Defining aquatic and semi-aquatic snakes is complicated. The two categories are on continuums in many lineages. Aquatic is an easy label if the snake never comes out of the water, like some sea snakes or the Tentacled Snake (Erpeton tentaculatus). They are easily labeled semi-aquatic if they are like the common Northern Water Snake (Nerodia sipedon) or the sea kraits of the genus Laticauda, which enter the water to hunt or search for mates but bask in bushes and rocks close to the water.
Freshwater snakes have evolved many times and to varying degrees in dozens of snake lineages. Snakes living in still, turbid water and feed on fish can be expected to have a different suite of adaptations than snakes living in bedrock streams with boulders, fast-moving water, and feed on crustaceans.
Pauwels et al. (2008) proposed freshwater snakes are those that 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 that excluding water from the upper respiratory system and allowing the snake to inhale at the surface without being seen by birds and other 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 aquatic snakes are often reduced in size, and sometimes the iris matches the color of the surrounding scales. At the same time, other species have a brightly colored iris that makes the eyes stand out compared to the surrounding scales. Also, aquatic snakes frequently have a dorsal pattern of transverse bands and keeled scales.
None of these characters are shared by all freshwater 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 tentaculum, is highly aquatic, possibly the most specialized freshwater snake, and may never leave the water. Its eyes are relatively large and lateral, not dorsolateral or dorsal, and it has longitudinal stripes, not transverse bands. The ancestors of modern aquatic snakes may have been adapted for life in the trees or living on the ground or fossorial. Some freshwater snakes have evolved secondarily from marine species.
Extant snakes are organized into about 45 family and subfamily level clades that extend backward in time, about 170 MYA. During this time, the snake lineages branched. Some existed for a while before becoming extinct. Others survive to the present day. Living species are mostly terrestrial, arboreal, or fossorial, while relatively few are adapted to live in freshwater and marine environments.
The evolution and adaptations of marine snakes have received considerable attention (Dunson 1975; Heatwole 1999; Sanders et al. 2008; Brischoux and Bonnet 2009, Ukuwela et al. 2016, Sherratt et al. 2018). Most marine 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. Living marine elapids have three different ancestors and have adapted to life in the oceans in different ways. They form three lineages that have invaded the oceans – the sea snakes (Hydrophis + Apisuryus = hydrophiids) with 62 species and the eight species of sea kraits 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. In fact, 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 in areas that have heavy rainfall. Freshwater coming off the land into the ocean stays on the surface before it mixes with seawater, and rain will also float on the ocean’s surface and form a lens of freshwater before mixing with the heavier seawater.
Ambush predators conceal themselves in water and retain 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 prey from a perch over the water. Arboreal snakes can use aquatic resources without morphological modifications to aquatic habitats, a reminder of the remarkable plasticity we see in snakes. Asad et al. (2020) reported the mock vipers, Psammodynastes pictus, is restricted to Sundaland hunting 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. 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 aquatic snakes. However, we are including those snakes that hunt from the water – even though many of them are likely habitat generalists.
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 that 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 while hunting.
So the questions become – are the snakes that use water filled crayfish burrows for hibernation semi-aquatic snakes? Are snakes that are terrestrial or semi-aquatic that spend the winter submerged in a pond or stream – are they aquatic? Are snakes that use water-logged soils semi-aquatic? Is a arboreal viper that hunts prey underwater semi-aquatic? I have identified about 380 species that are aquatic or semi-aquatic using an expanded definition of those terms.
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Jowers MJ, Simone Y, Herrel A, Cabezas MP, Xavier R, Holden M, Boistel R, Murphy JC, Santin M, Caut S, Auguste RJ. 2022. The Terrific Skink bite force suggests insularity as a likely driver to exceptional resource use. Scientific Reports. 12(1):1-9.