Dehydration and drinking responses in a pelagic sea snake

Recent investigations of water balance in sea snakes demonstrated that amphibious sea kraits (Laticauda spp.) dehydrate in seawater and require fresh water to restore deficits in body water. Here, we report similar findings for Pelamis platurus, a viviparous, pelagic, entirely marine species of hydrophiine ("true") sea snake. We sampled snakes at Golfo de Papagayo, Guanacaste, Costa Rica and demonstrated they do not drink seawater but fresh water at variable deficits of body water incurred by dehydration. The threshold dehydration at which snakes first drink fresh water is -18.3 ± 1.1 % (mean ± SE) loss of body mass, which is roughly twice the magnitude of mass deficit at which sea kraits drink fresh water. Compared to sea kraits, Pelamis drink relatively larger volumes of water and make up a larger percentage of the dehydration deficit. Some dehydrated Pelamis also were shown to drink brackish water up to 50% seawater, but most drank at lower brackish values and 20% of the snakes tested did not drink at all. Like sea kraits, Pelamis dehydrate when kept in seawater in the laboratory. Moreover, some individuals drank fresh water immediately following capture, providing preliminary evidence that Pelamis dehydrate at sea. Thus, this widely distributed pelagic species remains subject to dehydration in marine environments where it retains a capacity to sense and to drink fresh water. In comparison with sea kraits, however, Pelamis represents a more advanced stage in the evolutionary transition to a fully marine life and appears to be less dependent on fresh water.     

Moving in two worlds: aquatic and terrestrial locomotion in sea snakes (Laticauda colubrina,Laticaudidae)

Yellow‐lipped sea kraits (Laticauda colubrina) are amphibious in their habits. We measured their locomotor speeds in water and on land to investigate two topics: (1) to what degree have adaptations to increase swimming speed (paddle‐like tail etc.) reduced terrestrial locomotor ability in sea kraits?; and (2) do a sea krait’s sex and body size influence its locomotor ability in these two habitats, as might be expected from the fact that different age and sex classes of sea kraits use the marine and terrestrial environments in different ways? To estimate ancestral states for locomotor performance, we measured speeds of three species of Australian terrestrial elapids that spend part of their time foraging in water. The evolutionary modifications of Laticauda for marine life have enhanced their swimming speeds by about 60%, but decreased their terrestrial locomotor speed by about 80%. Larger snakes moved faster than smaller individuals in absolute terms but were slower in terms of body lengths travelled per second, especially on land. Male sea kraits were faster than females (independent of the body‐size effect), especially on land. Prey items in the gut reduced locomotor speeds both on land and in water. Proteroglyphous snakes may offer exceptional opportunities to study phylogenetic shifts in locomotor ability, because (1) they display multiple independent evolutionary shifts from terrestrial to aquatic habits, and (2) one proteroglyph lineage (the laticaudids) displays considerable intraspecific and interspecific diversity in terms of the degree to which they use terrestrial vs. aquatic habitats.     

Abundance of sea kraits correlates with precipitation

Recent studies have shown that sea kraits (Laticauda spp.)--amphibious sea snakes--dehydrate without a source of fresh water, drink only fresh water or very dilute brackish water, and have a spatial distribution of abundance that correlates with freshwater sites in Taiwan. The spatial distribution correlates with sites where there is a source of fresh water in addition to local precipitation. Here we report six years of longitudinal data on the abundance of sea kraits related to precipitation at sites where these snakes are normally abundant in the coastal waters of Lanyu (Orchid Island), Taiwan. The number of observed sea kraits varies from year-to-year and correlates positively with previous 6-mo cumulative rainfall, which serves as an inverse index of drought. Grouped data for snake counts indicate that mean abundance in wet years is nearly 3-fold greater than in dry years, and this difference is significant. These data corroborate previous findings and suggest that freshwater dependence influences the abundance or activity of sea kraits on both spatial and temporal scales. The increasing evidence for freshwater dependence in these and other marine species have important implications for the possible impact of climate change on sea snake distributions.     

