Morphology

Pectens or scallops have two valves, held together along the hinge line by a triangular shaped elastic ligament (= risilium). When the animal closes the shell by the pull of the internal muscle, the resilium becomes compressed. By relaxation of the muscle the valves gape again.

At the edge of the scallop we find the most notable of the soft animal parts: the mantle with numerous tentacles. These are in constant motion and have sensatory organs for detecting movement. Even a slight disturbance in the water is enough to make the valves contract. Alternating with the tentacles, we can see a row of small circular points which are the lightsensitive eyes. On the mantle there are also tentacles which provide the sense of smell, and which react to minute chemical changes of the seawater.

When the valves gape the mantle with all the tentacles form a sort of screen through which water is entering into the gill of the animal. Oxygen and food will be filtered by the gill and then be conveyed towards the mouth. The water current will leave the shell on either side near the ears of the pecten.

We can also notice a big reproductive organ that is divided in a male gland and a female gland. Depending on factors such as age and season either the female or the male part will be more developed. Aequipecten opercularis will become sexual mature at the age of two to three years but this is different for other species. Spawning happens from January to August and mostly around the time of each full moon. Also the temperature of the seawater seems to be important. When the eggs of the female pectens and the sperm of the male scallops are released in the seawater, the fertilization can take place. The fertilized eggs develop into a larva. This larva will loose its swimming membrane and settles down on a hard substrate. It will stay at this substrate by its byssus threads so the scallop can continue to grow. Some species are only temporarily attached by its byssus to a substrate (for example Aequipecten opercularis). After a few weeks this type of shell will break its byssal anchors and starts its free swimming life. Others stay their whole life byssaly attached to a hard substrate (for example Mimachlamys varia) while other types (for example Talochlamys pusio) become cemented onto the hard object.