Distributed worldwide, approximately 450 species of water fleas have been identified. The vast majority of the organisms are only found in freshwater, but a small handful can survive in the sea. These marine species may, however, appear quite different than their inland relatives. Typically, water fleas feature a discrete head, a compound eye, pronounced antennae, and a protective carapace that surrounds the trunk and abdomen. The carapace, which appears to be composed of two separate sections although it is actually formed from a single piece, may sometimes enclose the head as well, and in at least one species is significantly reduced in size. Reaching lengths as great as 18 millimeters, the predatory water flea Leptodora seems to have outgrown the traditional carapace, but retains the truncated structure as a brood sac.