AbstractOf all the metazoan groups discussed in this book, the crustaceans are the most diverse and ubiquitous. Among them, the copepods are dominant. They, jointly with the monogeneans, are the most diverse group of metazoan ectoparasites of marine fishes; in addition, they infect a wide range of marine invertebrates. Thousands of species are already known, but many potential host groups have not been examined, and for this reason even approximate estimates of species numbers are impossible. Reflecting the diversity of hosts, copepods show an amazing variety of adaptations which secure infection of and survival on the hosts. Many copepods have great economic importance as agents of disease in wild and aquacultured fish populations. Isopods are primarily found in warm waters, they infect fish but also other crustaceans. Larval isopod parasites of the family Gnathiidae are abundant on the gills of tropical marine fish and represent a primary source of food for cleaner fish. Most branchiurans occur in fresh water, but a few species of the genus Argulus are ectoparasites on the skin of marine fish. The tiny tantulocarids are ectoparasites of other crustaceans. To date only 28 species have been described, and little is known about their biology. Thoracica and Rhizocephala are included in the Cirripedia. Few species of the Thoracica are parasitic (on dogfish and polychaetes), whereas the rhizocephalans parasitize other crustaceans. The latter are particularly fascinating because of their extreme sexual dimorphism, the extreme reduction of morphological complexity in the parasitic female, and their ability to change the behaviour of host crabs which benefits the parasite. The Ascothoracida infect various echinoderms and cnidarians. Amphipoda use many groups of marine animals as hosts, including medusae, siphonophores, ctenophores, and thaliaceans. Others (including the whale-lice) infect various marine mammals. When occurring in large numbers, whale-lice may even damage very large humpback whales.