Pecten maximus, common names the great scallop, king scallop, St James shell or escallop, is a northeast Atlantic species of scallop, an edible saltwater clam, a marine bivalve mollusc in the family Pectinidae. This is the type species of the genus. This species may be conspecific with Pecten jacobaeus, the pilgrim's scallop, which has a much more restricted distribution.
Description The shell of Pecten maximus is quite robust and is characterised by having "ears" of equal size on either side of the apex. The right, or lower, valve is convex and slightly overlaps the flat left, or upper, valve. Larger specimens have a nearly circular outline and the largest may measure 21 cm in length. The "ears" are prominent and are a minimum of half the width of the shell, with the byssal notch situated in the right anterior ear being slight and not serrated. The sculpture of the valves is distinctive and consists of 12 to 17 wide radiating ribs and numerous concentric lines, which clearly show the scallop's growth history, while the "ears" show a few thin ribs, which radiate from the beaks. The radiating ribs reach the margins of the valves and this creates a crenulated form. The left valve is normally reddish-brown whilst the right valve varies from white through cream to shades of pale brown contrasting with pink, red or pale yellow tints; either valve may show zigzag patterns and may also show bands and spots of red, pink or bright yellow. The colour of the body of Pecten maximus is pink or red with the mantle marbled brown and white. When young they are attached to the substrate by a byssus but mature animals are capable of swimming by the opening and rapid closing of the valves. The adductor muscle which is used to close and open the valves is very large and powerful. The foot is a finger-like organ, which spins the byssal threads, which pass through the byssal notch on the ears. The margin of the mantle has two layers: the inner layer is finely fringed whilst on the outer is lined with long tentacles with two series totalling 30–36 dark blue or green simple eyes or ocelli in two rows at their base.
Distribution Pecten maximus occurs in the eastern Atlantic along the European coast from northern Norway, south to the Iberian Peninsula, it has also been reported off West Africa, off the Macaronesian Islands. In Great Britain and Ireland it is distributed all round the coast but it is uncommon and localised on the eastern North Sea coast. It prefers offshore waters down to 100 metres (330 feet; 55 fathoms) depth.
Biology
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