#mangrove saltmarsh snake

[snototter]

A mangrove saltmarsh snake (Nerodia clarkii compressicauda) in Key West, Florida, USA

by John Williams

[aquariuminfobureau]

Fishes do not drink, so it's not an issue for them, to provide drinking water. The ambient water is enough. Frogs do not drink either, according to studies of their physiology and behavior, including the dry skinned bufonids, as well as moister skinned ranoids and pipids. Nearly all of their water intake is actually transdermal, the rest of it being swallowed during feeding or under stressful conditions.

Among the line leading to the amniotes, which include the mammals and reptiles, true drinking behavior first appears. Usually the means of drinking is hyoid pumping, but some reptiles and mammals have lost this ability, for example most birds use gravity to swallow water by throwing their heads back, and it's thought that cetaceans rarely if ever drink. Other methods of water intake do exist among mammals and reptiles, including obtaining moisture from their foods.

Some reptiles are able to take up water through the wall of their cloaca, or by way of their skins, although the structure of reptile skin is less permeable than is that of amphibians. Arboreal reptiles do not easily have access to pools of water, and for this reason, their environmental humidity is important to them. Keeping them in an aquaterrarium, although they might not use the water as living space, helps arboreal species to stay hydrated. They should also have water droplets to lick at.

Special mentions should be made of reptiles inhabiting saltwater environments, and of the extra drinking behaviors of those reptiles that can easily excrete excess salts from ingested seawater. Sea snakes and some seabirds are physiologically adapted to cope with salts, yet they persist in drinking rainwater, which is of course freshwater, from the ocean surface. Thus captives of these species need to be given freshwater to drink, for example by pouring it on the water surface.

Brackish water habitats, such as mangrove forests and saltmarshes, are frequented by a number of reptile and even amphibian soecies, although amphibians as a group are quite averse to salt in their environment. Only some of these animals possess salt glands, and of those that do, it may be less developed than in marine reptiles.

Some of these animals, like snapping turtles, survive in these brackish waters by moving between brackish and freshwater water bodies. Others like Australasian snake necked turtles, simply possess physiological adaptations, to survive without drinking between rains, as an alternative to evolving a salt gland. In any case, the survival of most estuarine reptiles and amphibians, depends on freshwater, and freshwater should be provided for them.

[aquariuminfobureau]

It's a received wisdom that amphibians are intolerant of salinity in their water. With the usual caveat that Rana (or Fejervarya) cancrivora, a superficially ordinary Rana-like frog from Asia, is 'the only marine amphibian' and is presented as almost a miracle of nature. In fact, prehistoric amphibians were preserved in saltwater environments, and a few species of salamanders and frogs are tolerant of mildly brackish conditions.

One study investigated the salinity tolerances of tadpoles, belonging to a few species of North American frogs. All of the species investigated had near-perfect rates of survival at 5 parts per thousand of salinity. Where estuaries are of such a low salinity, it is clearly the freshwater components of estuarine biota that predominate. Nonetheless the salinity is elevated sufficiently at only 5 ppt, to qualify such water as brackish. The tadpoles of all the species suffered and died at higher percentages of salinity.

Thus it can only be remarkable when amphibians survive, and thrive, at higher salinities than 5 ppt. A large meta-analysis of amphibian salt tolerance, revealed that the majority of amphibian occurrences in brackish environments, predictably involved low salinities of about 5 ppt or below. Of the others, most involve salinities up to about 10 ppt. For contrast, seawater has a salinity of 33-38 ppt, and is usually communicated as 35 ppt, as though it were the same everywhere.

A salinity of 10 ppt is still at the low end of brackish salinities, below a specific gravity reading of 1.01. Therefore it is below the salinity expected to be maintained I a standard mangrove tank. A salinity of merely 5 ppt, is below 1.004. The mangrove associated R. cancrivora, is however able to complete it's life cycle, where the specific gravity would be 1.02.

It is also more significant when aquatic life stages are tolerant of saltwater, because amphibious, terrestrially motile animals may move to find freshwater. A number of species of amphibian adults, such as toads and mole salamanders, have been recorded in the field, in and around waters more saline than their proven or probable physiological tolerance.

Rana cancrivora are observed in the field to be at least temporarily present in waters with full marine salinity. Tadpoles of this species tolerate this, but the adults seem to have an upper limit of tolerance that is lower than their tadpole stages, at 28 ppt. Other amphibians with a fairly good physiological tolerance of salinity, seem limited to water where it is no more brackish, than 10 ppt or close readings.

These include the American bullfrog (Rana catesbiana), green treefrog (Hyla cinerea), Cuban treefrog (Osteopilus septentrionalis), yellow-bellied toads (Bombina variegata), common platanna (Xenopus laevis), and tiger salamander (Ambystoma tigrinum). Few amphibians tolerate higher than 10 ppt.

Adult cane toads (Rhinella marinus) live around waters with a salinity close to 15 ppt, although its life cycle is known to be impaired at salinities as low as 8 ppt, and 12 ppt is lethal to their tadpoles. Thus R. marinus, although they forage in coastal areas where the salinity is higher, are really among those amphibians bound to lower salinities of around 5 ppt or lower.

Which is to say, the aquatic life stages of an amphibian with amphibious or terrestrial adults, may be essentially a freshwater element that is present in certain estuarine environments. Some snakes and turtles similarly frequent mangroves and saltmarsh habitats, but require environmental freshwater, for example, moving to freshwater in order to drink, but feeding in brackish water.

This is the hybridity of the estuarine biota, with its marine and freshwater components, and also the bivalence of most amphibian life cycles, being able to breed in one environment, even microenvironment, whilst exploiting elsewhere. Typical amphibian life cycles are not inferior to those of reptiles and mammals, or the amphibians would not be so ubiquitous.

Which does in fact raise the question, of why there aren't any truly marine frog species. R. cancrivora and at least two other ranids, demonstrate that amphibians can actually evolve physiological tolerance to elevated natural salinities. Although this seems more common in some clades than in others. Asking the question is like asking why certain successful fish clades have remained essentially freshwater animals, such as cyprinids, loaches, tetras, and for the most part, catfishes.

Surely more than one factor is responsible, and possible explanations won't be mutually exclusive, when it comes to explaining why or why not. But competitive exclusion by clades that are established and dominant in saltwater environments, is a limit upon the range of niches, that are available for colonization, by intrusive pioneers moving down from upriver.

Adult frogs do not do very well in fast flowing waters, so they are unlikely to do very well where the tide is powerful. Some anurans have specialized tadpoles, bearing derived lifestyles and morphologies, that are appropriate to habitats such as fast flowing streams. But in such environments, fish competitors are relatively few. In the sea, frog tadpoles without mobile, paired hydrofoils, would need to compete against fishes with them, and other successful competitors.