Lake Songkhla (above) looks like many other wetlands from the shoreline (Figure 1-1), but it is the largest wetland complex in Thailand and is composed of three shallow basins. One hundred and fifty years ago, Songkhla was a bay open to the ocean. Since then, a series of barrier islands were expanded with human assistance and gradually enclosed the basins. The islands now form the Sathing-Phra Peninsula and almost wholly isolate Lake Songkhla from the Gulf of Thailand. Only one opening to the sea is located at the south end of the lake near Songkhla. Annandale (1916) described the lake as no more than 4.9 m deep. Since then, the lake has continued to accumulate silt, and today the three basins have a mean depth of 1.2 m and a maximum depth of 2.4 m (Leelawathanagoon et al. 1997). The water levels and chemistry changes accompanying the lake’s formation left an unusual mixture of freshwater and marine fauna.
Lake Songkhla has perhaps the most diverse assemblage of water-dwelling snakes globally. One sea snake (Hydrophis brookii family Hydrophilidae), one file snake (Acrochordus granulatus, family Acrochordidae), the pipe snake (Cylindrophis sp. family Cylindrophiidae), at least one species of keelback (Fowlea piscator – family Natricidae), and at least four species in four genera of Asian-Australasian Mud Snakes (family Homalopsidae) live in the lake or along its wet edges.
Aquatic is an easily applied label if the snake never comes out of the water, like some sea snakes or the Tentacled Snake (Erpeton tentaculatus). And they are easily labeled semi-aquatic if they are like the North American water snakes of the genus Nerodia, the European Natrix, or the sea kraits of the genus Laticauda. All of these enter the water to hunt or search for mates but bask in bushes and on rocks close to the water, and some mate and lay eggs on land. Then terrestrial or arboreal snakes escape predators by diving into the water, such as Philodryas olfersii (Abegg and Mario-da-Rosa 2018).
Snakes have evolved aquatic lifestyles many times and to varying degrees in dozens of different lineages. Snakes living in still, turbid water and feeding on fish can be expected to have other adaptations than snakes feeding on crustaceans living in bedrock streams with boulders and fast-moving water. Making this more complex and confusing are snakes that evolved aquatic lifestyles and then returned to a terrestrial existence.
Pauwels et al. (2008) proposed that freshwater snakes forage in the water and cannot survive without aquatic prey and frequent submersion. They suggested that subtle external anatomical characters betray aquatic habits in freshwater snakes. Morphological specializations include dorsally oriented nostrils, often close together, some of which have valved nares that exclude water from the upper respiratory system and allow the snake to inhale at the surface without being seen by predators. The eyes, too, tend to be dorsal oriented so that a snake lying in the water can view the sky (or the water) above for potential prey or predators.
The eyes of snakes have evolved aquatic lifestyles many times and to varying degrees in dozens of different lineages. Snakes living in still, turbid water and feeding on fish can be expected to have other adaptations than snakes feeding on crustaceans living in bedrock streams with boulders and fast-moving water. Making this more complex and confusing are snakes that evolved aquatic lifestyles and then returned to a terrestrial existence.
Pauwels et al. (2008) proposed that freshwater snakes forage in the water and cannot survive without aquatic prey and frequent submersion. They suggested that subtle external anatomical characters betray aquatic habits in freshwater snakes. Morphological specializations include dorsally oriented nostrils, often close together, some of which have valved nares that exclude water from the upper respiratory system and allow the snake to inhale at the surface without being seen by predators. The eyes, too, tend to be dorsally oriented so that a snake lying in the water can view the sky (or the water) above for potential prey or predators. The eyes of aquatic snakes are often reduced in size, and sometimes the iris matches the color of the surrounding scales. Other species have a brightly colored iris that makes the eyes stand out compared to the surrounding scales. Aquatic snakes frequently have a dorsal pattern of transverse bands and keeled dorsal scales.
None of these characteristics are shared by all aquatic snakes, and some traits, like transverse bands or keeled scales, are shared with many terrestrial, arboreal, or exclusively marine species. The freshwater Tentacled Snake, Erpeton tentaculatum, is highly aquatic, possibly the most specialized freshwater snake, and may never leave the water or do so only on rare occasions. Its eyes are relatively large and lateral, not dorsolateral or dorsal, and it has longitudinal stripes, not transverse bands.
