Giant Moray Eels
The giant moray (Gymnothorax javanicus) is in terms of body mass the largest of the moray eel species, from the family Muraenidae.
General Description
As the name suggests, this is a large eel, reaching up to 3 metres in length and 30 kilograms in weight. While juveniles are tan in colour with large black spots, adults have black specks that grade into leopard-like spots behind the head and a black area surrounding the gill opening, there is a greenish base colour with dark speckling, and a more pale area around the face. In some species, the inside of the mouth is also patterned.
The body is long and heavy set but despite this is very flexible and moves with ease.
The dorsal fin extends from just behind the head along the back and joins seamlessly with the caudal and anal fins. Most species lack pectoral and pelvic fins, adding to their serpentine appearance. Their eyes are rather small so giant moray eels tend to rely on their highly developed sense of smell, lying in wait to ambush their prey.
Their jaws are wide, framing a protruding snout. Most possess large teeth that are designed to tear flesh or grasp slippery prey items and they are capable of seriously wounding humans.
Giant moray eels secrete a protective mucus over their smooth, scaleless skin which in some species contains a toxin. Morays have much thicker skin and high densities of goblet cells in the epidermis that allows mucus to be produced at a higher rate than in other eel species. This allows sand granules to adhere to the sides of their burrows thus making the walls of the burrow more permanent due to the glycosylation of mucins in mucus. Their small circular gills, located on the flanks far posterior to the mouth, require the moray to maintain a gap in order to facilitate respiration.
Typically, only its head is visible sticking out of the reef, but occasionally it will spend time with its head and much of its body extending up into the water column. It is usually a solitary species, but on occasion two giant morays may share the same crevice or cave.
The giant moray eel is frequently thought of as a particularly vicious or ill-tempered animal. In truth, they hide from humans in crevices and would rather flee than fight. Giant morays are shy and secretive, and attack humans only in self defense or mistaken identity. Most attacks stem from disruption of a giant moray’s burrow (to which they do react strongly), but an increasing number also occur during hand-feeding of giant morays by divers, an activity often used by dive companies to attract tourists. They have poor vision and rely mostly on their acute sense of smell, making distinguishing between fingers and held food difficult; numerous divers have lost fingers while attempting hand feedings. For this reason the hand feeding of giant moray eels has been banned in some locations.
The moray’s rear-hooked teeth and primitive but strong bite mechanism also makes bites on humans more severe, as the eel cannot release its grip even in death and must be manually pried off.
Distinguishing Features
As the name suggests, this is a large eel, reaching up to 3 metres in length and 30 kilograms in weight. While juveniles are tan in colour with large black spots, adults have black specks that grade into leopard-like spots behind the head and a black area surrounding the gill opening, there is a greenish base colour with dark speckling, and a more pale area around the face. In some species, the inside of the mouth is also patterned.
The body is long and heavy set but despite this is very flexible and moves with ease.
The dorsal fin extends from just behind the head along the back and joins seamlessly with the caudal and anal fins. Most species lack pectoral and pelvic fins, adding to their serpentine appearance. Their eyes are rather small so giant moray eels tend to rely on their highly developed sense of smell, lying in wait to ambush their prey.
Their jaws are wide, framing a protruding snout. Most possess large teeth that are designed to tear flesh or grasp slippery prey items and they are capable of seriously wounding humans.
Giant moray eels secrete a protective mucus over their smooth, scaleless skin which in some species contains a toxin. Morays have much thicker skin and high densities of goblet cells in the epidermis that allows mucus to be produced at a higher rate than in other eel species. This allows sand granules to adhere to the sides of their burrows thus making the walls of the burrow more permanent due to the glycosylation of mucins in mucus. Their small circular gills, located on the flanks far posterior to the mouth, require the moray to maintain a gap in order to facilitate respiration.
Typically, only its head is visible sticking out of the reef, but occasionally it will spend time with its head and much of its body extending up into the water column. It is usually a solitary species, but on occasion two giant morays may share the same crevice or cave.
Feeding Habits
Giant moray eels are carnivorous and feed primarily on other fish cephalopods, molluscs and crustaceans. It also engages in cooperative hunting with the roving coral grouper. The invitation to hunt is initiated by head-shaking. The rationale for this joining of forces is the ability of morays to enter narrow crevices and flush prey from niches not accessible to groupers. This is the only known instance of interspecies cooperative hunting among fish. Cooperation on other levels, such as at cleaning stations is well-known.
Morays rest in crevices during the day and hunt nocturnally, although they may ensnare small fish and crustaceans that pass near them during the day.
The giant moray eel has an unique way of swallowing their prey, this is due to their alien-like second set of jaws (or pharyngeal jaws) which they have in the back of their throat (very nicely visible in the video above!). When they lash out at a prey, they grab it with their first set of jaws first, then with the second while holding it with the first set. When the second set of jaws has a hold on the prey it pulls the prey into the digestive system. The giant moray eel is the only animal in the world which uses its pharyngeal jaws while hunting.
Reproduction
Reproduction
Giant moray eels travel up to 4,000 miles to breed, a journey which can take up to seven months. During that time it is believed that the eels do not eat. Instead, they use their body fat and muscle tissue for nutrients. Adults die after breeding.
Courtship among compatible morays begins when water temperatures reach their highest, and they begin sexual posturing in the form of gaping widely. Then the morays will wrap each others’ long slender bodies together, either as a couple or 2 males and a female. They simultaneously release sperm and eggs in the act of fertilisation, which signals the end of their relationship.
Scientific studies have shown hermaproditism in morays, some being sequential (they are male, later becoming female) and others are synchronous (having both functional testes and ovaries at the same time) and can reproduce with either sex depending on species in a natural life cycle.
Life Cycle
On hatching, the eggs take the form of leptocephalus larvae, which look like thin leaf-shaped objects, that float in the open ocean on ocean currents for around 8 months. Then they swim down as elvers to begin life on the reef and eventually after three years become a moray eel, living between 6 and 36 years
Predation
Its natural prey consists mainly of fishes, but it also eats crabs, shrimp and octopuses. This species will also eat other species of eel.
Distribution
Giant moray eels are found worldwide in tropical and temperate seas, particularly in relatively shallow water among reefs and rocks, in lagoons and seaward reefs, as well as in estuarine areas.
This immense moray hides in reef interstices, under ledges and in caves on reef walls and they live at depths of up to several hundred metres.
We encounter Giant Moray Eels on many of our trips, but they are particularly common on dives done from our Similan Liveaboard trips
Ecological Considerations
This species of moray are fished, but are not considered endangered. This is due in no small part to their toxicity. Ciguatoxin, the main toxin of ciguatera, is produced by a toxic dinoflagellate and accumulated up through the food chain, of which moray eels are top, making them dangerous for humans to eat. This fact was apparently the cause of death for King Henry I of England, who expired shortly after feasting on a moray eel.
