Arca noae

Description

The shell of Arca noae grows to about 10 cm (4 in) in length. It is shortened at the anterior end and elongated posteriorly. It is irregularly striped in brown and white and has fine sculptured ribs running from the umbones to the margin. The hinge is long and straight and the shell is attached strongly to the substrate by byssal threads. There are pallial eyes on the edges of the mantle, especially at the posterior end.

Distribution

Arca noae is found in the Mediterranean and Adriatic Seas. It used to be common in the Adriatic but in 1949/50 there was a sudden unexplained, catastrophic decline in numbers. Since then populations have been creeping back upwards and in 2002, densities of up to 13 individuals per square metre (11 square feet) were recorded but, because of lack of records, it is unclear whether a return to prior population levels had been reached.

Biology

In the lower part of the intertidal zone in the Adriatic, Arca noae often grows in association with Modiolus barbatus. The shells are often heavily encrusted with epibionts. Water is drawn into the shell mainly at the posterior end. Plankton and fine organic particles are filtered out as the water passes over the gills and inedible particles are rejected at the same time.

Uses

Arca noae is fished commercially in the Adriatic Sea, either by divers gathering individual shells by hand or from boats using specially adapted rakes which are pulled along the seabed. The shellfish are then sold in local markets.