Thermal biology of sea snakes and sea kraits1

Temperature probably had no direct effect on the evolution of sea kraits within their center of origin, a geologically stable thermal zone straddling the equator, but may have indirectly affected expansions and contractions in distributions beyond that zone through global fluctuations that caused alternation of higher and lower sea levels. The northern limit of the Laticauda colubrina complex seems to be the 20°C isotherm; in the south, the range does not reach that isotherm because there is no land (also a habitat requirement of sea kraits) within the zone of suitable temperature. The relationship of temperature to the pattern of geographic variation in morphology supports either the hypothesis of peripheral convergence or the developmental hypothesis but does not distinguish between them. Quadratic surfaces relating cumulative scores for coloration and morphological characters to global position showed a strong latitudinal component and an even stronger longitudinal one in which the direction of the latitudinal effect was reversed between east and west. A multivariate analysis revealed that while morphological characters vary significantly by location and climate when tested separately, when the influence of location on morphology is taken into account, no residual relationship between climate and morphology remains. Most marine snakes have mean upper temperature tolerances between 39°C and 40°C and operate at temperatures much nearer their upper thermal limits than their lower limits but still avoid deleterious extremes by diving from excessively hot water to deeper, cooler strata, and by surfacing when water is cold. At the surface in still water in sunlight, Pelamis can maintain its body temperature slightly above that of the water, but whether this is significant in nature is questionable. As temperature falls below 18-20°C, survival time is progressively reduced, accompanied by the successive occurrence of cessation of feeding, cessation of swimming, and failure to orient. Acclimation does not seem to be in this species' repertoire. In the water column, marine snakes track water temperature; on land, sea kraits can thermoregulate by basking, selecting favorable locations, and by kleptothermy. Laticauda colubrina adjusts its reproductive cycle geographically in ways that avoid breeding in the coldest months. Mean voluntary diving time is not temperature-dependent within the normal range of temperatures experienced by marine snakes in the field, but is reduced in water colder than 20°C. On land, much as while diving in the sea, sea kraits maintain long periods of apnea; intervals between breaths are inversely related to temperature.     

Sexual divergence in diets and morphology in Fijian sea snakes Laticauda colubrina (Laticaudinae)

In the Fiji Islands, female yellow‐lipped sea kraits (Laticauda colubrina) grow much larger than males, and have longer and wider heads than do conspecific males of the same body length. This morphological divergence is accompanied by (and may be adaptive to) a marked sex divergence in dietary habits. Adult female sea kraits feed primarily on large conger eels, and take only a single prey item per foraging bout. In contrast, adult males feed upon smaller moray eels, and frequently take multiple prey items. Prey size increases with snake body size in both males and females, but the sexes follow different trajectories in this respect. Female sea kraits consume larger eels relative to predator head size and body length than do males. Thus, the larger relative head size of female sea kraits is interpreted as an adaptation to consuming larger prey items. Our results are similar to those of previous studies on American water snakes (natricines) and Australian file snakes (acrochordids), indicating that similar patterns of sex divergence in dietary habits and feeding structures have evolved convergently in at least three separate lineages of aquatic snakes.     

Thirsty sea snakes forsake refuge during rainfall

Abstract Vertebrates living in dry or salty habitats spend significant amounts of time and energy maintaining their hydro‐mineral balance; any opportunity to drink fresh water should reduce such expenses. However, to find fresh water, individuals are generally forced to leave the safe and buffered conditions that prevail in their refuges. Therefore, they must face harsh environmental conditions and increased predation risks. We examined this trade‐off in the field, using a very large dataset gathered on amphibious sea snakes that confront periodic droughts. The study was carried out on different islets in the lagoon of New Caledonia. As expected, sea kraits remain concealed most of the time under rocks or within burrows during dry weather. They select precise time windows to move rapidly between the sea and the land, thereby minimizing the time spent in the open. Rainfall triggered massive disruptions of this rhythm: many snakes quickly forsook their shelter to drink in the open, and remained almost motionless until satiety. Interestingly, they adopted specific and unusual postures to drink. They perched on the top of rocks that were benefiting from running and less salty water during downpours. They also drank the droplets attached to vegetation during drizzle.     