Thus, the ancestors of modern aquatic snakes were adapted for life underground, on the ground, or in the trees. Some of their descendants moved into freshwater, brackish water, or marine environments and evolved traits for those environments. Some returned to life on land, and others evolved into more specialized aquatic lifestyles.
Extant snakes are organized into about 45 family and subfamily-level clades extending backward. Estimated dates for snake origins are complicated by the evolution of squamate snake-like morphs that appear in the fossil record. Snakes and lizards shared an ancestor in the lower Jurassic, about 190 MYA(Harrington and Reader 2017). During this time, snake lineages branched. Some existed for a while before becoming extinct. Others survive to the present day.
The evolution and adaptations of marine snakes have received considerable attention (Dunson 1975; Heatwole 1999; Sanders et al. 2008). Sea snakes are in the subfamily Hydrophiinae – a clade nested in the larger elapid clade, all of which share fixed front fangs and venom glands and include the coral snakes, cobras, and mambas. Marine elapids have three different ancestors and have adapted to life in the oceans differently. They form two or three lineages the sea snakes (Hydrophis + Apisuryus = hydrophiids) with 62 species and the sea kraits with eight species in the genus Laticauda. Both clades have paddle-like tails to improve their swimming ability, but the hydrophiids give birth to young in the water and have lost the broad ventral scales. Sea kraits are oviparous, laying eggs in terrestrial situations, and retain their broad ventral scales. The only thing sea kraits do in the water is hunt. Despite living in the oceans, sea snakes and sea kraits still rely on freshwater for drinking (Lillywhite et al. 2008; Bonnet, Brischoux 2008; Lillywhite et al. 2014). This requires the sea snakes to stay close to river deltas or areas of heavy rainfall. Freshwater draining off the land into the ocean remains on the surface before it mixes with seawater, and rainwater floats on the ocean’s surface and forms a lens of freshwater before mixing with the heavier seawater.
Ambush predators can use water for concealment while retaining much of the terrestrial morphology for life on land. Other snakes hunt shorelines and search small pools without entering the water, and a few snakes hunt from branches and strike aquatic prey from a perch over the water. Arboreal snakes can use aquatic resources without the apparent morphological modifications to aquatic habitats, a reminder of the remarkable plasticity we see in snakes. Asad et al. (2020) reported that the mock viper, Psammodynastes pictus, is restricted to Sundaland and hunts fish of the genus Rasbora from branches overhanging the water. Trobisch and Gläßer-Trobisch (2011) observed hunting behavior in captive juvenile Natrix tessellata and reported hunting from branches over the water. The Burmese Vine Snake, Ahaetulla fronticincta, is highly arboreal but feeds exclusively on fish (Figure 1-4). It inhabits bushes and other low vegetation in mangrove forests in coastal Myanmar (Wogan and Vogel, 2020).
Snakes that hunt from branches or shorelines without entering the water are not included here because, technically, they are not spending much time in the water. However, snakes that hunt from the water are included – even though many of them are habitat generalists. And maybe taking prey from the shoreline.
Not surprisingly, the largest snakes often use water. The Green Anaconda (Eunectes murinus) and its relatives are highly aquatic. The largest pythons (Python sebae, P. natalensis, P. bivittatus, P. molurus, and Malayopython reticulatus) are less dependent on water than the anacondas (aquatic boas). Yet the pythons still spend substantial time in the water, hunt from the water, and use the water for thermoregulation. Not only does water provide buoyancy for massive bodies, but it allows the snakes to conceal themselves from prey and predators.
One sea snake, the Yellow-bellied Sea Snake (Hydrophis platurus), has become pelagic, drifting with ocean currents. The only time it is seen on land is when it washes up on beaches from Africa’s east coast to the coastlines of western North and Central America. The Yellow-bellied Sea Snake aggregates along slicks or drift lines. Floating debris accumulates in the slicks, and the snake may remain for days or weeks before a change in wind speed or current direction breaks them up. Aggregations of Hydrophis platurus drift, the aggregate, may contain just a few animals or several thousand, and they include juvenile and adult snakes. Other animals inhabiting the drifts are jellyfish medusa, fish, porpoises, sea turtles, and sea birds often following floating debris lines. Snakes aggregate here because the slicks are a place to locate food and mates. The degree to which these snakes have adapted to the marine environment is significant, given that they represent a recent evolutionary radiation.
Was the snake ancestor aquatic?
