Environment

Moray Eels Knot Themselves to Pry Prey

The tricks moray eels use to hunt may mean that they can have a bigger impact on their ecosystems than any other predator of similar size.

A fimbriated moray eel. Credit: elevy/Flickr, CC BY 2.0

A fimbriated moray eel. Credit: elevy/Flickr, CC BY 2.0

Animals do the most amazing things. Read about them in this series by Janaki Lenin.

Moray eels are a large group of 200 species. At night, many of these snake-like creatures sniff out fish and crustaceans hiding among rock crevices. Their long tubular bodies pose a challenge when they hunt large prey. Without razor-sharp, cutting teeth to tear off chunks of flesh, they jerk and shake their heads to rip pieces off. If this isn’t successful, they perform what crocodile people call a ‘death roll‘. They grab a mouthful and spin their bodies rapidly along their lengths until the piece comes detached. One species, the American eel, rotated as many as 14 times per second. Other creatures with a similar body shape such as the amphibious caecilians that live underground also use the same tactic.

Seven species tie knots in their bodies to either compress prey or pin it while they take bite-sized morsels. Tracy J. Miller of Michigan University described this behaviour for the first time in 1987. The “knotting behaviour is a complex series of movements in which the eel, in a backward motion, moves the tail under the middle of its body to form a loop, passes the tail back up and over the body, down through that loop to form a second loop, as the tail again passes up and over the body. The loops are tightened and the eel quickly draw its head backward through them, resulting in the facilitation of the prey item being swallowed whole, or decapacitation, or removal of a piece of the prey, which is then consumed.”

This eel behaviour is reminiscent of constriction in snakes. Snakes like pythons that eat enormous meals have elastic jaws. The bones of left and right side of their jaws are not rigid. Instead, muscles and ligaments connect them. They sink the recurved teeth of one side into the prey while moving the other side forward. With no tongues to aid the swallowing process, snakes “walk” over their prey until they envelop it with their bodies. Eels cannot open their mouths wide like snakes and they live in tight places, so swallowing large prey whole is not an option. Knotting may help them overcome these restrictions.

Shanta Barley, then a doctoral student at the University of Western Australia, and her colleagues lowered a metal frame with a pair of remote video cameras in underwater housings. Such systems called ‘stereo-BRUVS’ are becoming popular tools to study cryptic fish like moray eels. But the researchers’ intention was not to study moray eels; they wanted to study the role of sharks in coral reefs. The cameras sat 30 centimetres above the seabed, since more sharks cruise at this level. Fishing had reduced shark numbers at Scott Reefs, off the north-west Australian coast. But the Rowley Shoals marine reserve abounded with them. Comparing the two locations would show the role sharks played. To attract fish to the camera rig, the researchers attached a plastic mesh bag stuffed with a kilogram of crushed pilchards.

At Scott Reefs, the researchers saw moray eel behaviour never seen before. “It was purely serendipitous that we observed moray eels performing these unusual behaviours,” Shanta Barley told The WireOne laced moray spent more than five minutes struggling to rip the bait bag. It swished its tail like a paddle to push water towards the bag. This gave it the leverage to rip the bait bag.

Another, a fimbriated moray, held the mesh bag with its mouth while its tail looped a loose overhand knot. The knot moved up its body, over its head, and pushed hard against the bag while it tugged with its mouth. Then the knot unravelled and the moray looped another knot. It repeated this over and over again for 90 seconds in its efforts to break the bait bag or squeeze the crushed fish bits through the mesh.

While researchers have observed morays used the paddle and knot tricks before, this is the first time eels were seen putting them to different use. “The knotting and paddling behaviours were so dramatic and unusual that I noticed them immediately,” says Barley.

A fimbriated eel attacks the bait bag (top left), forms a “knot” in its tail (top right), and rapidly pushes the knot into the bait bag (middle left). A laced moray attacks the bait from two angles (middle right, bottom left) before using its tail as a “paddle” to rip open the mesh bag (bottom right). Credit: Shanta Barley

A fimbriated eel attacks the bait bag (top left), forms a “knot” in its tail (top right), and rapidly pushes the knot into the bait bag (middle left). A laced moray attacks the bait from two angles (middle right, bottom left) before using its tail as a “paddle” to rip open the mesh bag (bottom right). Credit: Shanta Barley

“I was particularly excited when Shanta showed me the videos,” says Rita Mehta, University of California-Santa Cruz and a co-author. “I’ve observed quite small individuals knotting in the lab.” But these eels looped their bodies to break down a large meal into bite-sized pieces.

Another marine creature with a similar tubular body plan uses the same technique for another purpose. In the absence of any objects in the deep sea against which to rub, the yellow-bellied sea snake knots itself to remove barnacles and slough its skin. Barley and her team suggest that use of the knotting and paddling tactic to pry prey wedged between rocks might be more widespread.

“As stereo-BRUVS become more common in marine research, we will undoubtedly start to uncover a host of exciting new behaviours in ocean wildlife that will revolutionise our understanding of marine ecosystems and show how important it is to conserve this environment,” says Barley.

Is knotting a superior technique to rip the bag? After all, the laced moray paddled for five minutes to achieve its aim while the fimbriated moray by knotting itself took less than two minutes. “That’s a hard question to answer,” replies Barley. “However, it is possible that knotting is overall a more versatile behaviour than paddling as it can be used in multiple ways to achieve different objectives in different contexts.”

These tricks may mean that morays can have a bigger impact on their ecosystems than any other predator of similar size. They can eat prey that is larger than their mouths, a trick most fish cannot perform. “As sharks are removed from coral reefs, moray eels may increasingly become the top predators,” says Barley. “It is, therefore, crucial that we develop a better understanding of how moray eels regulate reef ecosystems through their diet and feeding behaviours.”

Just how widespread are knotting and paddling behaviours among the moray eel group? “Are the behaviours ‘one offs’ or are they in fact employed regularly by moray eels as they hunt for prey in the crevices of the reef? Do moray eels ‘transmit’ these behaviours to each other?” Barley suggests more research is urgently needed to understand the effects of over-fishing on marine predators.

The study was published online on October 8, 2015, and included in the September 2016 issue of the journal Marine Biodiversity.

Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.