Head to head
Sea Snake vs Eel
Sea snakes and eels overlap on tropical reefs and are easy to confuse from a distance, both being long, sinuous, and swimming with an S-shaped motion. One is a reptile that must surface to breathe, though, and the other is a fish that never leaves the water.
| Sea Snake | Eel | |
|---|---|---|
| Animal group | Reptile (cobra family) | Fish (Anguilliformes) |
| Breathing | Air; must surface | Water, through gills; never surfaces |
| Body covering | Dry scales | Slimy skin, fine or no scales |
| Tail and fins | Flattened paddle tail, no other fins | Continuous fin around the tail |
| Venom | Yes, potently neurotoxic | None (morays bite hard) |
| Tongue | Forked, flicked to smell | Not forked, not used to smell |
| Young | Live birth at sea in fully marine species | Eggs; drifting larvae |
| Salt handling | Salt gland under the tongue | Gills and kidneys |
Sea Snake vs Eel: common questions
How can I tell a sea snake from an eel while diving?
Watch the tail and the surface. A sea snake has a distinctly flattened, oar-like tail that ends in a paddle, its body is often banded, and it will periodically swim up to the surface, poke its head out for a breath, and go back down. An eel has a fin that runs continuously along the rear of its body and around the tail with no paddle, it stays on or near the bottom or in a crevice, and it never goes to the surface for air. A moray eel in particular usually shows only its head sticking out of a hole, opening and closing its mouth to pump water over its gills. If in doubt, keep your distance from anything long and snake-like on a reef.
Are sea snakes or eels more dangerous?
They are dangerous in different ways, and both risks are low for a careful diver. Sea snakes have among the most toxic venom of any snake, so a serious envenomated bite can be life-threatening, but sea snakes are placid, rarely bite unless grabbed or trodden on, often give dry bites, and have short fangs that struggle through a wetsuit, so bites are uncommon and mostly happen to fishers handling nets. Eels are not venomous, but a large moray has powerful jaws and backward-pointing teeth and will bite hard if cornered, threatened, or hand-fed, and the wound can be deep and prone to infection. In both cases, not touching and not feeding the animal removes almost all the risk.
Is a sea snake just an eel that breathes air, or the other way round?
Neither; they are unrelated animals that evolved a similar shape for similar reasons. The long, thin, finless-looking body with side-to-side swimming is an efficient way to move through water and to thread into reef crevices after hidden prey, and it has arisen many times in fish and in reptiles. Sea snakes came from land snakes, themselves from lizards, and only entered the sea a few million years ago, keeping their lungs and venom. Eels have been fish the whole time, with gills and fins, for hundreds of millions of years. The resemblance is convergence, the same solution reached from opposite starting points, much like the resemblance between a shark and a dolphin.