Evidence from morphology and molecules leaves little doubt that snakes evolved from lizards. However, the transition between lizards and snakes is amongst the most controversial topics in evolution, in part because of the lack of well-preserved fossils. Three competing hypotheses for the lifestyle of the ancestral snake — burrowing, aquatic, and terrestrial origins — have been debated for more than a century.
Cope (1869) implied the snakes’ ancestors could have been aquatic when he placed snakes and mosasaurs (Fig. 1-6) in the Pythonomorpha. An aquatic snake ancestor was also suggested by Caldwell and Lee (1997) and Caprette et al. (2004), and others who have worked on Cretaceous marine lizards. Vidal and Hedges (2004) tried to rule out an aquatic origin for snakes based on the molecular results suggesting snakes were not related to varanoid lizards – the mosasauroids’ closest living relatives.
Camp (1923) proposed snakes evolved from a grassland anguimorph lizard (a terrestrial ancestor). Today, anguimorph lizards remain possible candidates for the snake ancestor, and there are living grass-swimming anguimorphs. Anguimorph lizards and snakes share paired male reproductive organs with ornamentation and venom glands.
Mahendra (1938) proposed that the snake ancestor was a burrowing lizard. This hypothesis gained broad support and was the most well-accepted idea regarding the snake’s ancestral lifestyle. Mahendra noted that a ring of scales surrounded the eye in most advanced snakes. Burrowing forms like the threadsnakes and blindsnakes (Leptotyphlopidae and Typhlopidae), the shield-tailed snakes (Uropeltidae), and the Amazonian pipe snakes (Anilius) have the eye covered by a single large scale. Mahendra viewed all these snakes as primitive or basal to more advanced snakes. He placed them at the evolutionary tree’s base and concluded that the ancestral snake must have been fossorial. Today, these snakes are recognized as some of the oldest lineages, the dawn snakes, threadsnakes, and blindsnakes — have been called scolecophidians – but they are not monophyletic (Heise et al. 1995).
An early terrestrial relative of snakes was the 95-million-year-old Najash rionegrina, which had two rear legs and a sacrum. The remains of Najash included partial skulls. However, other fossil snakes possessed hindlimbs, but their pelvic bones were not directly attached to the vertebrae. These are older than Najash, and all were marine (Haasiophis, Pachyrhachis, and Eupodophis).
The widespread acceptance of the burrowing ancestor hypothesis was partly due to Walls’ (1940) work on snake eye anatomy. In a list of differences between snake and lizard eyes, Walls clarifies that snake eye anatomy is quite distinct from lizard eye anatomy. The most dramatic difference between the lizard and snake eyes is the snakes’ inability to change the shape of their lenses to focus an image onto the retina. Muscles attached to the lens for focusing in lizards are absent in snakes. Snakes must move the lens towards or away from the retina to focus their eyes using the enlarged peripheral iris muscles. Also, most snakes lack a fovea, although some Asian tree snakes have re-evolved the structure. Walls also explained the cone-like structure of rods in snake retinas (and those of other tetrapods) resulting from the loss of rods in response to diurnal activity and then secondarily redeveloping rods from cones. He thought cone cells were transformed into rod cells, which became known as the transmutation hypothesis. The modifications Walls (1940) found in snake eyes suggested to him they resulted from a burrowing lifestyle.
Shine and Wall (2008) found two ecomorphs for lizards that had lost their legs: legless burrowing lizards have elongated trunks, small heads, short tails, and constant body widths; and surface-active legless species have short trunks, broader heads, long tails, and more variable body widths. They suggested that a long tail in surface dwellers is helpful for escaping predators if the tail can be automized and that long tails are probably beneficial for locomotion when the animal is using lateral undulations (side-to-side movement) to move. On the other hand, burrowing lizards need to push through the substrate, which requires a different arrangement of muscles and bones in which the rib cage and trunk muscles form a rigid but flexible cylinder. This pipe-like morphology transfers force to the digging head, and a long tapering tail is not well suited for this lifestyle. Regarding the aquatic origins of snakes, Shine and Wall observe living semi-aquatic lizards have well-developed limbs and long tails. The tails are essential in swimming and, therefore, are unlikely to be lost. Many living lizard species are burrowers, and by implication, Shine and Wall infer that snakes were more likely to have evolved from a burrowing ancestor than an aquatic one.
