Shape of attachment structures in parasitic isopodan crustaceans: the influence of attachment site and ontogeny

Many fields of modern systematic biology are adult-centred. This is unfortunately also the case for Cymothoidae, an ingroup of parasitic forms of Isopoda, with fishes as hosts. Different ingroups of Cymothoidae have specialised appendages that enable their fish associated lifestyles, attaching to different sites on the body of the host. The extent to which these structures vary among species and in relation different sites of attachment, and between different ontogenetic stages, is explored here. This study presents the detailed descriptions, illustrations, comparisons, and analysis of a variety of attachment structures of 13 adult and immature specimens representing three major groups Ceratothoa, Elthusa and Anilocra, along with full focus, detailed photographs of all the examined life stages. The three groups exhibit different strategies attaching to mouth, gill and externally, respectively. A statistical representation of the morphology of the dactyli, used for attaching to the host, was performed. This included a critical comparison of 10 additional species documented in literature. This is the first comprehensive description and photographs of specialised appendage morphology of immatures of Ceratothoa, as well as the first detailed micrographs of embryonic stages of Cymothoidae, and the first lateral and ventral views of immature stages of the examined species. Immature specimens possess morphological characters that can be used to distinguish between different species, but cannot be accurately identified based on diagnostic characters of adults. Quantitative analysis indicates that ontogeny plays a major role in the shape of the attachment structures (e.g. dactyli).