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 snakelike 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 on the next page.