Molecular clock dates produced by Vidal et al. (2007) suggest lizards last shared an ancestor with snakes 166.4 MYA. Other authors using similar methods estimated this ancestor lived 194–145 MYA. Molecular clock dates suggest the scolecophidians (leptotyphlopids, typhlopids, and anomalepidids ) shared an ancestor with all other snakes about 155.6–151.9 MYA. Hugall et al. (2007) placed the snake-Anguimorph lizard divergence at about 155 MYA and the separation of the scolecophidians from the rest of the snakes at about 105 MYA. Sanders et al. (2008) place the anguimorph-snake divergence date at about 140 MYA and the divergence of the scolecophidians from the rest of the snakes at about 110 MYA.
Yi and Norell (2015) argued in favor of snakes’ burrowing origin based on inner ear morphology. They noted that the fossil snake Dinilysia is a stem snake close to the most recent common ancestor of living snakes; according to most phylogenetic analyses, it has inner ear morphology like that of some burrowing squamates. Their model predicted both Dinilysia and the reconstructed ‘hypothetical ancestor of crown snakes’ are classified as burrowing forms based on a comparison with modern species of known ecology. Burrowing is a predominant lifestyle in most basal lineages of crown snakes; therefore, snakes must have had a burrowing origin.
Again, looking at inner ear structure, Palci et al. (2017) used a larger dataset with more taxa and subdivided the species into five traditional ecological categories. They recognized that the closest outgroup to snakes might be iguanians, anguimorphs, or mosasauroids. The results placed Dinilysia close to the existing snake Xenopeltis and found that homalopsids were close to Dinilysia. The placement contradicts the claim that a large spherical sacculus (their ‘vestibule’) is diagnostic of burrowing forms. Such morphology can also be found in semi-aquatic snakes, and it is absent in some typical obligate burrowers like scolecophidians or semifossorial snakes such as Calabaria and Brachyurophis.
Scolecophidians are widely considered the most basal lineage(s) of crown snakes historically and in recent phylogenetic analyses (Figure 1.7). Supposing this is an accurate phylogenetic position for these snakes, it might be expected that Dinilysia would display an inner ear morphology like scolecophidian snakes if burrowing were indeed primitive for snakes. However, the sharp differences between the inner ears of Dinilysia and scolecophidians are not easily understandable if all share a burrowing ancestry.
The ancestral snake’s lifestyle remains uncertain; it could have been a burrower, a swimmer, a terrestrial species, or a species using a combination of these ecological lifestyles. Further discussion of this topic awaits additional fossils discoveries and studies of extant species.
Ancient Aquatic Snakes
The Eocene began at 55.8 MYA and ended at 33.9 MYA. During this time, the continents were drifting toward their present positions. It was a time when the Earth was a “greenhouse,” with global warming due to increased atmospheric carbon dioxide (Bohaty and Zachos 2003). The distribution of Eocene species was remarkably different than what we see today. Palm trees in Alaska and the northern Rockies, crocodiles on Elsmere Island above the Arctic Circle, and forests covering much of Antarctica. Primates spread from Asia to Europe and North America during this time, and the rivers, estuaries, and oceans contained snakes, including giant aquatic snakes.
The snake family Palaeopheidae is known exclusively for their vertebrae and ribs. They inhabited both hemispheres, from the Upper Cretaceous to the Eocene (McDowell 1987). Their fossils are always associated with rock formed in watery environments. The vertebrae tend to be tall and narrow, and the ribs are only slightly curved, characteristics found in most aquatic snakes living today, the sea snakes. Those who study Palaeopheidae fossils consider the group relatives of the boids, but others have suggested that they are close to the file snakes (the acrochordids). Size estimates for the paleophids range from 0.5 to 9 m or more, and while some lived in near-shore environments such as estuaries and mangroves, others were using open ocean habitats far from shore.
Holman (2000) and Rage et al. (2003) have reviewed the family, known chiefly from vertebrae and ribs, and have described paleophids as booid-like. Some species were highly modified for life in the water. They had laterally compressed vertebrae and bodies for swimming, high neural spines on all vertebrae (to aid the lateral compression of the body), synapophyses for rib articulation low on the vertebrae, and ribs that were only slightly curved (assisting lateral body compression). The vertebrae of at least one species contained marrow cavities, which may have been involved in regulating buoyancy, increased erythrocytes, or both. And, like some modern sea snakes, the laterally compressed species likely could not move on land. Other family members showed only slight modifications for aquatic life and were probably somewhat terrestrial. The extinct colubroid families Anomalophiidae and Russellophiidae are also suspected of being aquatic (Rage et al. 2003).
