Cerastoderma edule | Common Cockle

Introduction

The common cockle (Cerastoderma edule) is a plump, strongly ribbed bivalve that lives just beneath the surface of sheltered sandy and muddy flats around the coasts of north-west Europe. Vast beds of cockles build up in estuaries and bays, where they are harvested by hand and by dredge, and where they form one of the most important food supplies for wading birds. The shell’s rounded, heart-like outline gives the whole cockle family its scientific name.

Taxonomy

Cerastoderma edule belongs to the family Cardiidae within the class Bivalvia and the phylum Mollusca, the same great group of shellfish that includes Mytilus edulis (Common Mussel). The family name comes from the Greek kardia, “heart,” because the shell is heart-shaped when seen end-on, and the genus name means “horny skin.” Like other cockles it is a shallow burrower with short siphons, unlike the mussel which attaches to rocks by threads.

Physical Characteristics

The shell is thick, rounded, and slightly triangular, typically 3–5 cm across, with a strongly convex, heart-like profile. Its surface carries 22–28 prominent radiating ribs crossed by finer concentric ridges, giving a distinctly sculptured, cross-hatched appearance. Color ranges from dirty white and cream to yellowish or grey-brown, often with a darker periostracum that wears away. Two short siphons project just above the sediment, and a large, powerful, bent foot is used for burrowing and for making short hops across the surface.

Habitat

Cockles live in the intertidal zone and shallow subtidal waters, burrowing a few centimetres into clean sand, muddy sand, or mud in sheltered bays, estuaries, and tidal flats. They favour areas with muddy sand and stable sediment, and can occur at extraordinary densities—hundreds per square metre in the best beds. Because they sit so close to the surface, they are exposed at low tide and are vulnerable both to feeding birds and to cold winter weather.

Diet

The cockle is a filter feeder. It draws water in through one short siphon, passes it across the gills, and traps microscopic algae, chiefly diatoms, along with bacteria and organic particles in mucus, which is then carried to the mouth. Cleaned water leaves through the second siphon. Dense cockle beds filter enormous volumes of water, helping to keep estuaries clear, and the animals themselves convert that plankton into food for birds, fish, and people.

Reproduction

The sexes are usually separate, and spawning takes place in spring and summer when water temperature rises. Eggs and sperm are released into the open water, where fertilisation occurs. The larvae drift as plankton for two to five weeks before settling onto suitable sediment, where they burrow and grow. Settlement success varies enormously from year to year, and a whole cockle bed may be dominated by a single successful year class until a good spatfall replaces it.

Conservation

The common cockle supports valuable hand-gathered and dredged fisheries, and it is not threatened overall. The concern is competition: cockle beds are the principal food of oystercatchers, knot, and other waders, and heavy commercial harvesting can seriously reduce the food available to birds, particularly in winter. Most estuaries now manage the fishery through permits, seasonal closures, and stock assessments designed to leave enough cockles for both the fishery and the birds.

Fun Facts

• The shell is heart-shaped when viewed end-on, which is why the family is called Cardiidae.
• Cockles can hop short distances across the sand by flicking their strong, bent foot.
• Enormous beds can hold hundreds of individuals in a single square metre of mud.
• Oystercatchers are specialist cockle feeders, using their strong bills to hammer or prise shells open.
• Cockles are still gathered by hand with rakes on many tidal flats, just as they were centuries ago.