Eels of the Caribbean

Dr Anjani Ganase, marine ecologist, tells us about the eels that inhabit our coral reefs. Research shows, she says that "...eel species that occur in different coral reefs around the world have strikingly similar genetics, even though their distances apart..."

Eels have an unfortunate association with snakes because they look like them. However, eels are not related to snakes at all; rather they are specially adapted fish that have developed without fins. To move in their underwater world, they undulate their long bodies to create propulsion which moves them through the water. This means that they swim easily in the water column and do not just slither along the bottom. They have flattened bodies and tails that allow them to swim with considerable grace - like a colourful ribbon rippling in the wind.

On coral reefs around the world, eels are a common sight in bright colours - greens, yellows with exotic spotted or reticulated patterns. Unlike their freshwater look-alikes, eels that live on coral reefs are similar to other fish species in that they tend to be more brightly coloured to match the surrounding colour patterns of the coral reef environment that occurs in shallow well-lit water. 

Resident Green Moray Eel at Pirates Bay reef in Charlotteville. Photo credit Anjani Ganase, Maritime Ocean Collection

One of the most common types of eel on coral reefs is the moray eel. Moray eels are predators, and during the day they often hide out in the crevices of reefs with usually just the head sticking out. Divers often observe morays baring their teeth, but this is not a defensive or aggressive move; they open and close their mouths to allow water to flow over their gills for breathing. In the night, however, moray eels roam the reefs as they hunt for food. While moray eels are voracious predators, they are not a threat to divers. They may hunt on their own but can also team up with a grouper or another eel, as they tend to share similar prey, to coordinate chasing prey out of the reef for capture. The eel or grouper would drive the fish out of its hideout for the other to eat and then switch turns.

The more haunting thing about moray eels is how they feed. Moray eels have two sets of jaws to assist in prey capture; and it looks like something out of the movie Alien. When a moray eel bites, it latches on with both sets of jaws. The inner jaw can retract, pulling the food further into the mouth while the outer jaw can adjust its bite on the prey for a better grip as the prey is being pulled into the mouth. Moray eels are also not top predators so while they are hunting they have to seek their own safety from larger predators, such as sharks and larger species of groupers.  They are incredibly important on the reef because they control the population of invertebrates and prey species.

In the Caribbean, there are about ten species of eels that are commonly found in our marine and coastal habitats - coral reefs, seagrass beds and mangroves. Seven of these species are moray eel species including the green/yellow moray, the spotted moray, purple mouth, golden tail moray, chestnut moray, viper moray and the chain moray eel. The distribution of most morays finds them throughout the Caribbean region as well as across the Atlantic off the coast of West Africa. Unfortunately, very little is known about their reproduction and spawning patterns and lifespan. One  captive eel is recorded as living for 85 years.

Interestingly, studies done on eel species that occur in different coral reefs around the world have found strikingly similar genetics, even though their distances apart should have resulted in some geographic variability. This is likely the result of the long dispersion phase of eel larvae where the larvae would ride ocean currents across long distances before settling down on a coral reef far away. Eels are also most vulnerable during this drifting phase, where they can easily be eaten because of the small size. Some species of moray eel larvae in the Caribbean can drift for eight months, therefore the Moray eels observed on our coral reefs may have been born farther up the island chain.

The long larval drift phase also explains why eels are common to the Hawaiian Islands, a remote island archipelago in the Pacific, while other species of fish are absent as they are less able to survive transportation across such great distances. This also begs the question that if eels can easily be distributed across different regions in the world with little local geographic adaptations, what actually drove the existence of numerous species of Moray eels around the world? This, among many aspects of the physiology and ecology of moray eels, is still unknown.

Other species of eels include the snake eels and the garden eels, which are relatively smaller in size. Snake eels, as you may guess, look even more snake-like in appearance, and have a distinctive head. Snake eels forage the reef and sandy areas. They can dig up prey by blowing water jets into the sand to remove particles, and then using the tail to whip and dig and displace the sand when digging for prey. The last group of eels common to our reefs are the brown garden eels. Often located at the base of the reefs, these eels stick out of the sand in a stand that makes them look like part of a seagrass meadow. However, as you approach, the eels will retreat slowly back into their holes for safety. You have to be quite still to encourage them to peak out from their holes again. The skin can secrete mucus which lines the hole and keeps it from caving in. Although it may be difficult to distinguish their coloration, they are brown with tiny yellow dots. They also feed in this manner, picking out zooplankton from the water column.

Eels are fascinating to watch underwater, and important to the ecology of coral reefs. Their dispersal over far distances indicates how connected all coral reefs may be. As we protect and manage coral reefs around our islands more diligently, we have no doubt that we will learn more about these reef inhabitants.

Moray Eel in Chagos, Indian Ocean. Photo credit Underwater Earth, XL Catlin Seaview Survey

Reference

Reece, J. S., Bowen, B. W., Joshi, K., Goz, V., & Larson, A. (2010). Phylogeography of two moray eels indicates high dispersal throughout the indo-pacific. The Journal of heredity, 101(4), 391–402. https://doi.org/10.1093/jhered/esq036