Sea snakes (Laticauda spp.) require fresh drinking water: implication for the distribution and persistence of populations

Dehydration and procurement of water are key problems for vertebrates that have secondarily invaded marine environments. Sea snakes and other marine reptiles are thought to remain in water balance without consuming freshwater, owing to the ability of extrarenal salt glands to excrete excess salts obtained either from prey or from drinking seawater directly. Contrary to this long-standing dogma, we report that three species of sea snake actually dehydrate in marine environments. We investigated dehydration and drinking behaviors in three species of amphibious sea kraits (Laticauda spp.) representing a range of habits from semiterrestrial to very highly marine. Snakes that we dehydrated either in air or in seawater refused to drink seawater but drank freshwater or very dilute brackish water (10%-30% seawater) to remain in water balance. We further show that Laticauda spp. can dehydrate severely in the wild and are far more abundant at sites where there are sources of freshwater. A more global examination of all sea snakes demonstrates that species richness correlates positively with mean annual precipitation within the Indo-West Pacific tropical region. The dependence of Laticauda spp. on freshwater might explain the characteristically patchy distributions of these reptiles and is relevant to understanding patterns of extinctions and possible future responses to changes in precipitation related to global warming. In particular, metapopulation dynamics of the Laticauda group of sea snakes are expected to change in relation to projected reductions of tropical dry-season precipitation.     

Fine scale site fidelity in sea kraits: implications for conservation

The shores of coral reef islands are major sites for biodiversity, but unfortunately they are also subject to strong anthropogenic disturbances. Indeed vast arrays of organisms live exclusively in these very narrow and well structured zones, many others depend on the rich and diverse micro-habitats for essential part of their life cycle (to reproduce, forage, etc.). Sea kraits are sea snakes that depend on the shore of coral islets; they forage at sea but digest, reproduce and rest on land. They have been killed in extremely large numbers in many places, causing local extinctions. In the current study we demonstrate through recapture and translocation studies that these snakes exhibit a strong and fine-scale fidelity for particular segments of the shore. Consequently, these specific areas should be under strong protection, as it the case for the breeding beaches used by marine mammals, birds or turtles.     

Morphological adaptations to marine life in snakes

We investigated morphological adaptations to aquatic life within animals that exhibit a structurally simple, elongate body form, i.e., snakes. This linear body plan should impose different biomechanical constraints than the classical streamlined body shape associated with propulsion by fins, feet, or wings. Our measurements of general body shape of terrestrial, amphibious, and marine snakes (all from the same phylogenetic lineage, the Elapidae) show that seasnakes display specialized morphological attributes for life in water. Most notably, the cross‐sectional body shape is circular in terrestrial snakes but dorso‐ventrally elongated in seasnakes (due to a prominent ventral keel); amphibious species (sea kraits) exhibit an intermediate shape. The tail of amphibious and marine species (a major propulsive structure during swimming) is higher and thinner than in terrestrial snakes (i.e., paddle‐shaped) but shorter relative to body length. The evolution of a laterally compressed shape has been achieved by an increase in body height rather than a decrease in body width, possibly reflecting selection for more effective propulsive thrust, and for an ability to maintain hydrodynamic efficiency despite the minor bodily distension inevitably caused by prey items and developing offspring. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc     

Sea snakes rarely venture far from home

The extent to which populations are connected by dispersal influences all aspects of their biology and informs the spatial scale of optimal conservation strategies. Obtaining direct estimates of dispersal is challenging, particularly in marine systems, with studies typically relying on indirect approaches to evaluate connectivity. To overcome this challenge, we combine information from an eight-year mark-recapture study with high-resolution genetic data to demonstrate extremely low dispersal and restricted gene flow at small spatial scales for a large, potentially mobile marine vertebrate, the turtleheaded sea snake (Emydocephalus annulatus). Our mark-recapture study indicated that adjacent bays in New Caledonia (<1.15 km apart) contain virtually separate sea snake populations. Sea snakes could easily swim between bays but rarely do so. Of 817 recaptures of marked snakes, only two snakes had moved between bays. We genotyped 136 snakes for 11 polymorphic microsatellite loci and found statistically significant genetic divergence between the two bays (F(ST)= 0.008, P < 0.01). Bayesian clustering analyses detected low mixed ancestry within bays and genetic relatedness coefficients were higher, on average, within than between bays. Our results indicate that turtleheaded sea snakes rarely venture far from home, which has strong implications for their ecology, evolution, and conservation.     

