution and habitat. The species ranges from northern Norway to the Gulf of Mexico and the Azores into the Mediterranean; it is also present in the northern Pacific. The species dwells on mud and sand from the intertidal zone to below 2000 m depth, being both chemosymbiotic and suspensivore (Nordsieck 1969; Barash & Danin 1992; Poppe & Goto 1993; Oliver et al. 2016).

Fossil record. Miocene to Recent (Monterosato 1872; Monegatti & Raffi 2001); Pliocene? of northern Italy (Tabanelli 2008 sub T. biplicata); Pleistocene and Holocene of the North Sea and Denmark (Petersen 2004).