“Look at this! How really weird,” I exclaimed to a colleague. We were standing on the tilted deck of an old, wrecked merchant ship partly submerged off the coast of Papua New Guinea, near the capital city of Port Moresby. At my feet, a hundred or more sea snakes lay on the rusty deck, some stretched out side by side, others in tangled clusters. All were more or less still. Yellow-lipped sea kraits they were (Laticauda colubrina), their two- or three-foot-long bodies dressed in alternating bands of gray-blue and black. It wasn’t their aggregation on dry land, as it were, that surprised me—sea kraits are amphibious and known to gather in large groups occasionally. But, like shipwrecked sailors, nearly all of them appeared emaciated, and I could not imagine the reason. It would be more than passing strange for so many in a single population to be unable to find enough fish to eat.
[ad:51 1094]That was in 1975. In the intervening decades I’ve replayed the scene in my mind now and then, each time returning to the question: what was wrong with those sea snakes? In hindsight, and with the benefit of additional research into sea snake physiology, I’m almost certain it wasn’t hunger plaguing them. Although surrounded by the vast waters of the Pacific Ocean, they were most likely severely dehydrated. They might even have been early harbingers of climate change.
Of the myriad and diverse creatures of the sea, most evolved right there in the saltwater. But a handful of them, including sea snakes, are secondarily marine, having evolved from terrestrial ancestors. The evolutionary transition from one medium to another is difficult, and the switch from air on land to seawater presents special problems.Chief among them is obtaining enough water to stay properly hydrated: the high concentration of salts in seawater poses a challenge to maintaining the less-salty body fluid that most terrestrial organisms and their marine descendants possess.
Ancient mariners learned that we humans become seriously dehydrated if we drink seawater (an act called “mariposa”). Our kidneys cannot concentrate urine sufficiently to conserve enough water while eliminating the excess ingested salt. Marine mammals, by contrast, can excrete more concentrated urine than ours, and they have digestive-system adaptations that enable them to extract the maximum liquid from their food. As a result, marine mammals have no need for freshwater. It remains unknown whether they, and most other marine vertebrates, drink seawater directly.
Marine birds and reptiles have come up with a different solution for eliminating excess salt: specialized glands that secrete concentrated fluids of sodium chloride, the principal salt constituent of seawater. Desert animals, too, are subjected to osmotic stresses, and some excrete potassium salts as well. The list of species known to possess salt glands includes desert birds and reptiles, along with seabirds, marine turtles, the marine iguana, some crocodilians, sea snakes, and terrestrial reptiles living in coastal zones. In marine birds and iguanas a salt gland near each eye excretes through the nostrils; in marine turtles the gland is in the eye socket and excretes salty tears; crocodilians have salt glands in the tongue; and sea snakes have them beneath the tongue.
Salt glands have been studied almost exclusively in the laboratory, largely by infusing excess salt into an animal—either intravenously, or by pumping saltwater into the stomach—and demonstrating that salt glands secrete highly concentrated salt solutions in response. But there is little information concerning when and how effectively salt glands work in free-ranging animals. Physiologists have assumed that animals possessing salt glands are able to maintain water balance by excreting excess salts ingested in salty substances, such as marine prey or seawater—no freshwater required. It has been standard textbook dogma, for example, that sea snakes drink seawater and, in essence, distill it with their salt glands. But there is always drama in science, and my recent work shows that at least some sea snakes’ salt glands are insufficient to that task, and their water balancing act more complicated than expected.
In the 1990s, my colleagues and I studied an unusual marine reptile called the little file snake (Acrochordus granulatus). It is the sole marine species in the file snake family, the Acrochordidae, which also contains two freshwater species. The file snakes are only distantly related to the group herpetologists call sea snakes, which includes the sea kraits. Through a series of observations and experiments, we discovered that the little file snake not only drinks freshwater but requires it to maintain water balance. Most populations of little file snakes live in tropical southern Asia among mangroves or in other nearshore marine habitats. They spend their entire lives in seawater, where they can potentially dehydrate despite possessing a functional salt gland.
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We further demonstrated that little file snakes eliminate much of their nitrogenous waste in the form of ammonia or ammonium rather than uric acid, as terrestrial reptiles typically do. That is important because the ammonia—a product of protein metabolism—is highly toxic and cannot be allowed to accumulate in body fluids. Nor can it be concentrated or precipitated, as uric acid can be, and it requires comparatively more water to eliminate via the kidneys. Thus the high protein load of the little file snakes’ diet of fish exacerbates their need for freshwater. Indeed, little file snakes that are partly dehydrated cease to eat, presumably to conserve water that they would otherwise expend ridding the body of ammonia.
