SISTERS PARASITIZING SISTERS: ON THE ONCHOBOTHRIIDEAN TAPEWORMS OF HAMMERHEAD AND REQUIEM SHARKS

The onchoproteocephalidean cestodes reported on herein were obtained from requiem (Carcharhinidae) and hammerhead (Sphyrnidae) sharks during fieldwork conducted over the past several decades off Australia, Japan, Malaysian and Indonesian Borneo, Mexico, Mozambique, Senegal, Taiwan, the United States, and Vietnam. Sequence data were generated for the D1-D3 region of the 28S rDNA gene for 36 specimens of 23 species of onchoproteocephalideans. The phylogenetic tree resulting from a Maximum Likelihood analysis revealed 3 subclades consistent with major morphological and host differences among included taxa. One subclade consisted of species of Triloculatum, which have bothridia with 3 subloculi, basal prongs on their hooks that are at least half as long as the axial and abaxial prongs, and a uterus that extends essentially the length of the proglottid. These species parasitize requiem sharks. Another subclade consisted of a subset of species currently assigned to Phoreiobothrium, all of which came from hammerhead sharks. These species have bothridia with 5-11 subloculi, basal prongs on their hooks that are less than half as long as the axial and abaxial prongs or are entirely absent, and a uterus that extends only to the level of the terminal genitalia. The final subclade consisted of species of Phoreiobothrium that have bothridia with 8 or more subloculi, basal prongs on their hooks that are more than half as long as the axial and abaxial prongs, and a uterus that extends much of the length of the proglottid, all of which parasitize requiem sharks. Sphyrnacestus n. gen. is erected for species in the second subclade, and the diagnoses of Phoreiobothrium and Triloculatum are emended. Four new species of Sphyrnacestus n. gen. are described, and the 5 species from hammerhead sharks currently assigned to Phoreiobothrium that are consistent with the morphology of members of the second subclade are transferred to the new genus. Six new species of Phoreiobothrium are also described. Evidence of 16 undescribed species of Phoreiobothrium and 1 undescribed species of Triloculatum is presented. The family Phoreiobothriidae is resurrected for the 3 genera, which all possess bothridia with tri- or bi-pronged hooks and subloculi. Members of this family are predicted to be found in other species of Carcharhinidae and Sphyrnidae. Furthermore, we predict they will not be found to parasitize members of other families of carcharhiniform sharks because phoreiobothriids appear to be restricted to sharks that possess the scroll type rather than the conicospiral type of spiral intestine.