The Book

Preface
“I shall not pretend to assert that they are as numerous as land snakes; but it is very likely that one hundred species at least of this tribe exist in the waters of the ocean, lakes, and rivers.”
C. S. Rafinesque 1817. Dissertation on water snakes, sea snakes and sea serpents. Philosophical Transaction

When Rafinesque wrote this, 14 species of snakes were considered water snakes. The idea for a global overview of aquatic snakes has attracted my attention since I finished the Homalopsid Snakes, Evolution in the Mud in 2007. Books on sea snakes are available: Malcolm Smith’s classic The Sea-Snakes, William Dunson’s The Biology of the Sea Snakes, Harold Heatwole’s Sea Snakes, and Ivan Ineich and Pierre Laboutes’ Sea Snakes of New Caledonia, to name a few. However, most information on aquatic snakes remains scattered in hundreds of journals and books.

Snakes living in fresh and brackish waters had been mostly overlooked. Snakes inhabiting marine environments were restricted to a relatively few lineages, while snakes using freshwater were found in many more lineages. Furthermore, snakes use aquatic habitats in many ways that may not cause them to be easily recognized as aquatic. For example, some North American snakes spend the winter in water-filled crayfish burrows and are terrestrial or semi-aquatic the rest of the year.

This volume is perhaps best considered an illustrated, annotated checklist of aquatic and semi-aquatic snakes in marine and freshwater environments. The biggest challenge was deciding what species to include and which to exclude. While the intent was to have accounts for all aquatic snakes, where do you draw the line on the continuum? How much evidence do you need to label a species semi-aquatic? I am confident this volume covers the range of snakes that are traditionally considered semi-aquatic. However, knowledge of some of these animals is poor or absent, while others are some of the best-studied reptiles.

Snakes are derived from lizards, but surprisingly few saurians are semi-aquatic, and no extant lizards are known to be aquatic. Bauer and Jackman (2007) found 73 species in 11 families using freshwater habitats. They found no aquatic lizards inhabiting the Nearctic or Palearctic regions. Still, the Neotropics, the Oriental, and Australian regions support the greatest diversities of freshwater forms, particularly in the families Gymnophthalmidae, Scincidae, and Varanidae. Pauwels et al. (2008) listed 153 snakes (about 5% of all snakes recognized at the time) that inhabit freshwater. This number covers 44 genera in six families (Acrochordidae, Boidae, Colubridae, Elapidae, Homalopsidae, and Viperidae), with the greatest numbers in the Oriental (64 species) and Neotropical Regions (39 species).

Aquatic Snake Diversity and Natural History is a survey of the aquatic and semi-aquatic snakes of the world

LATEST STORIES

The most basal Thamnophiine clade recovered by Zaher et al. (2019) included the Liodytes, Clonophis, Haldea, and Virginia genera. These are a mixture of fossorial, terrestrial, and semi-aquatic species. Clonophis exploits an unusual aquatic habitat – crayfish burrows; both Haldea and Virginia are small fossorial species. Liodytes contain some of the most aquatic North American snakes.
The ancestor of North American natricids immigrated from Eurasia sometime before the early Miocene (~19–18 Ma). We know this because the earliest known Western Hemisphere natricid fossils date to the early Miocene, and we know they shared an ancestor because of shared anatomy and DNA.

There are no highly specialized aquatic pythons that converge with the anacondas – the highly aquatic boas. However, some python species are best considered semi-aquatic. The largest pythons (Malayopython reticulatus, Python sebae, Python natalensis, Python bivittatus) use aquatic situations to hunt and thermoregulate. The largest pythons are habitat generalists, but they are also frequently associated with water. The large body size may be possible because of their buoyance in water, making movement more efficient.

The highly aquatic and unique file snakes have loose, baggy skin and minor, rough scales, each with sensory organs. Their short tails are round, prehensile, flat, and sword-like; the eyes and valvular nostrils are dorsally or dorsolateral. Their lack of muscle tone is spectacular and can only be fully appreciated by handling one, a physiological feature restricting them to an entirely aquatic lifestyle. The rough, file-like surface of the skin and its loose folds are used in catching and holding slippery, mucus-covered fish. Their maxillary teeth are similar in size, long, and fluted on the sides and posterior surfaces. The fluted condition is found in many fish-eating snakes.

The two species that compose the genus Farancia are considered some of the most aquatic snakes in North America, but both species are known to wander overland (Steen et al. 2013). Droughts may trigger their movement from a wetland, and excessive rainfall may also trigger their movement. Females will also move overland to nest. Both species take aquatic prey. Semlitsch et al. (1988) have described juvenile emergence in both the Fall and Spring and note that hatchlings may benefit from Fall emergence by feeding before their first winter. However, Fall emergence may present the hatchlings with dried or reduced aquatic habitats, while spring emergence would have abundant habitat and food (larval salamanders).
The members of the genus are known to have maternal attendance of eggs, a highly specific diet of elongated prey, American eels (Anguilla rostrata), and the eel-like salamanders of the genera Siren and Amphiuma.

The largest boas, the highly aquatic anacondas (Eunectes), are relatively well known. The oldest and largest member of the clade (Titanoboa) was aquatic, and today, the most aquatic members of the family are the anacondas of the genus Eunectes.

Anacondas are Neotropical snakes found east of the Andes. Four extant species and one fossil species compose the genus. The clade includes the largest snake, the Green Anaconda, Eunectes murinus, which reaches at least 8 m and has the greatest mass (in females) of any extant snake (250 kg). Eunectes is derived from a Greek word that means “good swimmer.” The aquatic habitats of these snakes are well documented. Pizzatto et al. (2007) found an aquatic habitat to be a synapomorphy for this genus. The large body size, a relatively long tail, and the dorsal positioning of the eyes and nares are all traits associated with life in water.