White Abra

The production of a large number of small-sized eggs suggests that A. alba has a larval development with a long pelagic life. The optimal spawning period of the bivalve lies between May and August. The first postlarvae have a shell length of btween 200 and 300 µm. Specimens that settle in early summer grow rapidly, while another spatfall in late summer or autumn apparently does not grow until the following spring. Most specimens of A. alba live only one year, reaching modal lengths of 12-14 mm. Specimens that live for two years grow to 13-16 mm and the maximum length reached is 20-25 mm. The abundance of A. alba typically fluctuates widely from year to year due to variation in recruitment success or adult mortality. A. alba is capable of rapidly exploiting any new disturbed substratum suitable for colonisation, either through larval recruitment, secondary settlement or post-metamorphosis juveniles or re-distribution of adults following storms. A thin-shelled selective deposit

A. alba and Tellina fabula are two species occupying the same feeding niche since both can feed on deposit as well as on suspended material. Competition between the two species may result in the slow growth of the later-settling T. fabula.

Small (up to 25mm long), thin and fragile shell. Oval shape, rather flat with thin concentric growth lines. The shells are shiny white and somehow transparent. Abra alba lives rather deep in the bottom and feeds on sediment by means of long, individually separated stretchable siphons.

In the investigated area A. alba is very abundant in the south-eastern part of the Oyster Ground and the Frisian Front area. The bivalve is also present near the Dutch coast and in the Voordelta area, where its distribution extends into the Westerschelde and the Grevelingenmeer. The highest densities in the Delta area are found in the Oosterschelde. In the western part of the Wadden Sea and at the Balgzand the species is present in low densities. Highest biomass is found along the Dutch coast and in the central part of the Frisian Front area. Other investigations found the species to be confined to a more or less narrow strip of sea bottom near the shore (Eisma, 1966; Tebble, 1966; Wolff, 1973; Hayward & Ryland, 1990;< Rees & Dare, 1993).

During both periods Abra alba is a common species in the near-coastal zone. The species is only rarely found further than 30 km off the coast. Abra alba locally reaches very high densities: up to about 14,000 ind./m2 in the 1976-1986 period and appr. 5,000 ind./m2 in the 1994-2001 period. The highest densities are observed near the western coastal zone in both periods.

Abra alba can be found in all sediment types. However, the species does tend to prefer (> 50% of the samples) fine-grained sediments with a median grain size between 50 and 250 µm and a mud content of 10-50%. In coarse sediments (median grain size > 300 µm) with a low mud content (< 10%) the A. alba is rather rare.

The shell is thin and brittle, broadly oval in outline. It is up to 25 mm long and has a sculpture of fine concentric lines. The growth stages are clear. The shell is white in colour, covered with a light brown periostracum. The interior of the shell is white.