[2] They were originally classified in the former order Conchostraca, which later proved to be paraphyletic, because water fleas are nested within clam shrimps.

Clam shrimp are now divided into three orders, Cyclestherida, Laevicaudata, and Spinicaudata, in addition to the fossil family Leaiidae.

The animals react to danger by contracting the muscle, so that the valves close tightly and the crustacean, as if dead, lies motionlessly at the bottom of the pool.

In the common genus Lynceus, which can open its spherical valves wide, the thoracic legs move in an oar-like manner along with the antennae.

In females, the outer lobes of several middle legs are modified into long, upward-bending threadlike outgrowths, used to hold the eggs on the dorsal side of the body under the shell.

In freshwater deposits, generally poor in fossils, the well-preserved clam shrimp shells are found quite often.

Many extinct species, mostly Triassic specimens, once lived in marine environments, where no extant clam shrimp inhabit today.