Clamming up

This large clam is the Smooth Trough Shell (Mactra glabrata) that often washes up on Fish Hoek Beach. While alive, the clam burrows just below the surface of the waves in fine sand and like most bivalves, it is a filter feeder - sucking water through an inhalant siphon, sieving it through sheet-like gills, and then expelling the waste water through an exhalant siphon. Much of the body consists of gonad and enormous amounts of egg and sperm are shed into the water where they develop into planktonic larvae.
One seldom sees them alive, but apparently they are good to eat.

Info from Branch, G.M., Griffiths, C.L., Branch, M.L. and Beckley, L.E. 1994. Two oceans. A guide to the marine life of southern Africa. David Philip, Cape Town.