Abra alba
Early juveniles are capable of fast and frequent burrowing (Aabel, 1983). Adults are less active in the sediment, but possess a highly mobile inhalant siphon which bends and twists in all directions. This siphon often arches over with the aperture searching the surface of the sediment from which it actively collects material (Yonge, 1949).
Larval shell: obliquely oval in outline. The anterior end is larger than the posterior end and is expanded ventrally. The length of the prodissoconch I falls in the range 0.075-0.095 mm; the prodissoconch II reaches 0.26-0.31 mm at metamorphosis. In the late veliger stage the larval hinge comprises twelve to fifteen small denticles and an anterior larval ligament. During the pediveliger stage, two cardinal teeth are formed in the right valve and one in the left valve (Webb, 1986).
Siphons: very long (inhalant extending for more than 5 times the length of the shell), separate, formed by fusion of the inner folds of the mantle margin, with the apertures fringed by only six blunt lobes (Yonge, 1949)
Gills: outer demibranch comprising only one lamella, reflected to the dorsum. Food groove absent (Yonge, 1949).
Foot: very large and laterally compressed (Yonge, 1949).
Gonads: yellowish in color and situated in the anterior part of the body, posterior/ventral
to the pericardial chamber. Female gonad granular. Male gonad smooth and root-like in appearance (Nott, 1980).
Sigurdsson et al. (1976) found that early juveniles may secrete a long byssal thread (length greater than 3 cm in a specimen measuring 1 mm in shell length) which, by increasing the drag of the animal, enables it to be transported along relatively weak currents.
Constitutes an important item in the diet of flatfish (Nott, 1980). It is also eaten by dab (Limanda limanda), flounder (Platichthys flesus), plaice (Pleuronectes platessa) and small fishes such as gobies. Other known predators are polychaetes, such as Nephtys and phyllodocids, as well as the brittlestar Ophiura albida and the starfish Asterias rubens (Rainer, 1985).
Occurs in great abundance in the coastal waters of north-west Europe, being often the dominant species of shallow water benthic communities (Nott, 1980).
In tanks with the water temperature kept at approximately 17 degrees Celsius, fertilized eggs of A. alba develop into trocophores in less than 24 hours. They reach the early veliger stage in less than 48 hours and metamorphose after approximatelly 4 weeks (Nott, 1980).
While Nott (1980) reported a mainly annual life cycle for A. alba in the Irish Sea, Stephen (1932) found that the average age of individuals in Loch Striven, Scotland, is of 3 or 4 years. Rainer (1985) gives the life span of the species in Kiel Bay as varying between a little more than one year and two and a half years.
Nott (1980) studied the reproductive biology of the species. It is strictly dioecious, with external fertilization. Attains sexual maturity when the shell length is at least 7-9 mm. Produces a large number of small eggs (15000-17000 eggs of 60 micrometers diameter), suggestive of planktotrophic larval development with a long pelagic stage. The spawning period commences as early as July and continues until after October.
Feeds on deposited material brought into the mantle cavity via the inhalant siphon (adults) or the pedal aperture (early juveniles; shell length < 1.5 mm) (Aabel, 1983).