Species Account

Arca (Tetrarca) tetragona Poli, 1795

Arcoidea : Arcidae

Tebble name: Arca tetragona Poli
Smith & Heppell name: Arca tetragona Poli, 1795

To size: To 40mm. Shell Structure: Strong but not thick, opaque. Equivalve: Equivalve. Equilateral: Inequilateral, beaks about 1/4 of dorsal margin length from anterior. Tumidity: Becoming greatly expanded. Outline: Trapezoidal, dorsal margin very long straight, posterior margin oblique straight, ventral margin straight or sinuate with a prominent byssal gape, anterior margin rounded. Contour: Dorsal area flat, becoming very wide. Posterior carina sharp, median area flattened or depressed.

Sculpture: Primary sculpture of closely spaced radial riblets these interrupted by growth stops and overall nodulose; posterior area with a few broad ribs these also nodulose. Margin: Inner margin weakly crenulated in correspondence to riblets. Ligament: Duplivincular, initially between beaks but extending over the whole of the dorsal. Hinge: Taxodont: Of numerous small teeth in two series, set on a narrow straight hinge plate; anterior teeth mostly vertical small up to 30 in number; posterior teeth larger, mostly oblique up to 15 in number although those closest to the centre may be eroded or lost. Pallial Musculature: Anterior adductor slightly smaller than posterior, both with a weak myophoric flange. Periostracum: Of simple dark brown, short bristles, those on the posterior carina longer. Colour: White to dull pale brown.

Distribution & Ecology

Attached to hard substrates by a stalked byssus in the intertidal and over the shelf. In all areas, except southern North Sea and eastern end of English Channel.

Depth Range
Continental Shelf (to 200m)

Additional Information & Related Species

Juvenile Description

Shells not distorted, posterior indented, carina very sharp, sculpture prominent. Ligament initially in a simple pit directed posteriorly. Juvenile Striarca with weaker sculpture, amphidetic ligament pit and straight posterior margin.

Key Features & Similar Species

Ligament as v-shaped chevrons and the only one with these initially between the beaks and then covering entire dorsal area. Only shallow water species with ligament in chevrons.

Related Species


Listed are literature citing Arca (Tetrarca) tetragona Poli, 1795. Reference containing the species Type Description is highlighted.

Poli J X 1795. Testacea Utriusque Siciliae eorumque historia et anatome. vol. 2 Parma, Regio Typographeio. i-lxxvi. 75-264, pls 19-39.


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Record last modified: 22/03/2016