Sea snakes are renowned predators that display a suite of adaptations, including paddle-like tails for swimming and glands to expel excess salt, which equip them for the marine realm. Yet, as the snakes still require air to breathe, they remain reliant on excursions to the surface, which expose them to predators and limit their ability to hunt prey underwater. Aquatic snakes somewhat alleviate this need by having thin skin, which permits some degree of oxygen uptake across the body wall into their blood. But gas exchange via this route is restricted, as skin on the whole must retain its integrity to protect the body, so it remains a barrier to the diffusion of gases. However, a new discovery by Alessandro Palci from Flinders University, Australia, and colleagues sheds light on how some species have developed an additional specialised respiratory organ fit for the purpose.
It has long been known that some species of sea snake have a peculiar foramen, or hole, in the top of their skull. This attracted the attention of Palci's team, who pondered why the snakes need this hole in their head. To resolve its function, they began studying the anatomy of blood vessels in the head of the annulated sea snake (Hydrophis cyanocinctus) using CT scans of preserved specimens, as well as with microscopic analysis of dissected tissues. To provide a comparison, they also investigated the same anatomy in a closely related land snake, the taipan Oxyuranus scutellatus, as well as two other sea snakes, the olive sea snake (Aipysurus laevis) and Stoke's sea snake (Hydrophis stokesii), which do not have the unusual enlarged openings in their skulls.
In the annulated sea snake, the team saw a dense bed of blood vessels, termed a plexus, on the top of the skull that was absent in their terrestrial cousin, the taipan, and the other sea snakes. This plexus primarily consists of thin-walled veins, which eventually converge in a single vessel that penetrates the skull, via the conspicuous foramen, and passes into the brain. To ascribe a role to the vessels, the authors considered a number of hypotheses – including the possibility that the bed of blood vessels supplies the salt secretion glands – but none except for a role in respiration made anatomical sense. The team believe that the dense meshwork of blood vessels, lying just beneath a thin layer of skin, provides a route to boost oxygen in the blood in order to keep the brain alive. To strengthen their theory, the team made calculations, which suggest the additional oxygen that diffuses into the blood via the plexus could satiate the brain three times over.
It remains to be established whether this unusual labyrinth of blood vessels successfully prolongs dive duration and why only some species of sea snakes require it, but for now this study describes a curious, novel adaptation for cerebral oxygenation that may assist these diving specialists to prosper underwater.
