Morphological comparison of the buccal apparatus in two bivalve commensal Teleostei : Encheliophis dubius and Onuxodon fowleri (Carapidae, Ophidiiformes)

Onuxodon fowleri and Encheliophis dubius are two Carapidae species that live in bivalve hosts and, their diet is made of the same type of prey. The aim of this study is to compare their cephalic morphology to see whether 1) the head anatomy of both species is related to the constraints of their way of life, and 2) there are difference between these species and commensal carapids that shelter in other invertebrates. The components of their skeletons and muscles are similar, but differ in size and are arranged differently. In O. fowleri, the buccal cavity is smaller than in E. dubius, the jaws (bearing very large anterior teeth) are larger, the quadrato-mandibular joint lies further to the rear, the fibres of muscle bundles A3a, A2a and A2b are more vertical and insert higher on the neurocranium. The buccal system of O. fowleri appears better suited for ingesting food by biting and grasping. That of E. dubius seems better adapted to a feeding mechanism where sucking would have a more important role. The E. dubius head morphology is more similar to the cephalic anatomy of non-bivalve commensal species than to O. fowleri features. Diet constraints may have greater influence than the different host constraints on the head construction. A simulated backward rotation of the posterior part of the E. dubius suspensorium around the posterior joint between the hyomandibular and the neurocranium brings the jaws and the cheeks to coincide with those of O. fowleri. This model could be indicative of how structure modifications and their influences on annex pieces could in part have a role in the biodiversity.