Ontogenetic development and redescription of the whale louse Cyamus boopis Lütken, 1870 (Crustacea: Amphipoda: Cyamidae), ectoparasite of humpback whale Megaptera novaeangliae (Mammalia: Cetacea: Balaenopteridae)

Crustacean amphipods known as whale louse parasitise cetaceans across the world. Taxonomy of the group is generally based on individuals collected during whaling periods and from fresh stranded cetaceans. The only known parasite of Megaptera novaeangliae is Cyamus boopis. Specimens collected from humpback whales stranded on the coast of Brazil were counted, measured, and identified by sex. As C. boopis show a high variation on some diagnostic characters, a redescription of C. boopis is given based on the type series material obtained from the Zoologisk Museum, Copenhagen. A lectotype and four paralectotypes are designated herein. Variation of characters as presence of maxilliped palps and number of acute ventral processes within the Brazilian material is discussed, and should be used with caution for species identification. The proportion of the lateral and accessory gills proved to be good characters to separate C. boopis from the similar species C. catodontis and C. erraticus. Ontogenetic variation of C. boopis is described and remarks on population ecology are provided for the Brazilian material.