Palaeophis colossaeus was described by Rage (1983) based on vertebrae collected in Mali. The vertebrae were 34 mm long. We do not know how many vertebrae the snake had, but given that a typical boa or python has about 270, it is likely this snake could have been 9.180 m or more than 30 feet. Another huge paleophiid is Pterosphenus schucherti, described from coastal North America; it ranged from Texas to New Jersey. Remains from Florida indicate that an individual snake died at least 300 km from the nearest mainland during the late Eocene, where it was buried with cartilaginous fish, bony fish, and an ancient whale.
Ancient aquatic snakes disappeared near the end of the Eocene, and their extinction is likely linked to a changing climate. Global cooling at the start of the Oligocene occurred as oceanic circulation was altered with Antarctica’s disconnection from Australia and South America. The formation of the South Pole’s continental ice sheet followed the extinction of many warm-water species.
Four new fossil aquatic snake species ranging from 167–143 MYA in age were described by Caldwell et al. (2015). These four species push the earliest known snakes backward by nearly 70 million years – into the mid-Mesozoic. Thus, their origin coincided with the known radiation of most other major groups of squamates in the mid-Jurassic: the time of the final stages in the break-up of Pangaea into Laurasia and Gondwana. These new records for early snakes fill a significant chronological gap predicted by molecular phylogenetics.
In stratigraphic order, the fossil snakes Caldwell et al. recognized are: Parviraptor estesi, from rocks dated at ~167 MYA, from the Middle Jurassic, from Southern England, followed by the North American Diablophis gilmorei dated at ~155 MYA from the Upper Jurassic of Colorado, USA; which appears to be a contemporary of Portugalophis lignites from the Upper Jurassic of Guimarota, Portugal; the youngest species was Eophis underwoodi from rocks dated at ~150–140 MYA positioned in time at the Jurassic-Cretaceous border from an outcropping near Swanage, Dorset, Southern England. Portugalophis lignites remains were deposited in a coal swamp. Eophis and Parviraptor remains came from mixed coastal lake and pond systems and riparian environments, and Diablophis was recovered from an epicontinental deposit several hundred kilometers from the nearest shoreline.
Caldwell et al. wrote,
“It is also possible that snakes forming these island assemblages arrived as secondarily aquatic invaders. Secondary invasions of marine environments characterize the subsequent evolutionary histories of numerous clades of fossil and modern snakes.”
Thus, some of the earliest known snakes – were likely semi-aquatic or aquatic. Snakes known from marine deposits formed in the Late Cretaceous (91–95 MYA) were also aquatic. Pachyophis woodwardi and Pachyrhachis problematicus, had laterally compressed bodies, small heads, and pachyostoic ribs and vertebrae (thick, dense bone lacking marrow). These fossils came from carbonate rocks deposited in an inter-reef basin, suggesting they lived in shallow water on a carbonate platform (Caldwell and Albino, 2001). At least three species (P. problematicus, Haasiophis terrasanctus, and Eupodophis descouensi) had hind limbs. Their remains come from marine deposits in the Mediterranean area of the ancient Tethys Ocean or its immediate vicinity (Rage and Escuilié, 2002).
Not all Cretaceous snakes were aquatic. Apesteguia and Zaher (2006) describe Najash rionegrina, from the Cenomanian (Upper Cretaceous) of Rio Negro Province, Argentina. Najash is unique among snakes (fossil and living) in that it has two sacral vertebrae that separate the trunk vertebrae with ribs from the caudal vertebrae without ribs. Thus, this animal has a pelvis, and articulating with the sacrum were robust functional legs. Additionally, this species was from a terrestrial deposit, with the skull showing characteristics associated with an underground lifestyle. The authors wrote that it was perhaps “…a surface-dwelling species that would occasionally use tunnels produced by burrowers.” The fossil remains of the boid, Titanoboa cerrejonensis, from La Guajira in northeastern Colombia imply the snake reached a length of 12.8 m and a weight of 1,135 kg. It is the largest snake ever discovered, and it was aquatic. The remains were dated 58 to 60 MYA (Head et al. 2009).
