Size: Usually up to 45mm long, occasionally over 50mm and exceptionally, in Orkney, 75mm. Robust. Valves: Of similar shape and structure. Inequilateral, beaks slightly in front of midline. Globose; shell sub-spherical (except large ones) so rolls easily. Outline almost circular when <10mm, increasingly elongated at posterior with age and size . Outer surface: 22-28 flat topped or slightly rounded ribs. Intervening spaces almost as wide as ribs on juveniles, but relatively much narrower when large and old. Ribs bear many straight or slightly curved transverse scale-like spines which are often eroded down. On old shells, scale-spines waved. Annual growth lines distinct or eroded. Valve margin: Fairly smooth in outline, but crenulate inside and out when viewed from posterior. Ligament: Externally substantial to posterior of beak. Hinge: Two cardinal teeth centrally in each hinge. Right valve; 2 anterior and 2 posterior lateral teeth. Left valve; single anterior and posterior lateral tooth. Interior surface: Faint continuous pallial line runs parallel to margin to fairly rounded adductor scar at each end. No pallial sinus (indentation in line). Internally, ribs extend in only a short way from the edge and stop at pallial line. Periostracum: Brown, usually worn off except between close scale-spines and ribs on recent stages. Shell colour: Exterior yellowish and/or whitish. Juveniles often with more colourful pattern, sometimes retained near beak of older specimens. Interior white, often with some dark brown near edges, especially on posterior, and occasionally extending further in.
Flesh: Mostly translucent yellowish white. Siphons: Short, fused at base. Orange when contracted, diminishing to yellowish or peach as extend and colour is dispersed. Short rows of brown spots radiating from opening. Opaque white splashes inside openings. Siphons have tactile tentacles with camera-style eyes (single lens and retina). Mantle: Edge yellowish, with short pallial tentacles. Foot: Yellow-orange and robust when contracted within shell, but blade-like and white, faintly tinted peach, when extended. Can flex for rapid escape movement. Within shell: Thin transparent mantle with yellow edge lines interior of valves. Yellowish white body and yellow-orange foot. Pair of ctenidia, each folded into four lamellae, W in cross section, with fine ciliated mesh of fused filaments visible if magnified.
- Outer surface not smooth, with 22-28 radiating ribs.
- Internally, ribs extend in only a short way from the edge and stop at pallial line.
- Right valve has two posterior lateral teeth on hinge.
Ten other species of Cardioidea in Britain. Several share one or two of the three key identification features stated for C. edule, but none has all three in combination. In brackish water, 5-20 p.p.t.,consider C. lamarcki .
In stable sand, mud and muddy gravel below effect of strong wave action, to 200m, and from mid-tide level to ELWS and below in sheltered bays and outer estuaries, where up to 10 000 /sq. metre (small specimens). Does not prosper in brackish water below 20 p.p.t. Short siphons, so confined to near surface of substrate. Very large specimens lie exposed on surface. Water carrying oxygen, plankton, benthic diatoms and organic material enters inhalent siphon to be processed by ctenidium for respiration and food before exiting exhalent siphon.
Size distribution in populations varies with complex of factors including length of immersion /feeding period, competition, and numbers and capabilities of predators at different shore levels. Usually, average size increases and population density decreases from mid-tide level to ELWS. In Britain, average size increases from South to North, reflected in Fishery limits on riddle gauges; 16mm Thames, 19mm S.Wales, 20mm N.Wales, 30mm Outer Hebrides (limits subject to change). Largest specimens in Europe may be in Orkney; over thirty live specimens, 60mm –75mm length, found exposed on sand surface at ELWS on 300m stretch of beach. At this size immune to oyster catchers (major predator, preferred length 20-30mm), and numerous Buccinum undatum present. Beach rarely visited, so human predation minimal. Causal factor(s) of N-S cline uknown. Archaeological deposits suggest decline in size at places and periods of high human utilization of cockles.
Breeds in Britain late February – early July. Separate sexes release milt or ova into sea. After two or three weeks as a trochophore and veliger larva in the plankton, settles on substrate and metamorphoses into adult form.
Iceland and N. Norway to Senegal, Baltic, Black Sea and Caspian (see distribution on GBIF). All around Britain and Ireland. Often abundant in outer estuaries, but not often in brackish water below 20p.p.t.