The unexpected freshwater requirement of marine file snakes piqued my curiosity about how sea snakes manage to stay hydrated—particularly in light of that oddly emaciated group on the Port Moresby wreck. Whereas the little file snake is the only marine species in its family, sea snakes have diversified considerably in the sea. Scientists recognize about sixty species in two distinct lineages. The taxonomy is somewhat in dispute, but here, for simplicity, I’ll follow a popular classification that regards those two lineages as subfamilies within the family Elapidae. Sea snakes are thought to have evolved from terrestrial elapids, which today include cobras, land kraits, coral snakes, and numerous other venomous species in Australia. The subfamily Hydrophiinae contains more than fifty sea snake species that are entirely marine. A few are pelagic, but most live near shore, and they all give birth to live young in the water. The subfamily Laticaudinae contains another seven species, all in the genus Laticauda and called sea kraits. The group is amphibious: sea kraits come ashore to rest and to lay eggs in moist, rocky places along the seashore.
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[ad:51 1121]Sea snakes are widely distributed throughout much of the world’s marine tropics, primarily along coastlines and islands of the Indian and western Pacific oceans. A single species also occurs along the Pacific coast of the Americas between Baja California and Ecuador. In addition to their salt glands, sea snakes have other morphological adaptations to life in the sea. Valvular nostrils prevent the entry of water, and reduced ventral scales let the body compress laterally—which, in conjunction with a paddle-shaped tail, aids swimming. All sea snakes possess a single functional lung. They surface to breathe air, though certain species can also exchange a moderate amount of oxygen and carbon dioxide through the skin. Some species that feed or rest on the seafloor can dive as deep as 350 feet and can remain submerged for more than two hours. Sea snakes have highly toxic venom that most species use to immobilize their prey of fish or eels, and many are important top predators on coral reefs. A few species specialize on fish eggs.
Some intriguing clues hinted that sea snakes might need freshwater in their diet. In the 1970s, William A. Dunson, a biologist (now emeritus) at Pennsylvania State University in University Park, had observed the yellow-bellied sea snake, Pelamis platurus, a fully marine species in the subfamily Hydrophiinae, drinking freshwater in the laboratory. And in 1991, Michael L. Guinea, a biologist at Northern Territory University (now part of Charles Darwin University) in Darwin, Australia, reported observing yellow-lipped sea kraits drinking rainwater in Fiji; they would lick the water from vegetation or gulp it from depressions in coconut leaves. Snakes drinking freshwater in the wild are obviously thirsty, and therefore in negative water balance. Presumably that would not occur if the snakes could indeed get sufficient water from saltwater or from prey, using their salt glands.
To determine definitively whether sea snakes require freshwater to remain in water balance, I spent three field seasons, in 2005 through 2007, studying the question in three species of sea kraits that are common at Lanyu, a.k.a. Orchid Island, Taiwan. The three sea krait species represent a spectrum of habits: the yellow-lipped sea krait is semiterrestrial, the banded sea krait (L. semifasciata) is almost fully marine, andthe blue-banded sea krait is intermediate to the other two in its allocation of time between land and sea.
I collaborated with Ming-Chung Tu, a biologist at the National Taiwan Normal University in Taipei, and several students assisted us. To begin, we dehydrated sea snakes by keeping them in mesh bags exposed to laboratory air. After two weeks, they were moderately dehydrated—having lost between 10 and 16 percent of their body mass—and therefore thirsty. We then placed each snake individually into an aquarium with seawater and watched to see whether it would drink. We weighed each animal before and after to measure any water it might have ingested. After one hour, none of the snakes had drunk any seawater. Next, we left the snakes in seawater for about twenty hours, overnight. Again we weighed them; none had gained any significant mass. We then placed each snake into a container filled with freshwater and repeated the process. All the snakes drank the freshwater, opening their mouths and sucking it in, and many of them drank copiously within minutes of sensing it.
We also investigated whether the sea snakes would dehydrate when kept in seawater without a source of freshwater for drinking. The snakes lost body mass at a constant rate for more than a month, yet did not drink seawater. Other individuals were offered freshwater every third day; they drank variable amounts to rehydrate throughout their time in the seawater. We didn’t feed the snakes during either of the dehydration periods, which simplified the mass measurements. Snakes are intermittent feeders with relatively low metabolic rates, and they can go for several months and possibly longer in the wild without eating.
At the conclusion of the experiments, we tested the animals that had no access to freshwater to see what level of brackish water they might drink. We discovered that sea kraits will drink freshwater or very dilute seawater, but not brackish water more concentrated than 30 percent seawater. Our experiments showed conclusively that at least three sea snake species dehydrate in air and in seawater. Moreover, they voluntarily correct for body-water deficits by drinking fresh or dilute brackish water, but refuse seawater.
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What was true in the laboratory also seems to be true in the wild. We collected sea kraits at Orchid Island that appeared very thin, and they drank large amounts of freshwater in the laboratory. And in the 1970s and 1980s, in Papua New Guinea and in Fiji, I encountered numerous sea kraits with peculiarly dimpled scales. My team and I recently observed that the dimpling is a symptom of dehydration—though I didn’t realize it back then. A quick check of historical weather records for Papua New Guinea and Fiji confirmed my suspicion: I had observed the dimpled sea snakes during seasonal droughts. French scientists working in New Caledonia have witnessed numerous yellow-lipped sea kraits emerging from seclusion beneath rocks or vegetation in dramatic synchrony when rain fell after a period of drought. The snakes drank rainwater that dripped or pooled onto rocks. Thus, it seems certain that sea snakes can become severely dehydrated in the wild.
