Common Cockle

The common cockle is a suspension feeder, filtering plankton and other organic matter from the water (3). The sexes are separate, and adults typically begin to spawn in their second summer. Fertilisation is external, and a large percentage of a population spawns at the same time. Eggs and sperm are released into the water; the free-swimming larvae (veliger larvae) live for 3-6 weeks in the plankton before undergoing metamorphosis into juvenile cockles, which then settle to the substrate. Growth rates vary with the season; in winter there is very little growth, and this leads to the marked growth-bands on the shell, which have been used to age cockles (2). The typical life-span of this cockle is 2-4 years, although they may live for 9 years or more (3). Cockles are predated upon by oystercatchers (Haematopus ostralegus), the shore crab (Carcinus maenas), shrimps and flatfish (2).

In some areas, concerns about over-collecting have led to measures that control the numbers of cockles harvested and the methods used. In Scotland, for example, dredging with vehicles is banned, and hand gathering is the only method allowed in some parts of England and Wales (2).

This well-known edible cockle has a solid shell, consisting of two valves, which feature prominent ribs and concentric growth-lines (2). The outer surface of the shell is off-white, yellowish or brown, and the inner surface is white (3).

Inhabits the middle and lower shore, where it burrows into soft sand, mud and muddy gravel to depths of less than 5 cm (2). It is often found in huge numbers in estuaries and other sheltered inlets (3).

This cockle has a wide distribution around the coastline of Britain. Elsewhere, its range extends from the western Barents Sea and Norway in the north, to Spain and Portugal, and reaches as far south as Senegal in west Africa (2).

Common and widespread; not listed under any conservation designations (2).

The common cockle has been collected and sold for hundreds of years. Mechanised forms of collecting, using tractors and hydraulic dredging, have largely replaced more traditional methods such as hand raking. There are fears that without adequate management of cockle stocks, these new techniques could result in over-exploitation (2).