A new species of Phoreiobothrium (Cestoidea:Tetraphyllidea) from the great hammerhead shark Sphyrna mokarran and its implications for the evolution of the onchobothriid scolex.

Phoreiobothrium manirei n. sp. is described from the spiral intestines of 3 immature individuals of the great hammerhead shark Sphyrna mokarran collected off of the west coast of Florida. This cestode is unusual in that it possesses 4 muscular papillae on the anterior margin of the accessory sucker. Numerous small protrusions with central cilium-like projections were conspicuous throughout the posterior margin of each bothridium. This species possesses the unique combination of bothridia posteriorly subdivided into subloculi and hooks with extended bases and 2 rather than 3 prongs. In fact, the hooks are strikingly like those of species in the genera Dicranobothrium and Platybothrium. This combination of characters suggests that the bifid hook condition is plesiomorphic with respect to the trifid hook condition within the Onchobothriidae. These data further suggest that the triloculated bothridial condition is plesiomorphic relative to the biloculated condition.