Sea snakesHydrophiinae

A sea snake is a cobra that moved into the water. They sit inside the elapids, the family of cobras, kraits and mambas, and the true sea snakes went further into the sea than any other living reptile: they mate, give birth and die without ever touching land, and a body built for swimming can barely drag itself across a beach. The venom is potent even by elapid standards, though bites happen almost only to people who pick one up.

The body is rebuilt from end to end. The tail is flattened sideways into an oar. The nostrils sit on top of the snout and close with valves. A single lung runs most of the length of the animal and doubles as a buoyancy tank it can adjust. Salt leaves through a gland under the tongue, flicked out with the tongue itself. And the skin takes up oxygen directly from the water, enough of the animal's requirement that a dive can run far longer than the lung alone would permit.

What they cannot do is drink. A sea snake is surrounded by water it cannot use, and it depends on rain: after heavy rainfall a lens of fresh water floats briefly on the surface, and that is when they drink. Between rains they go thirsty for long stretches, and thirst, rather than food or temperature, sets the edge of where some species can live.

None of this happens in the 🌊 Red Sea. The one species listed in this section, the yellow-bellied sea snake, is the most widely distributed snake on Earth and still does not reach this basin, and the barrier appears to lie at the southern approaches. What divers here take for a sea snake is a banded snake eel or a moray out hunting in the open: an eel has gill openings, fins and no scales, and it never comes up to breathe.