image: This image shows a cleared and double stained barbeled dragonfish (Grammatostomias dentatus), in which bone is stained red, cartilage blue, and all musculature is mascerated. Barbeled dragonfishes (family Stomiidae) represent one of the dominant fish families in the deep-sea, exhibiting an array of specializations to a predatory existence in this environment, e.g. huge mouth gapes with prominent teeth, distensible stomachs, elongated dark bodies with photophores, and chin barbels with bioluminescent tissue. Some barbeled dragonfishes have from one to ten anterior vertebrae reduced or entirely absent and five genera exhibit a true functional head joint. Both characters allow a considerable degree of cranial elevation that is notably enhanced in the genera that share the functional head joint. view more
Credit: Nalani Schnell, MNHN
Some deep-sea fishes have a head joint that may enable them to open their mouths wide for consuming large prey, according to a study published February 1, 2017 in the open-access journal PLOS ONE by Nalani K. Schnell and G. David Johnson from the Muséum National d'Histoire Naturelle, Paris, France and the National Museum of Natural History, Smithsonian Institution.
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."
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In your coverage please use this URL to provide access to the freely available paper: http://dx.plos.org/10.1371/journal.pone.0170224
Citation: Schnell NK, Johnson GD (2017) Evolution of a Functional Head Joint in Deep-Sea Fishes (Stomiidae). PLoS ONE 12(2): e0170224. doi:10.1371/journal.pone.0170224
Funding: This study was initiated during a Smithsonian Predoctoral Fellowship of NKS in the Division of Fishes at the National Museum of Natural History and further supported by the Herbert R. and Evelyn Axelrod Endowment Fund for systematic ichthyology in the Division of Fishes (2008/2009).
Competing Interests: The authors have declared that no competing interests exist.
Journal
PLoS ONE