Eel vs Snake: How to Tell a Fish From a Reptile

Key differences at a glance

Long bodies and no legs make eels and snakes look alike, but the shared shape is another case of convergent evolution: a long, flexible body is a good design for sliding through mud, sand, crevices and water. In every other respect they belong to different worlds.

True eels form the order Anguilliformes, with about 1,000 species, including freshwater eels, moray eels, conger eels and snake eels. Snakes are reptiles of the suborder Serpentes, with about 4,000 species, most of which live on land. Their closest relatives are lizards, and some legless lizards, such as the slow worm of Europe, cause their own confusion.

Gills and fins: the quickest clues

The surest sign of an eel is a pair of small gill openings just behind the head, through which water leaves after passing over the gills. Snakes have no gill openings, only nostrils near the tip of the snout, and they must breathe air.

Look also for fins. Most eels have a long dorsal fin running along the back and an anal fin underneath, often joined around the tail into one continuous fin. Freshwater eels such as the European eel and conger eels also have small pectoral fins just behind the head, while morays and many snake eels have lost them. Snakes never have fins of any kind.

Skin, scales and tongue

Eel skin is covered in a thick layer of mucus that makes it slippery. Freshwater eels have tiny scales buried deep in the skin, and morays have no scales at all. Snakes are covered in dry, overlapping scales made of keratin, with wide belly scales in most land species, and they shed their whole skin in one piece from time to time.

A snake constantly flicks a forked tongue to collect scent particles and pass them to a smelling organ in the roof of its mouth; see why do snakes flick their tongues?. Eels do not do this. Snakes also have no eyelids, only a clear scale called a spectacle over each eye. Eels lack eyelids too, as most fish do, but their eyes are not covered by a shed scale.

How they move and where they live

Both move with S-shaped waves along the body. Eels swim this way so typically that scientists call it anguilliform swimming, after the eel’s scientific name. Many snakes swim well with the same motion, and on land they also use other methods, such as gripping with their belly scales and moving in a straight line, or sidewinding across loose sand.

Eels live only in water. Some, such as the European and American eels, spend years in rivers and lakes before migrating to the sea to spawn, and they can wriggle across wet grass on rainy nights to reach a new pond. But they cannot breathe dry air for long. Snakes are land animals by origin. Many hunt in ponds and rivers, but even water snakes must come up to breathe.

Reproduction and life cycle

Eels release huge numbers of tiny eggs into the sea. The larvae, called leptocephali, are flat, transparent and leaf-shaped, and they drift with ocean currents for months before turning into small, see-through glass eels. Freshwater eels breed only once, at the end of a long life, and then die. The European eel is believed to spawn in the Sargasso Sea in the western Atlantic, although no one has yet observed it doing so in the wild.

Snakes mate on land, and fertilization is internal. Most species lay leathery-shelled eggs in a warm, hidden place, while others, including most vipers and boas, give birth to live young. Baby snakes look like small adults and are independent from birth.

Confusing cases: sea snakes and snake eels

The trickiest pair is the sea snake and the snake eel. Sea snakes are true reptiles closely related to cobras, with about 70 species in the warm Indian and Pacific oceans. They have a flat, paddle-shaped tail for swimming, nostrils that close under water, and can take in some oxygen through the skin, but they still have lungs and must surface to breathe. Most give birth to live young at sea, while sea kraits come ashore to lay eggs. Sea snakes are venomous and should never be handled.

Snake eels, family Ophichthidae, are true eels that often look like sea snakes. Many are banded or spotted, and some, such as the harlequin snake eel, closely resemble venomous sea kraits, which may help protect them from predators. They burrow backward into sand using a hard, pointed tail tip. To tell them apart, look for the gill openings, fins along the back and the lack of a paddle-shaped tail.

Animals called eels that are not eels

Several fish carry the name eel without being true eels. The electric eel of South America is a knifefish, related to catfish and carp, and breathes air; see is an electric eel an eel?. Swamp eels and spiny eels belong to other fish orders, and the wolf eel of the North Pacific is a wolffish. Lampreys and hagfish, sometimes called lamprey eels and slime eels, are jawless fish that are not related to eels at all.

Which is bigger and which is dangerous?

The biggest snakes are much larger than any eel. The reticulated python is the longest snake, with well-measured individuals over 6 m (20 ft), and the green anaconda is the heaviest; see the largest snakes in the world. Among true eels, the European conger is reported to reach about 3 m (10 ft), and some slender morays grow to similar lengths, but most eels are well under 1.5 m (5 ft).

Most eels are harmless to people. Morays can bite if someone reaches into their crevice, and an electric eel’s shock can knock a person over. Only a minority of snakes are dangerously venomous, but where they occur, it is best to keep your distance and never try to handle any snake. If someone is bitten by a venomous snake, seek emergency medical help immediately by calling 911 in the US, 112 in the EU or 999 in the UK.