Why are eels slippery?

One of the creepiest animals you can encounter in the water is the eel. It's a slippery, slimy creature, and it doesn't fit neatly into the water creature categories we've set up in our brains, which only amplifies our fear of it. Is it a snake? A fish? An unholy hybrid of both?

Actually, the eel is just a regular fish -- that doesn't look like one. It has a serpentlike head and a snakelike body. And it also has a scary-looking face. If you've ever spotted one while snorkeling, you probably did an about-face after you got a glimpse of its snapping jaw and sharp teeth. That frightening mouth opening and closing, though, is just the eel "breathing" -- not a sign of aggression.

You'll find eels in nearly every ocean and sea. The green moray eel is the most common and well-known eel, but there are more than 600 species of eels around the world [source: Gerber]. Eels are fish of the order Anguilliformes. Anguilliformes propel themselves underwater by a means known as anguilliform swimming. Unlike other fish, an eel's body is elongated and flexible from end to end. When it swims, it moves in a series of waves. These waves cause each segment of the eel's body to oscillate in a figure-eight loop. This movement causes the eel to propel forward in the water. Anguilliform swimming differentiates the eel from other fish -- the eel swims with its entire body, while other fish mostly use just the tail-end.

Now that we've nailed down what an eel is, let's discuss what it's not -- for example, an electric eel. Although they resemble eels, electric eels aren't true eels at all. They actually belong to a family of fish called knifefish. This fish can generate an electric current in its own body -- enough to stun you or kill a small fish.

Another thing an eel is not is a slime eel. A delicacy in some countries, these bottom-dwellers look like small eels. And when agitated, they do indeed produce a very thick and slimy coat of mucous. However, slime eels aren't eels at all -- they're traditionally known as hagfish.

True eels, though, are certainly slimy. You may have even heard the expression "slippery as an eel" to describe someone who is particularly elusive or devious. But are eels more slippery than regular fish?

The Eel's Slime Layer

All fish have slime covering their body, which makes them very difficult to catch by hand. Produced from glands beneath the scales, this mucous gives the fish protection from a variety of dangers. It serves three main purposes:

Osmoregulation: Just like us, fish are mostly made of water. Fish constantly pump water in and out of their gills in order to keep an optimum biochemical balance in their bodies. We call this process osmoregulation. The slime level on the fish's body affects how much water the fish can take in and out -- a thinner slime layer allows more water in and out, and a thicker slime layer allows less. Some scientists believe this coating also allows the eel to move from ocean to freshwater -- and vice versa -- without trauma.

Physical protection: The slime layer physically protects the fish by making it slippery. The slimy surface helps to suffocate pathogens or parasites trying to enter through the fish's scales. The mucous also protects any open wounds from further external damage and lets the fish slip through barriers like coral or your hands with relative ease.

Aerodynamics: The slick layer of slime allows a fish to propel itself faster through the water. It fills up the tiny spaces between the scales to make the fish more streamlined -- much like the way competitive swimmers shave their bodies in order to reduce drag.

The slime on an eel can contain algae. This is why the common moray eel appears green, even though its skin is actually dark blue. Also, the scales of an eel are quite small compared to other fish. In addition to its long snake like shape, scale size might be a reason why eels are so much more slippery to the touch than other fish.

Removing the slime layer will kill the eel. Many commercial fishermen sprinkle salt on their caught eels in order to more quickly remove the slime and kill the fish. Also, eel blood is quite toxic. Although eel is a delicacy in many cultures, you should always cook it first in order to remove the toxic protein [source: Animal Planet].

Speaking of cooking eel, did you know that a freshly killed eel will continue to move for some time? Given that and its slimy skin, most chefs recommend nailing the dead eel's head to a board to make the skinning and filet process less problematic. Or, if you're not feeling particularly murderous, you can just go to your favorite sushi restaurant and let them prepare it for you!

To find out more about slimy eels and other fish, check out the links below.

Why are eels slippery?

The Eel's Slime Layer

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