Species Account

Thyasira equalis (Verrill & Bush, 1898)

Thyasiroidea : Thyasiridae

Tebble name: n/a
Smith & Heppell name: Thyasira equalis Verrill & Bush, 1898

To size: To 8mm. Shell Structure: Thin, fragile. Equivalve: Equivalve. Equilateral: Equilateral. Tumidity: Not inflated. Outline: Ovate-rhomboidal: inner anterior dorsal margin sloping steeply, depressed; distinct outer anterior dorsal margin short obliquely truncate; anterior narrowly rounded; ventral round to a little angulate; posterior margin truncate to slightly sinuous; posterior dorsal margin long sloping steeply. Contour: Submarginal sulcus long, edges sharply angled, no auricle; posterior area flattened or as a shallow sinus; when anterior dorsal truncation present accompanied by a low ridge. Larval shell 155-180microns with weak radiating wrinkles.

Sculpture: Smooth with growth lines only, often chalky and eroded. Margin: Smooth. Ligament: External, deeply sunken scarcely visible. Hinge: Hinge plate weak, indistinct cardinal protuberance in RV, corresponding depression in LV. Pallial Musculature: Muscle scars obscure. Periostracum: Not persistent. Colour: Shell white often with ferruginous deposits on anterior dorsal truncation and posterior. Additional Characters: 2 demibranchs.

Distribution & Ecology

A conservative position is taken here and regarded with certainty only from the central North Sea and shelf margin west of Shetland and Hebrides. The records from the Bay of Biscay in bathyal depths may be a different species.
Also from the shelf of Scandinavia.
Partial chemotrophic, suspension feeder, symbiosis partial, gills sometimes with bacterial symbionts but not highly adapted, 2 demibranchs. In British waters confined to deep shelf areas of the North Sea in muddy sands
Gross anatomy – Payne & Allen, 1991
Gill anatomy – Dufour, 2005
Symbiosis – Dando & Spiro, 1993

Depth Range
Continental Shelf (to 200m)
Continental Margin Zone (200 - 500m)
Bathyal (500 - 2000m)

Additional Information & Related Species

Additional Comments

Very similar to T. dunbari but this species not recorded south of Faroes, best identified from larval shell characters.

Other similar shells from upper slope of Atlantic margin not considered to be conspecific, see T. spA

Related Species


Listed are literature citing Thyasira equalis (Verrill & Bush, 1898). Reference containing the species Type Description is highlighted.

Dando P R & Spiro B 1993. Varying nutritional dependence of the thyasirid bivalves Thyasira sarsi and T. equalis on chemoautotrophic symbiotic bacteria, demonstrated by isotope ratios of tissue carbon and shell carbonate. Marine Ecology Progress Series . 92: 131-158.
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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Record last modified: 17/05/2016