Moray Eels: A slimy underwater alien you'll fall in love with!

Updated: Nov 2, 2020

Before I started scuba diving I knew nothing about moray eels, I don't think I even knew they existed! However, once I'd seen them underwater that was it, I was OBSESSED! There's nothing like seeing one of their little heads poking out from a hole in a reef, with their wide eyes and open mouths - they almost look like they are in shock at the sight of you passing by! So in honour of moray eels, and the fact that I'm currently landlocked due to coronavirus and miss them greatly, here is everything you need to know about moray eels!


Who and what are they I hear you cry!

Moray eels, like their name suggests, are a type of eel. There are around 200 different species of moray eels and they belong in the family Muraenidae. They vary in size, weight, colour and pattern. The largest moray eel is the giant moray (Gymnothorax javanicus) which can weigh a whopping 30kg and grow to 3m in length!! On the opposite end of the scale, the minute moray (Anarchias galapagensis) reaches a maximum length of 14cm and weighs just a few ounces. Not only do they vary in size and weight between species, but there is also a large difference in how long moray eels live for, they can live anywhere between 10 - 30 years. Although there are quite a few differences between species there are key similarities. One of them being that moray eels have a long fin that runs the entire length of their body it's, in fact, three fins merge together the dorsal, caudal and anal. This gives them a streamlined and serpent type look.


Two moray eels showing off their patterns by venturing outside of their hiding places


Where do you find them?... More like where don't you find them!

Moray eels are found pretty much worldwide! Most species live in marine environments, but a few are seen in brackish water and there are even some that live in freshwater! - the many-toothed moray (Gymnothorax polyuranodon) is the most well-known moray found in freshwater. They are also found in both tropical and temperate ocean environments. You'll often find them sheltering in dead patches of the reef, around coral rubble rocks or sometimes on live coral reefs.


They produce how many eggs?!

Like most other fish moray eels lay eggs, usually in dark secluded areas. However, some species release their eggs straight into the water to free float. Releasing eggs into the water column combined with the fact that moray eels leave their young to fend for themselves, once the larvae hatch, makes the death toll on these youngsters quite high. To combat this moray eels can release thousands of eggs at a time, for instance, the Mediterranean moray (Muraena helena) can lay around 60,000 eggs at once!


The snowflake moray eel (Echidna nebulosa) looks absolutely beautiful with its striking pattern!



Aggressively baring their teeth or just flashing you a smile?... Probably neither

If you've seen a moray eel in the water than you'll know that usually, they have their heads sticking out a small hole in a reef or rock with their mouths open. They constantly open and close their mouths so they can breathe! - The reason they have to do this is moray eels have particularly small gills for their body size, opening and closing their mouths keeps water flowing through them and allowing them to breath. So don't be alarmed if you see one biting away its not a sign of aggression they are just trying to stay alive!


Two white-eyed moray eels (Gymnothorax thyrsoideus) looking like I've cut them off mid-sentence when really they are just making sure they are getting enough oxygen



Their two sets of jaws are something straight out of Alien (1979)!

Bony fish usually rely on suction inside the mouth cavity to capture their prey. Moray eels don't have as good a suction mechanism as these other fish, which had previously confused researchers, how could these moray eels hold on to their prey long enough to swallow it? Well instead of suction moray eels have a second set of jaws in the throat (pharyngeal jaws) which they thrust forward to grab hold of the prey inside their mouth. Once the prey is captured by this second jaw it is then drawn towards the oesophagus and devoured. Though throat jaws aren't exclusive to moray eels, the use of them to capture the prey and pull it backwards is! Now doesn't that sound familiar? - okay so it might not be as dramatic as the extraterrestrial from Alien, which catapults its miniature set of teeth all the way out of its mouth, but it's still pretty impressive and it's part of the reason moray eels are such successful predators!


The morays second jaw is in its throat when resting (left) and comes forward to pull prey backwards when caught (right) - not too far off the extraterrestrial from Alien (middle) although maybe less dramatic!


Bad eyesight... Yet a pro hunter!

Moray eels have relatively small eyes in comparison to the rest of their body, meaning their eyesight isn't the best. Though they may be visually impaired it doesn't hold them back in the hunting department! Moray eels are mostly nocturnal meaning they can't really use their eyes to hunt in the ocean at night anyway. They do however have a very keen sense of smell which they use to sniff out their prey! Moray eels feed on an array of sea creatures ranging from fish to crustaceans and even octopuses. Species like the snowflake moray (Echidna nebulosa) have blunt teeth so they can feed on hard-shelled animals like crustaceans.


When hunting they usually lay in wait, with their head poking out of a crevice, to catch prey that swims past. Once they've got hold of their prey they sometimes tie their body into knots to help anchor them while breaking apart their food. They have also been seen pairing up with other predators and using cooperative hunting to catch their prey! The moray forces prey out from small holes and crevises to allow groupers to catch prey they otherwise wouldn't be able to reach and in return the grouper signals to the moray eel from above the location of prey that is hiding in crevices! (check out this cool video of them hunting together).


Why so slimy?

Morays, like other eels, secrete mucus that covers their skin to form a protective layer. However, moray eels produce quite a large amount of it! This is because they have a much higher density of goblet cells in the epidermis (outermost layer of the skin). The layer of mucus can be so thick that it can even make the moray look a different colour. The green moray (Gymnothorax funebris) produces a thick and toxic yellow-tinted mucus to cover its skin and ward off predators, the yellow tint makes the moray appear green, when in fact its skin is actually brown! Morays also use this sticky layer of mucus as a sort of glue to stick small pieces of sand on them to help build sand burrows.


Strikingly different patterns between the white-eyed moray (Gymnothorax thyrsoideus) (left) and the fimbriated moray (Gymnothorax fimbriatus) (right). Sadly the latter wasn't up for saying hello when I swam past it.


Hopefully, you now see how incredible moray eels are and like me, you'll be constantly looking in holes and under overhanging corals while diving or snorkelling trying to catch a glimpse of them!! Remember always respect underwater creatures by keeping your distance... wouldn't want those two sets of jaws grabbing hold of you!


Ruth x



Sources:

Hopkin, M., 2007. Eels Imitate Alien. [online] Nature. Available at: <https://www.nature.com/news/2007/070903/full/070903-11.html#B1> [Accessed 18 May 2020].


Mehta, R. and Wainwright, P., 2007. Raptorial jaws in the throat help moray eels swallow large prey. Nature, [online] 449(7158). Available at: <https://www.ncbi.nlm.nih.gov/pubmed/17805293> [Accessed 18 May 2020].


Winn, H. and Bardach, J., 1959. Differential Food Selection by Moray Eels and a Possible Role of the Mucous Envelope of Parrot Fishes in Reduction of Predation. Ecology, 40(2).



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