Moray Eels: Top Predators with an Arsenal of Defences
Photo StoryPublished : Dec 08, 2023Updated : Dec 15, 2023
Moray eels are unique in appearance, breaking all fish stereotypes. They are apex predators of coral reefs, with unique adaptations that give them a survival advantage
Text by: Wenzel Pinto
Moray eels are unique in appearance, breaking all fish stereotypes. They are apex predators of coral reefs, with unique adaptations that give them a survival advantage
On a typical fair-weather dive in almost any tropical coral reef, you will see a myriad of different fish. Surgeonfish and parrotfish roam widely in large schools. Butterflyfish and damselfish forage around coral heads. Large predatory groupers lurk, looking for their next meal. Within this high-diversity ecosystem, however, you might also see another fish that slinks into a crevice, shying away and looking nothing like your Average Joe fish. This fish, with a long, slender snake-like body, gaping maw lined with jagged teeth, and an absence of pectoral fins — in all its uniqueness — breaks all fish stereotypes. The fish is the moray eel.
Moray eels are from the family Muraenidae and are a diverse group of fish consisting of over 200 species globally. Some, like the Snyder’s moray, grow to a mere 11 cm in length, while others are quite literally giants — the giant moray, for instance, can reach over 3 metres in length and weigh over 30 kg.
Although most abundant across temperate and tropical coral reefs, moray eels are also found in estuaries and seagrass meadows. They seek out burrows and crevices in the reef, where they spend most of their day being cleaned by cleaner wrasses and shrimp. Although they are usually solitary, on occasion, you will find two or more individuals — sometimes of different species, too — sharing a burrow rather peacefully.
With apex predators like sharks on the decline globally, morays have taken up the mantle as the new apex predators on the reef. They routinely prey on other predators like lionfish, snappers, and even smaller sharks. In contrast, few creatures feed on morays besides sea snakes, certain large predators like barracudas, and, of course, other morays. Moray eels have maintained their high position in the coral reef food chain through an assortment of unique adaptations that set them apart.
Moray eels are scaleless and have tough, thick skin. They produce large amounts of mucous, which serves as a protective coat, keeping their skin moist even outside the water. (1) This allows them to venture to intertidal flats for short durations in search of prey, a habitat that would otherwise be inaccessible to them. (2) For the Indo-Pacific ribbon eel, the mucous serves an adhesive function, too. Their skin secretes mucous, which binds the sand of their burrow walls, glueing it in place. Photos: Sarang Naik (1), Sidharth Mahesh (2)
Cover photo: Wenzel Pinto
Coiled within its burrow, this honeycomb moray appears to be performing a threat display with its mouth wide open. But that’s not really its intention — it’s only breathing. Most fish flap their gill covers, pushing water over their gills to breathe more effectively even when they aren’t moving. Morays, however, have small circular gill openings and completely lack gill covers. Instead, they frequently open and close their mouths, pushing water from their mouth out through their gills. Since they spend large amounts of time with their mouths open, species like this honeycomb moray (Gymnothorax favagineus) have developed a camouflage pattern even on the inside of their mouths! Photo: Umeed Mistry
Like most morays, peppered morays are nocturnal and only leave their burrows to hunt at night. They have very small eyes for their size and, consequently, poor vision. They rely on their highly developed sense of smell instead. Morays have two pairs of nares (or nostrils) on their snout and above their eyes, which sense chemical cues from other fish in the water for their hunt. Photos: Wenzel Pinto (1), Umeed Mistry (2)
Moray eels do not always hunt alone. They often team up to hunt with groupers, another predatory fish. The hunting alliance is usually initiated by the grouper, who will swim up to the moray and shake its head in front of it as an invite. If the moray is interested, it will swim out of its burrow, and the duo will swim together in search of fish. While the grouper’s hunting tactic tends to scare fish to hide in crevices, the moray’s tactic of cornering prey in holes leads prey to swim out to open waters. When they hunt together, the moray scares fish out of crevices, allowing the grouper to swallow them. The grouper, in turn, scares fish into holes where the moray can grab them. The cooperative strategy provides better success for both predators. This intentional, cooperative hunting between species is rare across the animal kingdom and has shed new light on the extent of fish cognition and intelligence. Photo: Umeed Mistry
Morays have evolved teeth that fit well with the prey they prefer. (1) Snowflake morays (Echidna nebulosa), for example, specialise in eating hard-shelled molluscs and have blunt and rounded teeth that work like nutcrackers, crushing the shells of their prey. (2) The giant moray (Gymnothorax javanicus), in contrast, feeds only on fish and has evolved razor-sharp teeth with which it can grab and tear at its prey. Photos: Umeed Mistry (1), Dhritiman Mukherjee (2)
At times, the fish the moray hunts squeeze tightly into a crevice, making it difficult for the moray to get them out. The limbless morays may seem to be at a disadvantage in such a situation, but they have figured out a nifty solution. To extricate the fish from their hiding places, morays employ an inventive behaviour, also seen in some sea snakes. They create a knot towards the base of their tail and slide it down their body towards their head. As the knot reaches the coral, they use it as leverage to yank the fish out. Video: Evan Nazareth
Most predatory fish employ a common technique to swallow their prey. They first expand their mouth cavity, creating negative pressure within their mouth and then open their jaws to suck the prey in. But morays have very narrow heads, making it difficult for them to create the negative pressure needed to swallow in that manner. Instead, they use a pair of pharyngeal jaws located in their throats. When the moray eel grabs its prey in its mouth, these pharyngeal jaws extend outward towards the mouth, grab the prey, and pull it down the throat. A scene right out of the movie “Alien”! Photo: Sahil Lokhandwala
Being a top predator also means knowing how not to become prey. Morays pack an arsenal of defences to ensure this. The best defence is offence, and a moray’s sharp teeth are the perfect weapons. When required, they aggressively defend themselves from predators and other morays (1).
Morays also have more potent defences. Their thick skin produces copious amounts of mucous, which is highly sought after by cleaner shrimp and wrasses (3 & 2). This mucus contains low amounts of toxins called crinotoxins, which might make them less appetising a meal for their predators. If that were not enough, adult moray eels also possess high levels of ciguatoxins. These toxins are produced by microscopic algae, which small fish feed on, accumulating in significant quantities in top predators like morays higher up in the food chain. Although not harmful to most fish, ciguatoxins pose the greatest threat to humans. They are odourless, tasteless, and heat-stable and are the reason why fishing communities rarely consume large moray eels. Video: Umeed Mistry; photos: Sahil Lokhandwala (2), Umeed Mistry (3)
About the contributor
Wenzel Pinto
is a marine biologist at the Nature Conservation Foundation and is currently working on reef dynamics and island vulnerability in the Lakshadweep archipelago.
