Undulated and Laced Moray Eels and the Coral Reef

Undulated and Laced Moray Eel

and the Coral Reef

Coral reefs are referred to as the "tropical forests of the oceans" since they are inhabited by a vast number and variety of animals. There are an estimated 35-60 thousand animal species inhabiting the world's coral reefs, including 25 thousand fish species. The world of invertebrates is also incredibly rich. Aside from humans, mammals are rare on coral reefs, with visiting cetaceans such as dolphins being the main group. A few of these varied species feed directly on corals, while others graze on algae on the reef and participate in complex food webs.

The building blocks of coral reefs are the "skeletons" of generations of reef-building corals, and other organisms that are composed of calcium carbonate. The coral skeletons while alive house coral polyps (a tiny animal that looks like an upside-down jellyfish) and as a coral head grows, it lays down a skeletal structure encasing each new polyp. Waves, grazing fishes, sea urchins, sponges, and other forces and organisms break down the coral skeletons into fragments that settle into spaces in the reef structure. Reef-building corals are only found above a 50m depth (in the photic zone), the depth to which sufficient sunlight penetrates the water for photosynthesis to occur, and the clearer the water (which admits more sunlight), the faster the coral reef grows.

Although corals are found growing in most areas of a healthy coral reef, in general, only a small number of hardy coral species can thrive on the reef flat, and these cannot grow above a certain height because the polyps can withstand only limited exposure to the air at low tide. Coral reefs (shown in red) are estimated to cover 284, 300 square kilometers. Reef cavities are inhabited by shrimps, crayfish, starfish, snails, octopuses—and moray eels.

There are approximately 200 species in 15 genera of moray eel. The typical length for a moray is 5 feet, with the largest being the slender giant moray, Strophidon sathete, at up to 13 feet. Moray eels frequent coral reefs to depths of 200 m, where they spend most of their time concealed inside crevices.

Moray eels are bony fishes with bodies highly modified to suit their life style. The elongated head bears a large, gaping mouth and tube-like nostrils. The mouth is held open so that water can be pumped over the gills. The gill cover is reduced to a small hole for streamlining and protection of the delicate gills. The dorsal fin extends from just behind the head, along the back and joins seamlessly with the caudal and anal fin. Most species lack pectoral and pelvic fins, adding to their snake-like appearance. Lacking paired fins, eels swim by moving their entire body side to side in a S-shaped wave. Using this wave pattern an eel can move forward or backward, an important advantage if living within the narrow holes of the reef. The scales have either been lost completely or are buried within the skin.

Their eyes are rather small; morays rely on their highly developed sense of smell. Their bodies are a patterned camouflage which is also present inside the mouth. This camouflage works so well that most eels are never seen at all.  However, during the day, their head may be visible at the entrance to its shelter.

Morays are carnivorous and feed primarily on other fish, cephalopods, mollusks, and crustaceans. They hide in crevices in the reefs and wait until their prey is close enough for capture. They then jump out and clamp the prey in their strong jaws. They usually emerge at night to search within and over the reef for prey which they locate using a keen sense of smell. They often prey on inactive diurnal species that are resting within the reef framework at night. While armed with formidable teeth, most moray eels are not usually aggressive and will not attack prey larger than themselves. When approached by a swimmer or diver, most eels either disappear into their shelter, or open the mouth wide in a defensive posture toward the intruder. While unprovoked attacks on divers have been reported, most eel bites occur when divers place hands or feet too close to the eel's shelter area. The razor-sharp teeth can inflict muscle, tendon, or nerve damage if the bite is serious.

The Phoenix Zoo has 2 species of moray eel; Undulated moray (Gymnothorax undulatus) and Laced moray (Gymnothorax favagineus):

Undulated moray is found in the Indo-Pacific oceans from the Red Sea and East Africa to French Polynesia, and also the eastern Central Pacific from Costa Rica and Panama at depths down to 30 m. The undulated moray has a distinctive pattern of light undulating lines and speckles on a dark green background, with a snout that is often yellow. It can reach between 3-5  feet in length.  It is a common species on reef flats among rocks, rubble, or debris and it also occurs in lagoons and seaward reefs, commonly inhabiting caves. It is reported to be an aggressive species and prone to bite.

Laced moray can reach a length of 6-9 feet and is found in the Indo-Pacific oceans from the Red Sea and East Africa to Papua New Guinea, north to southern Japan, and south to Australia. This moray is basically white with black blotches and interspaces forming a honeycomb pattern. Some individuals have a near black overall appearance. Blotches vary between individuals and size, often in relation to habitat - those in clear coral reefs usually have proportionally less black than those found in turbid waters. Large adults may be aggressive.