Introduction to the symposium "New frontiers from marine snakes to marine ecosystems"

Interest in sea snakes and mythological "sea serpents" dates to ancient times and is represented in the writings of Aristotle, early voyagers, and explorers, and references in the Bible. Since then, awareness of the myriad species of snakes inhabiting the oceans has grown at a gradual pace. Scientific investigations into the biology of marine snakes-especially those in behavior, physiology, and other disciplines requiring living animals or tissues-have been comparatively challenging owing to difficulties in acquiring, transporting, handling, and husbanding these secondarily marine vertebrates. A broadening perspective with increasing interest in these animals peaked during the 1960s and 1970s, and literature from this period contributed to a growing knowledge that marine snakes comprise a very diverse fauna and are a significant part of marine ecosystems. Two persons figured prominently as influential drivers of research on sea snakes during this period, namely William Dunson and Harold Heatwole, and this symposium recognizes the contributions of these two individuals. Following a decline in scientific publications on sea snakes during the 1980s and 1990s, there has been a renaissance of scientific interest in recent years, and a wealth of new research findings has improved the understanding of phylogeny and diversity of marine snakes while simultaneously recognizing threats to marine ecosystems arising from climate change and other anthropogenic causes. The purposes of the symposium are to (1) illustrate the importance and relevance of sea snakes as contributors to better understanding a range of issues in marine biology, (2) establish and promote the use of marine systems as models for investigating conceptual issues related to environment, changing climate, and persistence of biological communities, with focus on marine snakes as novel or useful examples, (3) promote interest in sea snakes as useful organisms for study by scientists in a range of disciplines who might presently work with other organisms or systems, and (4) identify leading-edge topics for which studies of marine snakes might contribute uniquely to the advancement of research.     

Phylogeny of Australasian venomous snakes (Colubroidea, Elapidae, Hydrophiinae) based on phenotypic and molecular evidence

Scanlon, John D. & Lee, Michael S. Y. (2004). Phylogeny of Australasian venomous snakes (Colubroidea, Elapidae, Hydrophiinae) based on phenotypic and molecular evidence. — Zoologica Scripta , 33 , 335–366.
Phylogenetic relationships among Hydrophiinae (Australasian and marine elapid snakes) are inferred using 87 characters from external, skeletal, hemipenial and internal anatomy, ecology, and chromosomes as well as available sequences of two mitochondrial genes (cytochrome b and 16S rRNA). Parsimony analysis of the combined data retrieves many widely accepted clades; while observed bootstrap or branch (Bremer) support for these is often weak, most have never been corroborated previously by a rigorous numerical analysis. Sea kraits ( Laticauda ) and Solomon Islands elapids are basal to the remaining hydrophiines (Australian terrestrial forms and hydrophiin sea snakes). The latter clade includes three main lineages: a large-bodied oviparous lineage, a small-bodied oviparous lineage, and a viviparous lineage (which also includes the hydrophiin sea snakes, strongly reaffirmed as monophyletic). While the Solomons retain a relictual fauna, New Guinea has less endemism and has been invaded multiple times by Australian lineages, so there is no clear 'stepping stone' pattern supporting a northern (Asian, rather than Gondwanan) biogeographical origin.     

Comparison of freshwater discrimination ability in three species of sea kraits (Laticauda semifasciata, L. laticaudata and L. colubrina)

Three species of amphibious sea kraits (Laticauda spp.) require drinking freshwater to regulate water balance. The extent of terrestriality is known to differ among them. Species with higher extent of terrestriality would drink freshwater accumulated on land, whereas less terrestrial species would rely totally on freshwater that runs into the sea. Consequently, we predicted that the latter species might have a better ability to follow the flow of freshwater or lower salinity water in the sea than the former. We investigated the freshwater discrimination ability of three sea krait species, using a Y-maze apparatus. We found that Laticauda semifasciata and Laticauda laticaudata, less terrestrial species, followed freshwater significantly more frequently than seawater, whereas Laticauda colubrina, more terrestrial species, unbiasedly selected freshwater and seawater. This result supports our prediction and suggests that less terrestrial sea kraits more efficiently access freshwater sources in the sea than highly terrestrial sea kraits. It is likely that behavioral rehydration systems vary among sea kraits in relation to their terrestrial tendency.     

