Thyasira obsoleta (Verrill & Bush, 1898)
Thyasiroidea : Thyasiridae
To size: To 4mm. Shell Structure: Thin, fragile. Equivalve: Equivalve. Equilateral: Slightly to distinctly inequilateral, beaks behind the midline. Tumidity: Moderate. Outline: Obliquely ovate to pyriform (tear-drop shaped); anterior slightly to distinctly expanded; anterior dorsal margin sloping gently, weakly curved merging into broadly curved anterior; ventral margin evenly curved; posterior profile weakly to distinctly bi-angulate; posterior obliquely truncate, posterior dorsal angled by projecting auricle. Contour: Auricle low, rather long; submarginal sulcus depressed but edges weakly angled; lunule area a little flattened but no demarcating ridges.
Sculpture: Smooth with growth lines, often irregular. Margin: Smooth. Ligament: External, deeply sunken in auricle. Hinge: Hinge plate weak, slightly thickened below and anterior of beaks but lacking cardinal protuberance. Pallial Musculature: Muscle scars obscure. Periostracum: Not persistent. Colour: Shell white often with ferruginous deposits on posterior and anterior dorsal areas. Additional Characters: 2 demibranchs.
Distribution & Ecology
Upper slope specimens from the Atlantic margin are considered to be a different species.
Payne & Allen, 1991 record it from bathyal and abyssal rise depths in the bay of Biscay but we have not confirmed this. Partial chemotrophic, suspension feeder (by inference from gill morphology), 2 demibranchs, however Dufour found no symbionts.
Gill anatomy – Dufour, 2005
Continental Shelf (to 200m)
Continental Margin Zone (200 - 500m)
Additional Information & Related Species
Thyasiroidea : Thyasiridae
Listed are literature citing Thyasira obsoleta (Verrill & Bush, 1898). Reference containing the species Type Description is highlighted.
Dufour S C 2005. Gill anatomy and the evolution of symbiosis in the bivalve family Thyasiridae. Biological Bulletin. 208: 200-212.
Olabarria C. 2005. Patterns of bathymetric zonation of bivalves in the Porcupine Sea Bight and adjacent abyssal plain, NE Atlantic. Deep-Sea Research I. 52: 15-31.
Oliver P G & Killeen I J 2002. The Thyasiridae (Mollusca: bivalvia) of the British continental shelf and North Sea Oilfields. Studies in Marine Biodiversity and Systematics from the National Museum of Wales. BIOMOR Reports. 3: 1-73.
Payne C.M. & Allen J.A. 1991. The morphology of deep-sea Thyasiridae (Mollusca: Bivalvia) from the Atlantic Ocean. Philosophical Transactions of the Royal Society of London Series B. 334: 481-566.
Verrill A E & Bush K J 1898. Revision of the deep water Mollusca of the Atlantic Coast of North America with descriptions of new genera and species Part 1 Bivalvia. Proceedings of the United States National Museum. 20: 775-901.
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