Most bivalve mollusks are filter-feeders, straining water to get their food, usually microscopic plants. Exceptions to this include marine clams that prey on small creatures, such as crustaceans. Some of these clams, the septibranchs, use a flexible, muscular "wall", or septum, as a diaphragm pump that allows the animal to quickly and decisively suck prey in. The inch-long Specter Clam, Dilemma spectralis Leal, 2008 (below), a deep-sea dweller found off Vanuatu in the SW Pacific, is one of them. It is possible, given that Dilemma clams live permanently attached to rocks, that Specter Clams may use some form of chemical attraction to lure nearby prey.
The illustrations below show, from left, a whole preserved clam, the clam with shell removed, and drawing (by Kimberly Nealon) showing the stomach with its crustacean food, in this particular case an isopod crustacean. The drawing also shows the incurrent siphon, or opening through which prey is ingested. Read the paper here.