The influence of natural selection and sexual selection on the tails of sea-snakes (Laticauda colubrina)

Many phenotypic traits perform more than one function, and so can influence organismal fitness in more than one way. Sexually dimorphic traits offer an exceptional opportunity to clarify such complexity, especially if the trait involved is subject to natural as well as sexual selection, and if the sexes differ in ecology as well as reproductive behaviour. Relative tail length in sea-snakes fulfils these conditions. Our field studies on a Fijian population of yellow-lipped sea kraits ( Laticauda colubrina ) show that relative tail lengths in male sea kraits have strong consequences for individual fitness, both via natural and sexual selection. Males have much longer tails (relative to snout-vent length) than do females. Mark-recapture studies revealed a trade-off between growth and survival: males with relatively longer tails grew more slowly, but were more likely to survive, than were shorter-tailed males. A male snake's tail length relative to body length influenced not only his growth rate and probability of survival, but also his locomotor ability and mating success. Relative tail length in male sea kraits was thus under a complex combination of selective forces. These forces included directional natural selection (through effects on survival, growth and swimming speed) as well as stabilizing natural selection (males with average-length tails swam faster) and stabilizing sexual selection (males with average-length tails obtained more matings). In contrast, our study did not detect significant selection on relative tail length in females. This sex difference may reflect the fact that females use their tails primarily for swimming, whereas males also must frequently use the tail in terrestrial locomotion and in courtship as well as for swimming.     

Pelagic sea snakes dehydrate at sea

Secondarily marine vertebrates are thought to live independently of fresh water. Here, we demonstrate a paradigm shift for the widely distributed pelagic sea snake, Hydrophis (Pelamis) platurus, which dehydrates at sea and spends a significant part of its life in a dehydrated state corresponding to seasonal drought. Snakes that are captured following prolonged periods without rainfall have lower body water content, lower body condition and increased tendencies to drink fresh water than do snakes that are captured following seasonal periods of high rainfall. These animals do not drink seawater and must rehydrate by drinking from a freshwater lens that forms on the ocean surface during heavy precipitation. The new data based on field studies indicate unequivocally that this marine vertebrate dehydrates at sea where individuals may live in a dehydrated state for possibly six to seven months at a time. This information provides new insights for understanding water requirements of sea snakes, reasons for recent declines and extinctions of sea snakes and more accurate prediction for how changing patterns of precipitation might affect these and other secondarily marine vertebrates living in tropical oceans.     

First records of sea snakes (Elapidae: Hydrophiinae) diving to the mesopelagic zone (>200 m)

Viviparous sea snakes (Elapidae: Hydrophiinae) are fully marine reptiles distributed in the tropical and subtropical waters of the Indian and Pacific Oceans. Their known maximum diving depth ranges between 50 and 100 m and this is thought to limit their ecological ranges to shallow habitats. We report two observations, from industry‐owned remotely operated vehicles, of hydrophiine sea snakes swimming and foraging at depths of approximately 250 m in the Browse Basin on Australia's North West Shelf, in 2014 and 2017. These observations show that sea snakes are capable of diving to the dim‐lit, cold‐water mesopelagic zone, also known as the ‘twilight’ zone. These record‐setting dives raise new questions about the thermal tolerances, diving behaviour and ecological requirements of sea snakes. In addition to significantly extending previous diving records for sea snakes, these observations highlight the importance of university‐industry collaboration in surveying understudied deep‐sea habitats.     

Fijian sea krait behavior relates to fine‐scale environmental heterogeneity in old‐growth coastal forest: The importance of integrated land–sea management for protecting amphibious animals

The importance of terrestrial coastal ecosystems for maintaining healthy coral reef ecosystems remains understudied. Sea kraits are amphibious snakes that require healthy coral reefs for foraging, but little is known about their requirements of terrestrial habitats, where they slough their skin, digest prey, and breed. Using concurrent microclimate measurements and behavior surveys, we show that a small, topographically flat atoll in Fiji with coastal forest provides many microhabitats that relate to the behaviors of Yellow Lipped Sea Kraits, Laticauda colubrina. Microclimates were significantly related to canopy cover, leaf litter depth, and distance from the high‐water mark (HWM). Sea kraits were almost exclusively observed in coastal forest within 30 m of the HWM. Sloughing of skins only occurred within crevices of mature or dying trees. Resting L. colubrina were significantly more likely to occur at locations with higher mean diurnal temperatures, lower leaf litter depths, and shorter distances from the HWM. On Leleuvia, behavior of L. colubrina therefore relates to environmental heterogeneity created by old‐growth coastal forests, particularly canopy cover and crevices in mature and dead tree trunks. The importance of healthy coastal habitats, both terrestrial and marine, for L. colubrina suggests it could be a good flagship species for advocating integrated land‐sea management. Furthermore, our study highlights the importance of coastal forests and topographically flat atolls for biodiversity conservation. Effective conservation management of amphibious species that utilize land‐ and seascapes is therefore likely to require a holistic approach that incorporates connectivity among ecosystems and environmental heterogeneity at all relevant scales.     

