Cephalic morphology of Pythonichthys macrurus (Heterenchelyidae: Anguilliformes): specializations for head-first burrowing.

The Heterenchelyidae, a family of Anguilliformes, are highly specialized fossorial eels. This study was conducted to evaluate the cranial specialization in relation to head-first burrowing behavior in the heterenchelyid, Pythonichthys macrurus. Thereby, detailed descriptions are provided of the cranial myology and osteology of P. macrurus and its differences with that of representatives of three families: the Moringuidae (Moringua edwardsi), a head-first burrower; the Anguillidae (Anguilla anguilla), a nonburrowing representative and the Ophichthidae (Pisodonophis boro), a head and tail-first burrower. This comparison may help to get a better understanding of the cranial specialization of head-first burrowers in heterenchelyids and moringuids. We recognize as morphological adaptations to burrowing: reduced eye size, a caudoventral orientation of the anteromedial section of the adductor mandibulae muscle complex, the posterior position of the quadrate-mandibular joint, a solid conical skull, large insertion sites of epaxial and hypaxial muscle on the neurocranium, a widened cephalic lateral line canals extending into the dermal cavities, and a ventral position of the gill opening.