If you strip away the reputation, an eel is a fish that has taken one idea, length, as far as it will go. Almost every part of its body has been reshaped to suit life in narrow spaces: holes in coral, burrows in sand, mud under a riverbank or the interior of a shipwreck. That’s why eels have lost some fins, shrunk others, coated themselves in slime and moved their organs forward. This guide works through eel anatomy from the outside in, and compares true eels with the electric eel, whose anatomy is something else again.

Body plan: a fish built like a rope

Long, flexible and nearly the same thickness from head to tail.

A true eel’s body is a long cylinder, often slightly flattened from side to side toward the tail. Instead of a deep body and a forked tail like a typical reef fish, it has a slender trunk and a tail that tapers to a point or a rounded tip. That shape lets an eel move by sending waves of bending down the whole body, a style of swimming called anguilliform after the eels themselves. It’s slow compared with a tuna but very efficient, and it works equally well in open water, inside a crevice or across wet mud.

The secret is the backbone. Eels have far more vertebrae than most fish. Many species have well over a hundred, and some have over two hundred, each a short segment with flexible joints between. That gives the body the ability to bend in tight curves, coil into a hole, reverse out backwards and, in morays, tie itself in a knot for leverage when tearing prey. We look at the bones in detail on our eel skeleton page.

Fins: lost, merged and reduced

An eel’s fins are mostly one long frill.

True eels have no pelvic fins at all. In most species the dorsal fin along the back, the tail fin and the anal fin underneath have merged into one continuous fin that runs around the rear of the body. That ribbon is what an eel uses to fine-tune its swimming and steering. Freshwater eels and congers keep a small pair of pectoral fins just behind the head, which help with steering and holding position. Morays have lost even those, which gives them a smooth, snake-like profile with nothing to snag on coral as they slide into a hole.

Some snake eels go further still. Many have almost no fins, just a low ridge, and a hard, pointed tail tip that works like a drill for burrowing backward into sand. That tail tip is so effective that snake eels swallowed alive by predatory fish have been found to bore their way out of the stomach.

Skin, slime and scales

The “slippery as an eel” reputation is earned.

Eel skin is thick and packed with mucus cells. The slime it produces does a lot of work: it reduces friction, protects against scrapes from coral and rock, makes it hard for parasites and bacteria to get a hold, and in some species carries compounds that taste bad or irritate predators. Garden eels use it to cement the walls of their burrows. In the green moray, a yellowish mucus layer over dark skin is what gives the eel its green colour, as we explain in what color are eels.

Underneath that slime, most morays and congers have no scales at all. Freshwater eels do have scales, but they’re tiny, oval and buried deep in the skin, so small that many people assume eels are scaleless. Our page on do eels have scales has the details. Freshwater eels can also absorb some oxygen through their skin when it stays damp, which helps them survive short trips over land.

The eel’s face: mouth, nostrils and eyes

An eel’s face is built around its nose.

Look an eel in the face and the first thing you’ll notice is the mouth. In morays it’s long, reaching back well behind the eyes, and it’s often held slightly open, which shows off the teeth. That gaping is how a moray breathes, not a threat. In freshwater eels the mouth is smaller, with the lower jaw slightly longer than the upper. Congers have a heavy overbite and thick lips.

Eels have two pairs of nostrils. The front pair sit at the tip of the snout, often on short tubes, and the rear pair sit near or above the eyes. Water flows in one and out the other over a folded smell organ with a very large surface area. Eels have one of the keenest senses of smell among fish, and morays rely on it to find prey in the dark. The ribbon eel’s front nostrils are flared into leaf-like flaps, which gives it its distinctive look.

The eyes are usually small, especially in morays, which see poorly and hunt mostly by smell. Freshwater eels are an interesting exception: when they turn into silver eels before their ocean migration, their eyes enlarge dramatically and their visual pigments shift toward blue light, preparing them for life in deep ocean water.

Jaws and teeth, including the second jaws

Morays have a feature no other animal uses in quite the same way.

All bony fish have pharyngeal jaws, a second set of toothed bones in the throat used to process food. In most fish they stay in the throat. In morays they’re unusually mobile. When the front teeth have hold of a fish, the pharyngeal jaws shoot forward into the mouth, grab the prey and drag it back into the gullet. Researchers at the University of California, Davis, filmed this in 2007, and it lets morays swallow large prey inside a crevice where they can’t use suction the way most fish do. The details, with the skull structure, are on our moray eel skull page.

Teeth vary with diet. Fish-eating morays have long, sharp, backward-curved fangs, while crab-eating morays such as the zebra moray have flat, pebble-like crushing teeth. Freshwater eels have small, closely packed teeth in bands. Garden eels, which pick plankton, have small mouths and small teeth.

Gills and breathing

Small gill openings, and in some eels, a bit of help from the skin.

Eels breathe with gills like other fish, but the gill openings are small, often just a round hole or short slit on each side behind the head. Small openings keep sand and debris out and suit an animal that lives in holes. The downside is that eels have to work to move water over the gills, which is why morays rhythmically open and close their mouths: they’re pumping water in through the mouth and out through the gill openings. Eels also have a well-developed swim bladder that helps them control buoyancy, although bottom-dwelling species use it less.

Inside the eel: organs and blood

Most of the organs are packed into the front of the body.

In a long, thin animal, the internal organs can’t be spread along the whole length without getting in the way of flexing. In eels the heart, liver, stomach and most of the intestine sit in the front portion of the body, with the vent placed some distance back. The rest of the body is mostly muscle wrapped around the spine. That muscle is rich in fat in freshwater eels, which store energy for their long migration.

The digestive system changes with life stage. When a freshwater eel becomes a silver eel and stops eating, its gut shrinks while its gonads grow. Eel blood is also unusual: it contains a protein toxin that can irritate cuts and eyes and harm people if consumed raw. Heat breaks it down, which is why eel is always cooked. The life stages behind these changes are explained in our eel life cycle guide.

Senses beyond smell

Eels feel the water around them.

Like other fish, eels have a lateral line system, a row of pressure-sensitive pores and canals along the head and body that detect movement and vibrations in the water. In an animal hunting at night inside a reef, that’s almost as important as smell. Eels can also sense changes in temperature and salinity, and research suggests migrating eels may use the Earth’s magnetic field to help navigate. Freshwater eels appear to use smell and water chemistry to find river mouths when they arrive on the coast as glass eels.

Anatomy of an electric eel

An electric eel is mostly battery.

The electric eel isn’t a true eel but a South American knifefish, and its anatomy shows it. It has no dorsal fin. Instead, a long anal fin runs along the belly and does the swimming, rippling to move the fish forward or backward while the body stays fairly straight. About four-fifths of its body is taken up by three paired electric organs, the main organ, Hunter’s organ and Sach’s organ, built from thousands of modified muscle cells called electrocytes stacked like batteries in series. The vital organs are crowded into the front part of the body behind the head. It also breathes air, gulping at the surface and absorbing oxygen through a highly vascular mouth lining, so it will drown if it can’t reach the surface.

I find eel anatomy a good lesson in how far evolution will push a design. Everything about an eel’s body traces back to a long body in a tight space. If you’re interested in the skeleton specifically, see the moray eel skeleton and eel bones.