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 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.
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.
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.
In addition to the presence of pharyngeal jaws, there are other adaptations which enable morays to feed without negative pressure. The mouth opening extends far back along the edge of the moray's head, compared to other fishes which feed with suction. In the action of lunging at prey and biting down, water flows out the posterior side of the mouth opening, reducing waves in front of the eel which would otherwise displace prey. The result is that even with reduced bite times compared to other fishes, an aggressive approach to predation is still possible.
Differing shapes of the jaw and teeth reflect the respective diets of different species of moray eel. Evolving separately multiple times across the Muraenidae, short, rounded jaws and molar-like teeth allow durophagous eels (e.g. Gymnomuraena zebra and genus Echidna) to consume crustaceans, while other piscivorous genera of Muraenidae have pointed jaws and longer teeth. These morphological patterns carry over to teeth positioned on the pharyngeal jaw.
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.
The moray eel can be found in two separate aquatic environments: freshwater habitats and saltwater habitats. When concerning saltwater habitats, there is an extremely wide diversity, and large quantity, of moray eels which occupy these waters. An example of a saltwater moray eel, would be that of Gymnothorax vicinus. When concerning freshwater habitats, there is relatively little species abundance, or rather species richness, in these environments. The most widely known, and most relatively acknowledged, freshwater moray eel is Gymnothorax polyuranodon. Consequently, these moray eels can be found in habitats at depths of over roughly 80 centimeters.
The saltwater habitats are not uniform and have much variability, including shallow water nearshore areas, continental slopes, continental shelfs, deep benthic habitats, and mesopelagic zones of the ocean. In saltwater habitats, moray eels are considered "cosmopolitan," which is a loosely used term referring to the fact that the moray eel contains various species which can occupy two separate saltwater habitats: tropical oceans and temperate oceans. Tropical oceans are typically located near the equator, whereas temperate oceans are typically located away from the equator. That being said, the moray eel is capable of living in relatively warm water, despite the ocean being tropical or temperate. As a result, these moray eels can be found in habitats at depths greater than 10 meters.
Although the moray eel can occupy both tropical oceans and temperate oceans, as well as both freshwater and saltwater, the majority of moray eels occupy warm saltwater environments, which contain reefs. Within the tropical oceans and temperate oceans, the moray eel occupies shelters, such as dead patch reefs and coral rubble rocks, and less frequently occupies live coral reefs.
There are currently around 202 known species of moray eels, divided between 16 genera. These genera fall into the two sub-families of Muraeninae and Uropterygiinae, which can be distinctly defined by the location of their fins. In Muraeninae the dorsal fin is found near the gill slits and runs down the back of the eel, while the anal fin is behind the anus. The Uropterygiinnae, on the other hand, are defined by both their dorsal and anal fin being located at the end of their tails. Though this distinction can be seen between the two sub-families, there are still many varieties of genera within Muraeninae and Uropterygiinae. Of these, 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 :
The Moray Eel's change in shape within the species is due to an elongation process called pleomerism, which involves an increase in the number of vertebra. However, the development of vertebrae evolved independently from lengthened body and explains the high amount of diversity among Moray eel species. Additionally precaudal and caudal regions have differing modes of development and vertebra in these regions do not increase in a synchronous fashion.