Popular fiction has it that a giant clam (Tridacna gigas) can close its shells so quickly that it can trap a diver. Bad news indeed for the unfortunate with a foot in the clam. The bivalve won't let go while a big wedge of rubber fin is irritating it. And sooner or later the air will run out. Or a tiger shark will spot the easy meal.
It's a dramatic image—but not an accurate one. Tridacna clams, like most other bivalves, feed by filtering suspended material from the water. They supplement their diet with nutrients supplied directly by symbiotic micro-organisms (zooxanthellae) in the tissue of the mantle. (It's a similar relationship to that between reef-building corals and zooxanthellae.)
But not all bivalves are filter-feeders. Some vacuum up detritus from the mud or sand. And a few prey on other animals.
Most filter-feeding infaunal bivalves (those that live in sediments) extend muscular siphons up into the water so they can draw in fresh water and suspended food and eject waste material. Water entering through the inhalant siphon passes over the gills, which do double duty as respiratory organs and as particle sorters. Any food particles that are too big are chucked out, as are those that are too small. But particles that are just right are sent to the mouth.
Carnivorous bivalves feed by either sucking in prey or scooping them up with the inhalant siphon. Parilimyidae and Cuspidariidae have long, tubular, flexible siphons that can be moved around to hoover up passing crustaceans. More sinister are the Poromyidae and Verticordiidae, in which the siphon is expanded into a 'cowl' that encloses prey.
Once inside the siphon, muscular structures called labial palps direct the hapless victim into the mouth. Although carnivorous bivalves lack teeth, the oesophagus and stomach are heavily muscled and can easily crush small crustaceans. Digestive enzymes do the rest.
None of these carnivorous bivalves is large. Most species don't reach 3 cm. But wouldn't it be fantastic if they were the size of Tridacna? Then we might have something to worry about.