Moray Eels - Facts and Information for Scuba Divers

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Moray Eels Constantly Open and Close Their Mouths, Are They Aggressive?

moray eel breathing
Moray eels must open and close their mouths to breathe. ©
Some divers find moray eels frightening, but they have always looked a bit goofy to me! With tiny, bulbous eyes and flapping mouths, they remind me more of dopey canines than vicious snakes. To be fair, moray eels do tend to lurk creepily under dark overhangs and inside reef holes, and they often appear to be gnashing their teeth in an aggressive display of fangs. However, when a diver understands the reasons behind these behaviors, morays become less frightening. Moray eels hide themselves in the shadows during the daytime because they are typically nocturnal. They must continuously open and close their mouths to force water over their gills in order to breathe. What appears to be an aggressive display of teeth is merely respiration!
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Do Moray Eels Ever Attack Divers?

moray eel
Moray eels have poor eyesight. ©
Moray eels are reputed to have an excellent sense of smell, but very poor eyesight. Unfortunately, the moray's poor eyesight has led to diver injuries. While it is unusual, if not unheard of, for a moray eel to spontaneously attack a human, there have been cases in which a moray mistakenly nipped at a diver. While rare, this sort of accident almost always occurs in areas where morays are hand-fed. Morays have difficulty seeing the difference between a finger and a piece of food. To safely dive around moray eels, simply don't feed or threaten them (and don't wave your fingers in their faces).
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Morays Are Not Sea Snakes

full body of a moray eel
A rare glimpse of a free-swimming moray eel. ©
Morays are members of the Muraenidae family of fish. Despite their snake-like appearance, morays are fish, not reptiles or amphibians. An adult moray lacks both pectoral and pelvic fins, and has a long fin running from the back of its head, around its tail, and along its belly. Although this appears to be one fin, it is actually three; a fused combination of the eel's dorsal, caudal, and anal fins. A moray moves by swimming in snake-like undulations, and can propel itself through the water very rapidly.
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A Moray Eel Has Two Jaws

moray eel jaws
Moray Eels have a second set of jaws known as the pharyngeal jaws. © Zina Deretsky, National Science Foundation
Moray eels are piscivorous, meaning that they consume other fish (even smaller morays). Like some other piscivorous fish, morays have two jaws. They have a normal jaw in their mouths, called the oral jaw, and a second jaw in their throats, called the pharyngeal jaw. Unlike other fish with pharyngeal jaws, a moray eel's second jaw is highly mobile. Once a moray bites its food, the second jaw moves forward, grasps the food from the mouth, and drags it into the throat so that it may be swallowed. A fish trapped in this manner has almost no chance of escape. Interestingly, while the existence of pharyngeal jaws has been known for quite a while, the mechanism through which a moray consumes its food was only fully understood in 2007.
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Morays Are Slimey

green moray eel
A green moray's skin is not actually green!. ©

Divers who have closely observed moray eels will notice that they have smooth skin. Instead of scales, morays have skin cells that secrete a protective mucous coating, which guards them against infection and contact. Never touch a moray, as doing so will disturb its protective slim.

A moray's coating also serves a few other purposes. Sand-burrowing morays use mucous to stabilize their crumbly shelters by sticking sand particles together. The coating of some species also affects their color. Green moray eels are brown without their slime, but the yellow color of their mucous coating combines with their skin color and results in a brilliant shade of green.

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Pack Hunting

nuclear hunting
There is no eel in this photo. This was the only example I could find of nuclear hunting. Imagine a moray in the place of the grouper and you've got it!. ©

Moray eels may be observed hunting alone or in groups. When morays hunt in groups, they do not join forces with other morays, but with fish of other species. This type of hunting is known as nuclear hunting, and has been observed in a few other fish species such as trumpet fish and groupers. In "Reef Fish Behavior" by Paul Humann and Ned DeLoach, the nuclear hunting behavior of morays is described in detail:

"The eel nearly always waits for a grouper to position itself next to its body before making a break across the bottom. In every instance, the fish in contact at the beginning of the run shadows the moray's head. It appears that the two animals cooperate to insure that they arrive at the next coral head simultaneously so that the grouper can cover an escape route while the eel invades that dark interior."
One way or another, one of the animals gets fed.
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Moray Eel Habitat

ribbon moray
A ribbon moray eel shows off its brilliant colors. © Chika Watanabe, wikipedia commons
Moray eels can be found throughout Earth's tropical and temperate waters, and are most commonly associated with coral reefs. Most morays are marine, but some species have been known to tolerate brackish (mixed fresh and salt) water, and a few species live only in fresh water. Morays can be found as deep as several hundred meters in the ocean, and are usually observed hiding in shelter holes or under overhangs during the daylight hours. Only a few species of diurnal (active during the day) morays exist. It is an unusual treat to see a free-swimming moral eel jetting across the reef during a dive.
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Moray Eel Reproduction

A leptocephalus fish larvae.
A leptocephalus fish larvae. © wikipedia commons, Professor Uwe Kils
Female morays lay eggs and males fertilize them. Young morays hatch as a free-swimming lectocephalus larva. Lecotcephalus-type larvae are unusual among fish larvae in that they have flat, relatively large, transparent bodies and can already swim like eels. Only eels and closely-related fish hatch in this form. The moray larvae float among plankton for a period of time before becoming juveniles. Most species of morays can begin reproducing at about three years of age.
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Cleaning Behavior

moray eel
Cleaner shrimp remove parasites from a moray eel. ©

Divers who observe a resting moray with a hypnotized expression on its face should look closely to see if the moray is being cleaned by a small fish or shrimp. Cleaning is a symbotic behavior, benefiting both the moray and the cleaner fish. Parasites often attach themselves to morays near their gills and mouths. Small fish and shrimp remove and eat these parasites. Moray eels understand this cleaning behavior, and will not attack cleaner fish while they are at work. Such tiny shrimps and fish wouldn't be much of a snack for a moray anyhow!

Learn More About Aquatic Life:
•  Whale Sharks
•  Seahorses and PipeFish
•  Sea Turtles
•  4 Types of Rays
•  Yellowhead Jawfish