Cranial morphology of Ateleopus japonicus (Ateleopodidae: Ateleopodiformes), with a discussion on metamorphic mouth migration and lampridiform affinities

The cranial osteology and myology in the ateleopodiform Ateleopus japonicus were studied. Many free bony ossicles constitute the cephalic lateral line canals and are separated from the neurocranial roof by thick gelatinous tissue. The preoperculomandibular canal is unique in having a direct connection with the infraorbital canal owing to strong reduction in the size of the preoperculum. The neurocranium is largely cartilaginous, with 6 chondrocranial and 1 dermal element being absent (or not undergoing ossification). The left and right frontals are separated by a deep groove into which a long, mobile rostral cartilage is deeply inserted. Five pairs of cartilages, including 2 pairs of menisci, are associated with the ethmoid region, allowing premaxillary protrusion without involving maxillary rotation. The levator operculi is well developed and likely generates the primary force for depressing the lower jaw. The large interhyal is tightly attached to the entire ventral margin of the operculum, and the two elements appear to function as a single unit in mouth opening. The oral cavity is large because of the posterior position of the branchial arches the last (5th) arch is situated below the 3rd vertebra]. In pelagic individuals the head is flat with a terminal mouth and straight parasphenoid shaft, whereas in small, benthopelagic individuals the head is rounded with an inferior mouth and bent parasphenoid shaft. “Bending” of the parasphenoid with a dorsally elevated apex is considered the result of the posterior migration of the mouth during the habitat shift. Ateleopodiform characters are discussed phylogenetically and the deep insertion of the rostral cartilage into an open space in the ethmoid region is suggested as a synapomorphy of the order and Lampridiformes.