Chapter 21 – Copepoda

Publisher Summary Copepods are one of the most successful groups of animals on Earth. Their high densities in the world's oceans make copepods one of the most abundant metazoan groups. The Laurentian Great Lakes, which contain another 20% of the world's unfrozen freshwater, are also dominated by copepods, as are other notable lakes such as Lake Tahoe in the United States and many lakes in the Southern Hemisphere. The plankton of Arctic, Antarctic, and high-elevation alpine lakes around the world is often dominated by just one or two species of large red copepods containing pigments that protect them from damage by UV radiation. Free-living freshwater copepods can be distinguished from other small aquatic invertebrates by a variety of morphological characteristics. They have a somewhat cylindrical, segmented body with numerous segmented appendages on the head and thorax, and two setose caudal rami on the posterior end of the abdomen. They possess an exoskeleton, conspicuous first antennae, and a single, simple, anterior eye. The defining apomorphy of the Copepoda is the structure of their swimming legs, each pair of which is connected at the base by a “coupler” or “intercoxal sclerite.” This chapter focuses on the four orders that contain the widespread, abundant, and primarily free-living copepods (Calanoida, Cyclopoida, Harpacticoida, and Gelyelloida), of which there are over 5500 described species. The calanoids include about 2300 species, of which about 550 are freshwater; there are over 800 species of freshwater cyclopoids; and the harpacticoids include approximately 3600 species, of which some 1120 can be found inland, mostly in freshwaters. The remaining order, Gelyelloida, includes two free-living species in the genus Gelyella from subterranean habitats in Europe, and another from a single location in North America.