Extant Aquatic Snakes
Extant aquatic snakes of all lineages reach their highest diversity in tropical Southeast Asia and Australia (Heatwole 1999). However, among the advanced terrestrial snakes, some boodontines, colubrines, dipsadids, and natricids use coastal environments and enter brackish water or full seawater. Virtually all other marine species are restricted to Asia and Australia. The only exception is the pelagic hydrophiid Hydrophis (Pelamis) platurus, which has dispersed into the eastern Pacific and the western Indian Ocean).
Without a doubt, Asia has the most remarkable diversity of aquatic species. At first, the reason for this seems to be the abundance of shallow-water habitats in Asia. However, the answer may have more to do with the geographical origins of advanced snake lineages. The best-known and most committed aquatic snakes are the fixed front-fanged, paddle-tailed sea snakes (Hydrophiinae: Elapidae), which spend most of their life in water and have two or more or more terrestrial elapid ancestors. The unusual, semi-aquatic sea kraits (Laticauda: Elapidae) have paddle tails, broad ventral scales, and terrestrial tick parasites, and they only enter the water to feed. Instead, sea kraits bask, mate, and deposit their eggs in terrestrial locations.
The file snakes (Acrochordidae) seem to spend their entire lives in the water and show numerous adaptations to an aquatic lifestyle suggesting they, and their ancestors, have been living in the water for an exceptionally long time.
Some Oriental-Australian rear-fanged water snakes (Homalopsidae) give the impression that they spend slightly less time in the water than sea snakes and file snakes. And, as previously mentioned, many of the natricids (Colubridae: Natricidae) are semi-aquatic. The time spent in the water is known for a few species, but many are poorly understood. Within other genera and subfamilies of colubroids that are semi-aquatic, the time spent in the water. Thus, aquatic and semi-aquatic lifestyles have been derived independently numerous times throughout Earth’s history.
Life in the water changes the rules snakes face on land. Water supports an animal’s body weight and is slow to lose or gain heat. Salt in the ocean water renders the water undrinkable for snakes and many other animals. Salt also increases the density of water, making animals more buoyant. Warm water reduces the need for basking, but some aquatic snakes live in relatively cold water that originates on mountaintops. Water may lower the risk of predation from terrestrial and aerial predators. Of course, aquatic environments also expose their inhabitants to a new set of predators, parasites, and physical environmental challenges. Aquatic habitats provide abundant food resources in the form of invertebrates, fish, and amphibians and present a variety of new hiding places, such as the intertidal burrow system, the cavities under a riverbank, or the layer of detritus covering the bottom of the pond or stream.
All snakes are likely capable of swimming, but some species are much better at it than others. Many snake lineages have some species adapted to aquatic habitats. Still, five lineages (file snakes, the homalopsids, elapids, dipsadids, and the natricids) contain the most species adapted for life in water. Snake adaptations to water include modifications to many or all organ systems. However, all adaptations for an aquatic existence do not occur in every aquatic snake. While some species have extreme adaptations to life in water, others appear to have very few.
Dorsally positioned nostrils and eyes allow the snake to breathe and observe without exposing the head or body to the air. When diving, valves open and close the nostrils, and the trachea opens opposite the internal nares to exclude water from the respiratory tract. Lateral compression of the body increases the surface for swimming; the ribs may be less bowed and elevated to exaggerate the surface area on the snake’s sides. The tail may be flattened, compressed slightly at the base, or turned into a paddle with exaggerated fin-like flaps above and below the vertebrae. Ventral scales are broad in land-dwelling snakes so the snakes can grip surfaces; in some highly aquatic snakes, the ventral scales narrow (i.e., the file snakes) the belly scales are similar in size to the dorsal scales. This reduction in ventral scales aids the snake in flattening its body for an increased surface area when swimming. The snakes most adapted for aquatic life swallow food while submerged and give birth in the water.
Terrestrial and Freshwater Snakes visit the Oceans
Coastal populations of snakes that usually inhabit freshwater or terrestrial habitats may venture into brackish or full seawater. Snakes may get washed downstream into the ocean or brackish water habitats during flooding. Reports of Reticulated Pythons, Eastern Diamondback Rattlesnakes, and many other species swimming between islands are documented. They suggest that these species may intentionally enter the oceans (a hypertonic environment) because they will find food, mates, or a new territory.
