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

Onuxodon fowleri and Encheliophis dubius are two Carapidae species that liv e 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 differences between these species and commensal carapids that shelter in other invertebrates. The components of their skele tons and muscles are similar, but differ in size and are arranged different ly. In O. fowleri, the buccal cavity is smaller than in E. dubius, the jaws (bearing very large anterior teeth) are larger, the quadratomandibular joi nt lies further to the rear and the fibres of muscle bundles A(3)alpha, A(2 )alpha and A(2)beta 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 he ad morphology is more similar to the cephalic anatomy of non-bivalve commen sal species than to O. fowlevi features. Diet constraints may have greater influence than the different host constraints on the head construction. A s imulated backwards rotation of the posterior part of the E. dubius suspenso rium around the posterior joint between the hyomandibular and the neurocran ium brings the jaws and the cheeks to coincide with those of O. fowleri. Th is model could be indicative of how structure modifications and their influ ences on annex pieces could in part have a role in the biodiversity.