Previous studies suggest that cestodes (i.e., tapeworms) of the sister genera Symcallio and Calliobothrium attach in different specific regions of the spiral intestine of their triakid shark hosts, with species of Symcallio attaching in the anterior region of the spiral intestine and species of Calliobothrium attaching with a broader distribution centered around the middle of the spiral intestine. In the present study, we tested the generality of this pattern of site specificity in two additional species pairs: Symcallio peteri and Calliobothrium euzeti in Mustelus palumbes and S. leuckarti and C. wightmanorum in M. asterias. Finding that these cestodes also exhibit the aforementioned pattern, we investigated a series of functional explanations that might account for this phylogenetically conserved pattern of site specificity. The mucosal surface of the spiral intestine of both shark species was characterized, as were the attachment mechanisms of all four cestode species. Although anatomical differences in mucosal surface were seen along the length of the spiral intestine in both shark species, these differences do not appear to correspond to the attachment mode of these cestodes. We find that while species of Symcallio, like most cestodes, attach using their scolex, species of Calliobothrium attach with their scolex and, to a much greater extent, also with their strobila. Furthermore, attachment of Calliobothrium species appears to be enhanced by laciniations (flap-like extensions on the posterior margins of the proglottids) that interdigitate with elements of the mucosal surface of the spiral intestine. The role of proglottid laciniations in attachment in species of Calliobothrium helps reconcile a number of morphological features that differ between these two closely related cestode genera.