A Thamnophis ordinoides on the Oregon coast just north of Port Orford in August 2012 traveled a considerable distance to reach the ocean. The motivation was unknown. Was the snake lost or intentionally entering the marine environment? This individual snake has not washed down a stream during a flooding event (the evidence is the long track in the sand). Nor was it transported to the edge of the Pacific Ocean by humans.
Neill (1958) summarized the literature and included personal observations on snakes using brackish and seawater. In some cases, the snakes are directly observed in the water. In other instances, he assumes their saltwater use based upon their proximity to it. Many subsequent papers also discuss this phenomenon. Thus at least some semi-aquatic or mostly freshwater snakes can tolerate high concentrations of sodium ions in their blood (hypernatremia). This allows them to venture into brackish and marine environments to forage, find mates, or disperse into new habitats for at least a short time. Even some terrestrial species are occasionally seen swimming in the ocean. The list is extensive. The following list by family is based on Neill (1958) and other more recent literature.
Tropidophiidae. Neill also found the Cuban trope, Tropidophis m. maculatus, beneath debris at the edge of weed-grown tidal flats near Havana, Cuba.
Booids. Candoia carinata, Island distribution; Neil found one on Morotai Island, Moluccas, under masses of rooted coconuts at the bases of coconut trees growing above the tidal zone and notes one specimen was found swimming in the sea at Natterer Bay, New Guinea. An African Sand Boa (Eryx jaculus) was found beneath logs close to the beach in Bougie, Algeria.
Pythonidae Hart et al. (2012) found that the hatchling Python bivittatus could survive for five months in brackish water and for more than a month in full seawater.
Colubridae. The Florida kingsnake (Lampropeltis getula floridana) Carr (1940) listed it as “occasional” in a salt marsh. A close ally, the speckled kingsnake (L. g. holbrooki) was found to be as common in the salt marsh as in the woods of southeastern Texas.
Psammophiidae. The European Malpolon monspessulanus has been reported multiple times from brackish and saltwater habitats (Deso et al 2021).
Natricidae. Populations of these snakes adjacent to brackish or marine environments can be expected to occur in saline waters on occasion. The green water snake (Nerodia cyclopion), the Brown Water Snake, (Nerodia taxispilota), the Glossy Water Snake (Lithodytes rigida), and Graham’s Water Snake (Regina grahami) in brackish marshes. The European water snake (Natrix natrix) has been reported in the sea along the Mediterranean coast. The Viperine Water Snake (Natrix maura), the Eastern Garter Snake (Thamnophis s. sirtalis) the Ribbon Snake (T. sauritus) are frequent visitors to saline water. (Neill 1958).
Dipsadidae. The North American Mud Snake (Farancia abacura) and the Rainbow Snake (Farancia erythrogrammus) use tidal marshes. While the aquatic neotropical genus Tretanorhinus uses freshwater habitats, some inhabit mangroves.
Elapidae. The Indian cobra (Naja naja) is an adaptable but essentially terrestrial species in southern Asia’s forests, fields, gardens, and human-modified situations. It enters the water readily and has been observed swimming in the sea (Gharpurey, 1954, p. 54). The Australian black snake (Pseudechis porphyriacus) has been found swimming in the middle of Botany Bay. The Australian taipan (Oxyuranus scutellatus) is abundant around the shores of the Gulf of Carpentaria. And other Australian elapids (species not named) may swim across estuaries (Kinghorn, 1956). And, of course, marine hydrophiids can survive in full seawater as long as they have access to freshwater for drinking.
Viperinae. The Common Viper (Vipera berus) is terrestrial but swims readily and has been found many miles at sea off the coast of Ireland (M. A. Smith, 1951).
Crotalinae. Some rattlesnakes (Sistrurus and Crotalus) use saline habitats. These include the Dusky Pygmy Rattlesnake (Sistrurus miliarius barbouri) and the Eastern Diamondback Rattlesnake (Crotalus adamanteus).
Thus, freshwater snakes may first transition into brackish or salt water by exploring those habitats. In the reverse situation, brackish water or marine snakes may transition back to freshwater. Cerberus microlepis, Laticauda crockeri, and Hydrophis semperi are in marine genera, but those species live in freshwater
To the right. The major lineages containing numerous aquatic species are Homalopsidae, the elapid subfamily Hydrophiinae, some dipsadids, and the natricids. All clades have independently evolved a viviparous reproductive mode, income cases, and the evolution of viviparity occurred multiple times within a clade. To the right are oviductal eggs and embryos. from an Enhydris enhydris.
