A head joint found in deep-sea fishes may enable them to swallow large prey

Barbled dragonfishes, deep-sea fishes in the Stomiidae family, have a flexible connection between their first vertebra and the back of their head, a characteristic that is unique among deep-sea fish. Previous studies have suggested that stomiids consume significantly large prey, and hypothesized about the function of the top part of the vertebral column, but the precise anatomy of this structure was not known.

The authors of the present study describe a unique, functional head joint in multiple stomiid genera based on analysis of museum specimens. The joint comprises the back of the head and a flexible, rod running to the top of the vertebral column. The researchers found that, when the fish is in resting position and not elevating its head, this flexible rod has an additional ventral sheath that embraces the back of the head like a socket, but that when these fish open their mouths, the extra sheath gets stretched out and the bottom part of this rod is exposed, potentially allowing the fish to open its mouth up to 120 degrees. The researchers suggest that the additional flexibility provided by this functional head joint may have enabled the fish to consume larger prey, but recommend live observations of fish feeding to provide further support to this hypothesis. They also note that the distribution of this joint among taxa in the Stomiidae family may help to redefine their evolutionary relationships.

"In most other fishes, the head articulates directly with the first vertebra and thus does not offer a flexible joint between the two," says Nalani Schnell. "The functional head joint in those deep-sea fishes greatly reinforces the maneuverability of the head in order to engulf significantly large prey items."