There are many creatures that live in the water that are odd, fascinating, and frightening. But one creature that many people find to be downright creepy is the eel. Most eels are slipper and slimy, and often inspire horror or disgust in people because they seem like a cross between a snake and a fish.
The eel is actually a fish, although it doesn't look very fish-like. Its head and body look like a snake, and it has scales―but it is definitely a fish. The face of an eel can be scary, with its sharp teeth and snapping jaws. But when it flaps its mouth open and closed, as though priming itself for an attack, it's actually breathing, not being aggressive. Snorkelers are often scared when they encounter a snapping eel, but there is usually nothing to be afraid of.
Nearly every sea and ocean around the world is home to eels, and there are more than 600 species to be found. The most common and well-known is the green moray eel. The reason the moray appears green is that the slime on its skin can contain algae. The moray is actually colored dark blue. But the slime on an eel's body isn't only home to algae, it has a definite purpose, protecting the fish by way of making it slippery, and therefore it can easily slip among coral reefs and out of the hands and mouths of potential captors. The slimy skin also helps suffocate parasites or pathogens that may try to enter between the scales on the surface. The slime also protects it from sustaining further damage if there any open wounds or injuries on the skin.
Eels are aguilliformes, a type of fish that propels itself underwater by anguilliform swimming, which means swimming in a series of undulating waves. Unlike most fish, the body of an eel is elongated, and it is flexible from one end to the other. The waves caused by the anguilliform patterns cause each segment to oscillate as in a figure-eight pattern, and this loop causes the eel to be propelled forward through the water. This type of swimming is yet another distinction the eel has from other fish―it swims using its entire body, whereas other species of fish just use their tails for propelling them through the water.
Electric eels aren't really eels―they are part of a family of fish known as knifefish. These types of fish can generate electric currents within their own bodies, enough to stun a person or kill a small fish. Some countries eat a delicacy called a slime eel, but these are not true eels either, even though they resemble them. When they are agitated, they do produce a coat of thick, slimy mucous. But they are actually known as hagfish.
Actual eels are indeed slimy. In fact, the phrase 'slippery as an eel' is often used to describe someone who is devious or elusive. This mucous is produced from the glands just beneath its scales. The slime affects how much water that the fish can bring in and out of its body from the gills. Water is constantly pumped in and out in order to optimize the biochemical balance of its body, a process called 'osmoregulation'. The thinner the layer, the more water can move in and out. Some researchers believe that the amount of slime coating allows eels to move easily from seawater to freshwater without effect.
The layer of slime also fills up the minute spaces between the eel's scales to make it more streamlined, in the same way that competitive swimmers shave their entire bodies to reduce drag as they slip through the water. An eel's scales are much smaller than those of other fish, so that may also be a reason why they are much more slipperier than other fish. Removing the layer of slime will kill an eel. For this reason, many commercial fishermen douse eels with salt to remove the slime quickly and kill the fish.