What about exclusively marine species? Snakes living in coastal waters might have access to an underwater freshwater spring or to brackish or fresh water in estuaries. If not, their only source of drinking water appears to be rainfall. Because freshwater is less dense than seawater, it tends to remain on the ocean surface until currents and waves mix it in. In most cases, such freshwater “lenses” are thin and short-lived, but they can occasionally extend to depths of sixty feet and persist several days. Behavior I observed in the laboratory hints that saltwater-dwelling snakes are familiar with the phenomenon. Sprinkling freshwater over the surface of their tanks brought secretive little file snakes out of their PVC-pipe burrows. They rose quickly to the surface to drink the “rain.”
One might expect that a freshwater lens would persist longer after a rainfall in a sheltered location, such as a bay or a lagoon, than on the open ocean. Interestingly, that’s exactly where some notably large sea snake populations have been found. The distribution of sea snakes is characteristically patchy, and we are accumulating evidence that the patchiness might be explained, in part, by the distribution of rainfall both in time and in space.
During our field investigations at Orchid Island, we noticed that sea kraits were particularly abundant near a freshwater spring we had discovered while snorkeling near the coastline. Subsequently, we selected eight different coastal sites around the perimeter of the island, and sampled the abundance of the three Laticauda species there, searching at night, when the snakes are most active. Four of the sites have a known source of freshwater nearby. The other four have no apparent source other than rainfall. We confirmed the distinction by testing the salinity of water samples taken at each site. Our sampling showed that the freshwater sites sheltered several to seventy times more sea snakes than did the strictly marine sites, where we often encountered no snakes whatsoever.
In 2007, we sampled the two freshwater sites where the snakes had been most plentiful during the past two years. Our visit coincided with a period of local drought, and we noted that they were less abundant than in the previous, wetter years. The drought was so bad that some streambeds were totally dry, including the source of the underwater spring we’d discovered while snorkeling. Some of the villages on the island even ran out of drinking water. When I totaled the local numbers of snakes that we’d counted each visit during three different years, and plotted them against the total precipitation that fell during the six months preceding each visit, there was a positive correlation: more rain meant more sea snakes. On a local geographic scale at Orchid Island, then, the abundance of sea snakes related positively to the availability of freshwater, both spatially and temporally.
I set out to investigate whether the correlation would hold up at a much broader geographic scale. Sure enough, data in the scientific literature indicate that the distribution of sea kraits generally coincides with areas having low-salinity surface waters in the tropical Indian and western Pacific oceans. Indeed, using data from South Asia for the known distributions of all sea snake species combined, I discovered that in general, more sea snake species live in areas of greater precipitation.
The distribution of Laticauda species among small islands is also quite patchy, so one might hypothesize a dynamic model in which populations persist in certain areas that receive adequate precipitation and either die out or emigrate from others during droughts. Dispersing individuals might later repopulate the abandoned sites when the climate turned favorable once more. There are, of course, other factors that determine sea snake distributions—temperature and prey abundance, for example. But to the extent that evolving populations have a physiological requirement for freshwater, they are more likely to survive in regions of high compared with low precipitation. Moreover, the changing availability of freshwater potentially influences the dynamics of coral reef communities, because sea snakes can be important top predators there.
The irregularity and unpredictability of rainfall patterns is likely to limit the distribution of at least some marine snake populations. It might even explain, in whole or in part, certain declines and local extinctions that have recently been documented. Precipitation has generally decreased over the tropics since the 1970s, and climate models predict it will decrease further in tropical regions with seasonal drought. Because at least some sea snakes are dependent on freshwater, we may expect to see corresponding changes in their populations.
But little file snakes and the two sea snake subfamilies of the Elapidae (the Hydrophiinae and the Laticaudinae) are not the only snakes that live in saltwater. A few members of two other snake lineages, both in the family Colubridae, inhabit the brackish waters of salt marshes and estuaries. William Dunson has shown that those species probably also depend on freshwater. Thus, species representing four out of five distinct lineages of snakes that inhabit saltwater are now known to require freshwater to maintain normal water balance. The fifth, the Hydrophiinae, has not yet been thoroughly studied in that context. Although hydrophiines are the snakes most completely adapted to life in the sea, it does seem likely that they, too, need the sweet stuff: Dunson has observed that some hydrophiine sea snakes undergo a net loss of body water in seawater, and the pelagic species P. platurus not only will drink freshwater in the lab, but also reportedly dehydrates when fasting in seawater. My team and I are planning studies of representative hydrophiine sea snakes to settle the matter.
Understanding the water requirements of all sea snakes could prove to be crucial to their conservation. Some researchers have suggested that they may turn out to be indicator species for the health of coral reefs, which are in deep ecological crisis. Sea snakes’ thirst also raises the question of whether other marine reptiles, such as sea turtles, might turn out to be more dependent on freshwater than we’ve presumed.
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