Molecular phylogeny of elapid snakes and a consideration of their biogeographic history

Evolutionary relationships among the major elapid clades, particularly the taxonomic position of the partially aquatic sea kraits (Latkauda) and the fully aquatic true sea snakes have been the subject of much debate. To discriminate among existing phylogenetic and biogeographic hypotheses, portions of both the 16S rRNA and cytochrome b mitochondrial DNA genes were sequenced from 16 genera and 17 species representing all major elapid snake clades from throughout the world and two non-elapid outgroups. This sequence data yielded 181 informative sites under parsimony. Parsimony analyses of the separate data sets produced trees of broad agreement although less well supported than the single most parsimonious tree resulting from the combined analyses. These results support the following hypotheses: (1) the Afro-Asian cobra radiation forms one or more sister groups to other elapids, (2) American and Asian coral snakes form a clade, corroborating morphological studies, (3) Bungarus forms a sister group to the hydrophiines comprised of Latkauda, terrestrial Australo-Papuan elapids and true sea snakes, (4) Latkauda and true sea snakes do not form a monophyletic group but instead each group shares an independent history with terrestrial Australo-Papuan elapids, corroborating previous studies, (5) a lineage of Melanesian elapids forms the sister group to Latkauda, terrestrial Australian species and true sea snakes. In agreement with previous morphologically based studies, the sequence data suggests that Bungarus and Latkauda represent transitional clades between the elapine 'palatine erectors' and hydrophiine 'palatine draggers'. Both intra and inter-clade genetic distances are considerable, implying that each of the major radiations have had long independent histories. I suggest an African, Asian, or Afro-Asian origin for elapids as a group, with independent Asian origins for American coral snakes and the hydrophiines.     

Dwindling seagrasses: A multi-temporal analysis on Google Earth Engine

Seagrasses, a unique group of marine flowering plants, profoundly influence the marine environment by providing an array of critical ecological functions. They serve as the foundational habitat for several endangered and charismatic species, including sea cows, sea turtles, and sea horses, and are often referred to as coastal canaries. In comparison to boreal and tropical forests, they have an amazing ability for carbon storage. Despite their long evolutionary history, they are threatened by rapid environmental changes caused by climate change and human activity. Long-term monitoring is required to comprehend the changes in this fragile ecosystem. Conventional field survey methods for collecting long-term data are laborious, time-consuming, and expensive. Hence, this work builds a time-series dataset of the seagrass coverage in the Kalpeni lagoon from 2003 to 2020 by analysing Landsat data on Google Earth Engine. We also evaluated the temporal changes in the seagrass coverage of the study area and studied the influence of selected environmental factors on the seagrass coverage. We observed a negative relationship between sea surface temperature and seagrass coverage. The results revealed a decline in more than 99% of seagrass coverage, indicating an alarming threat to this seagrass ecosystem of the region. With such a drastic shrinkage in the seagrass coverage, the hysteresis must be strong, and the recovery of these meadows may require intensive interventions. By establishing a long-term time series database of seagrass coverage, our study also opens up new avenues for future ecological research on the seagrass meadows.     

Salinity influences the distribution of marine snakes: implications for evolutionary transitions to marine life

Secondary transitions from terrestrial to marine life provide remarkable examples of evolutionary change. Although the maintenance of osmotic balance poses a major challenge to secondarily marine vertebrates, its potential role during evolutionary transitions has not been assessed. In the current study, we investigate the role of oceanic salinity as a proximate physiological challenge for snakes during the phylogenetic transition from the land to the sea. Large‐scale biogeographical analyses using the four extant lineages of marine snakes suggest that salinity constrains their current distribution, especially in groups thought to resemble early transitional forms between the land and the sea. Analyses at the species‐level suggest that a more efficient salt‐secreting gland allows a species to exploit more saline, and hence larger, oceanic areas. Salinity also emerged as the strongest predictor of sea snake richness. Snake species richness was negatively correlated with mean annual salinity, but positively correlated with monthly variation in salinity. We infer that all four independent transitions from terrestrial to marine life in snakes may have occurred in the Indonesian Basin, where salinity is low and seasonally variable. More generally, osmoregulatory challenges may have influenced the evolutionary history and ecological traits of other secondarily marine vertebrates (turtles, birds and mammals) and may affect the impact of climate change on marine vertebrates.