Geography of Aquatic Snakes 1.

North America and the Caribbean

Aquatic North American snakes represent three family/subfamily level clades, Dipsadidae, Natricidae, and Viperidae.

Dipsadidae snakes are diverse in Central and South America. But relatively few species occur in North America, and only Farancia is aquatic. Grazziotin et al. (2012) found the phylogenetic placement of North American dipsadids unstable and controversial.
There are three semi-aquatic vipers in North America. First, the Massasauga Rattlesnake, Sistrurus cantenatus, may use an unconventional aquatic habitat – flooded crayfish burrows for over-wintering. But they also use mammal burrows and logs for over-wintering. Secondly, the two semi-aquatic viper species are the Northern Cottonmouth, Agkistrodon piscivorus, and the Southern Cottonmouth, A. conanti. These snakes spend much of their time in shallow water swamps, catching fish, frogs, and other prey. Wüster et al. (2008) found the New World pitvipers diverged from their Asiatic sister group at approximately 24 MYA, and the first radiation of the New World clade occurred at about 22 MYA.

Most North American aquatic snakes are in the family Natricidae, and aquatic and semi-aquatic taxa occur in five genera forming the clade Thamnophiini. Clonophis, Nerodia, Liodytes, Regina, and Thamnophis. These species shared an ancestor with the Eurasian genus Natrix about 29 MYA, and radiated about 12 MYA in North America (Depaak et al. 2022).

The Thamnophiini inhabit the most extensive wetlands in North America, the Gulf Coastal Plain, the western US, and Mexico. All of these species are viviparous
The Gulf Coastal Plain extends from the southeast coast of the US to southern Texas and into Mexico and expands inland at some locations for hundred miles. The region covers over 250 million square kilometers. The area has high biodiversity and was long overlooked as a global biodiversity hotspot. Noss et al. (2015) showed the Gulf Coastal Plain is older, more climatically stable than usually assumed, and rich in species, some of which are endemics. They consider the region a biodiversity hotspot. The Lower Mississippi Valley (including the delta) was North America’s largest forested wetland ecosystem. The floodplain extends from the confluence of the Mississippi and Ohio rivers at Cairo, Illinois, to the Gulf of Mexico. The drainage basin encompasses 41% of the continental United States. The floodplain is a mosaic of ridges, swales, meander belts, and swamps that support a diverse and ecologically rich forested wetland ecosystem.
The area also includes the Everglades of southern Florida – a shallow, slow-moving river from Lake Okeechobee to Florida Bay dominated by grass. Water levels drop during the dry season except for the deeper locations. As a result, the region supports rich biodiversity and many invasive species that arrived via the pet trade, including the highly publicized invasive Burmese Python.

The Cuatro Ciénegas Basin is a Chihuahuan Desert wetland in Coahuila, Mexico. It includes several natural springs that supply over 200 small ponds and wetlands, covering 1500 square kilometers. The wetlands are part of the UNESCO-designated Cuatro Ciénegas biosphere reserve. The basin has long been known for its exceptional biodiversity, including North America’s most significant number of endemic species (Stein et al., 2000). The region is a biological island on the Sierra Madre Oriental’s northern extension and ranges in elevation from about 740 m on the grassy basin floor to above 3,000 m in the surrounding mountains. Abundant freshwater originates in springs that emerge from hundreds of interconnected, spring-fed pools and spring-fed lakes with endemic fish, aquatic snails, and crustaceans. Semi-aquatic snakes found here are in the genera Thamnophis and Nerodia.

To the south, the trans-volcanic belt in southern Mexico supports seven endemic semi-aquatic Thamnophis eques populations that have been recognized as subspecies.

The Gulf of California has about 300 estuaries and other wetlands and is the northernmost limit for the distribution of mangroves in the Eastern Pacific. On the west coast of the Gulf of California, mangroves are distributed from the Cape region to the center of the Baja California peninsula. These are primarily in small bays, estuaries, and isolated pockets. On the Gulf’s eastern side, mangrove forests are distributed southward from Tiburon Island in Sonora to Sinaloa and Nayarit in large coastal lagoons with extensive mangroves. On the Pacific side, the largest mangrove forests occur inside the coastal lagoons of Magdalena Bay. (Aburto-Oropeza et al. 2008).

The region contains the endemic semi-aquatic garter snake, Thamnophis validus, a polytypic species that use mangroves and freshwater habitats. The area also supports the Yellow-bellied Sea Snake, Hydrophis platurus, an elapid. It is unclear if Hydrophis platurus is a resident population that migrates in and out of the area or is composed of waifs.

Relatively few semi-aquatic and aquatic snakes occur in Central America. The snake genera containing aquatic species are the tropidophiid genus Trachyboa is represented in Panama; the dipsadid genera: Hydromorphus, Tretanorhinus, and Erythrolamprus, and the natricid genera Nerodia, and Thamnophis. Central America covers about half a million km2 with steep topography concentrating wetlands in lowland and coastal areas. Lake Nicaragua is the largest body of fresh water in Central America. The limestone plain of the Peten in northern Guatemala has a mosaic of lakes, rivers, and wetlands, including the Laguna del Tigre, a vast expanse of palm forests, flooded forests, grasslands, and mudflats, which together form the largest Central American wetland. Coastal Belize has a chain of saltwater and freshwater lagoons surrounded by flooded forests, mangroves, and swamp forests, along much of its coastline. The Caribbean coasts of Honduras, Nicaragua, and Costa Rica have tidal lagoons with mangroves, swamp forests, and palm swamps that stretch inland and cover much of the landscape. Mangroves occur on the Atlantic and Pacific coasts, and the largest concentrations occur in the Gulf of Fonseca, Nicoya, and the Gulf of Panama, Hydrophis platura is a likely resident. Unconfirmed records of the sea krait Laticauda colubrina from the Pacific coasts of several Central American countries: Nicaragua (Villa, 1962), Mexico (Alvarez de Toro, 1982), and El Salvador are also noted by Cogger and Heatwole (2006).

There are no sea snakes in the Caribbean (but see Shuntov 1965; Hernández- Camacho et al. 2006) or Atlantic, and very few aquatic snakes on Caribbean islands. However, the Mangrove Water Snake, Nerodia clarki (a natricid), is in the Bahamas, and the tropical water snake, Tretanorhinus variabilis (a dipsadid), is present in the Cayman Islands, Cuba, and on some islands off the coast of Central America. Trinidad’s continental fauna has six aquatic species, the booid Eunectes murinus, the dipsadids Hydrops triangularis, Helicops angulatus, two species of Erythrolamprus (one of these is found on Tobago), and one species of Thamnodynastes.