Temporal range: Late Miocene-Recent 
|Moray eel in the Maldives|
Moray eels, or Muraenidae, are a cosmopolitan family of eels. The approximately 200 species in 15 genera are almost exclusively marine, but several species are regularly seen in brackish water, and a very few are found in fresh water.
The smallest moray eel is probably Snyder's moray (Anarchias leucurus), which attains a maximum length of 11.5 cm (4.5 in), while the longest species, the slender giant moray (Strophidon sathete) reaches up to 4 m (13 ft). The largest in terms of total mass is the giant moray (Gymnothorax javanicus), which reaches 3 m (9.8 ft) in length and 30 kg (66 lb) in weight.
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; morays rely on their highly developed sense of smell, lying in wait to ambush prey.
The body is generally patterned. In some species, the inside of the mouth is also patterned. Their jaws are wide, framing a protruding snout. Most possess large teeth used to tear flesh or grasp slippery prey items. A relatively small number of species, for example the snowflake moray (Echidna nebulosa) and zebra moray (Gymnomuraena zebra), primarily feed on crustaceans and other hard-shelled animals, and they have blunt, molar-like teeth suitable for crushing.
Moray eels' heads are too narrow to create the low pressure most fishes use to swallow prey. However, they have a second set of jaws in their throat called pharyngeal jaws, which also possess teeth (like tilapia). When feeding, morays launch these jaws into the mouth, where they grasp prey and transport it into the throat and digestive system. Moray eels are the only animals that use pharyngeal jaws to actively capture and restrain prey.
Morays secrete a protective mucus over their smooth, scaleless skin, which in some species contains a toxin. They 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 in sand-dwelling morays, 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 to facilitate respiration.
Morays are carnivorous and feed primarily on smaller fish, octopuses, squid, cuttlefish and crustaceans. Groupers, barracudas and sea snakes are among their few predators. Commercial fisheries exist for several species, but some cause ciguatera fish poisoning.
Reef-associated roving coralgroupers (Plectropomus pessuliferus) have been observed to recruit giant morays to join them in hunting for food. The invitation to hunt is initiated by head-shaking. The rationale for this joining of forces is the ability of the 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.
Most attacks stem from disruption of a moray's burrow (to which they do react strongly), but an increasing number also occur during hand feeding of morays by divers, an activity often used by dive companies to attract tourists. Morays 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, so the hand feeding of moray eels has been banned in some locations, including the Great Barrier Reef. 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. While the majority are not believed to be venomous, circumstantial evidence suggests a few species may be.
(video) Moray eel moving in an aquarium
Moray eels are cosmopolitan, found in both tropical and temperate seas, although the largest species richness is at reefs in warm oceans. Very few species occur outside the tropics or subtropics, and the ones that do only extend marginally beyond these regions. They live at depths to several hundred metres, where they spend most of their time concealed inside crevices and alcoves. While several species regularly are found in brackish water, very few species can be found in fresh water, for example, the freshwater moray (Gymnothorax polyuranodon) and the pink-lipped moray eel (Echidna rhodochilus).
There are currently around 202 known species of moray eels, divided between 16 genera. The genus Gymnothorax is by far the broadest, including more than half of the total number of species.
List of genera according to the World Register of Marine Species :