A group of crustaceans known as os-tracods can be very abundant in themesopelagic. Ostracods have a character-istic shell, or carapace, that makes themlook like tiny clams with legs (Fig. 16.5). They are crustaceans, however, and unre-lated to clams. Like copepods, most os-tracods are small, usually only a few millimeters (1/8 in) long. One group (Gi- gantocypris), however, can reach 1 cm(1/2 in). Amphipods and other crus-tacean groups are also part of the mid- water plankton.