A revision of Platybothrium Linton, 1890 (Tetraphyllidea: Onchobothriidae), with a phylogenetic analysis and comments on host-parasite associations.

A revision of Platybothrium Linton, 1890 is presented, based on available type and voucher material, as well as extensive new collections from elasmobranchs belonging to the Carcharhinidae (requiem sharks) and Sphyrnidae (hammerhead sharks). All 10 nominal Platybothrium species are treated or redescribed herein. Four of these 10 nominal species, in addition to one former member of Dicranobothrium Euzet, 1953, are considered valid members of Platybothrium. Five new Platybothrium species are described: P. angelbahiense n. sp. ex Carcharhinus leucas, P. coshtaprum n. sp. ex C. plumbeus, P. jondoeorum n. sp. ex Negaprion acutidens and C. melanopterus, P. kirstenae n. sp. ex C. obscurus, and P. tantulum n. sp. ex Sphyrna lewini and S. zygaena. Sixty-three morphological characters were employed in cladistic analyses of nine Platybothrium species and five outgroup species. Coding and analysis strategies were varied to assess the effects of coding inapplicable characters as missing or as a separate character state, and of excluding characters for which data are missing in more than 10% of the taxa. The analysis in which inapplicable characters were coded as a separate character state and no characters were excluded produced the best-supported and most conservative estimate of the interrelationships of Platybothrium spp. Platybothrium appears to be a monophyletic assemblage, with the most basal species being P. spinulifera Southwell, 1912. The group of species possessing an accessory piece between the hooks forms a clade within the genus. Species lacking an accessory piece, which had previously been placed in Dicranobothrium Euzet, 1953, do not appear to be each other's closest relatives; thus, Dicranobothrium is considered a synonym of Platybothrium. An examination of host associations indicated that Platybothrium species are broadly distributed among, and entirely restricted to, carcharhinid and sphyrnid shark species. Most Platybothrium species exhibit oioxenous host-specificity, with all but two species each parasitising only a single host species. In several host species, multiple Platybothrium congeners parasitise the same host individual, a phenomenon not previously reported for Platybothrium.