Osteology provides insight into the biology of the enigmatic Antarctic notothenioid fish Gvozdarus svetovidovi

Gvozdarus svetovidovi (Nototheniidae) is the rarest notothenioid fish, represented by only two specimens collected in 1970 and 1988. As it is little known, we present aspects of the cranial osteology of this species and use this, and other features of its morphology, to infer fundamental aspects of its biology. As in other pelagic nototheniids, G. svetovidovi has an elongated neurocranium and jaws but it does not have a paedomorphic skeleton. Although not unique to G. svetovidovi, the bones of the skull have some well-developed ridges for support and possibly for containment of subcutaneous fat. The skeleton is most similar to that of Dissostichus. G. svetovidovi has a high fat content, primarily within the trunk musculature and in a layer over the skull. It is probably neutrally buoyant in mesopelagic waters, where it has been captured at 295–550 m depth north of the Antarctic Slope Front (approximately the shelf break) and south of the southern boundary of the Antarctic Circumpolar Current. It likely relies on vision and mechanosensation to detect its prey, including bioluminescent, energy-dense myctophids and krill in this resource-rich area of the Southern Ocean. Strays near shelf waters feed on Pleuragramma antarcticum. G. svetovidovi is unlike all other notothenioids in fundamental aspects of its biology including its habitat, diet and distribution. Although it may be on the path to extinction, if our postulated life history of G. svetovidovi is accurate, it expands the current perception of the bounds of the notothenioid radiation into a locality in the Southern Ocean not previously known to be occupied by this group.