A Desert in the Sea: Water Relations of Marine Snakes (pp. 485-500)
Authors: (Harvey B. Lillywhite, University of Florida, FL, USA)
Abstract: One of the fascinating challenges confronting the evolution of living forms on Earth is the transition of secondarily-marine vertebrates from living on land to life in the ocean. Among the vertebrates that have undergone this transition, sea snakes comprise a successful group with diverse and numerous species that have evolved marine habits in the relatively recent past. These snakes have overcome numerous challenges including the foremost problem of acquiring fresh water and avoiding the intake of excess salt. Contrary to previous assumptions, current research demonstrates that various species of marine snakes require fresh water to remain in water balance and dehydrate in seawater if they are without a freshwater source. Fresh water is acquired sometimes from coastal springs or estuaries, or from rainwater that pools or drips from island vegetation. The latter is available to a few oviparous species that are partly terrestrial and remain tied to land for purposes of laying eggs. Most species of sea snakes, however, are viviparous and spend their entire lives at sea. For many of these species, the only source of fresh water is rainfall that accumulates as a freshwater lens on the surface of the ocean during intense storms. For species living in regions having seasonal drought, adaptations to resist as well as tolerate periodic dehydration are also important. Previously, it was thought that sea snakes drank seawater and remained in water balance by excreting the excess ingested salt via extrarenal salt glands. However, this seems not to be the case in the various species of sea snakes that my research group has studied, and not a single individual out of hundreds we have tested has ever drunk 100% seawater. Salt glands do play an important role in the osmoregulation of sea snakes, and their maximum rates of secretion correlate positively with the extension of species ranges into more highly saline oceanic waters. Nonetheless, the abundance and diversity of sea snakes correlate with sources of fresh water, both in space and time.