Clam shrimp

Clam shrimp are a taxon of bivalved branchiopod crustaceans that resemble the unrelated bivalved molluscs. They are extant, and known from the fossil record, from at least the Devonian period and perhaps before. They were originally classified in a single order Conchostraca, which later proved to be paraphyletic.

Characteristics

Both valves of the shell are held together by a strong closing muscle. 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 most species the head is dorsoventrally compressed. The sessile compound eyes are close together and located on the forehead; in the genus Cyclestheria they are truly fused. In front of them is a simple naupliar eye. The first pair of antennae is reduced and unsegmented. The second pair of antennae, however, is long and biramous. Both branches are covered with numerous bristles. The crustaceans swim primarily by swooping the antennae. 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.