Moray Den Fidelity and Behavior


This post is about the research project I did last semester while at the UC Berkeley Gump Station on Moorea in French Polynesia.

Moray eels make up a family within the order of true eels. They are found worldwide, including here in California, but are especially abundant in coral reef habitats. As top predators, they bioaccumulate toxins originally produced by dinoflagellates, usually in the form of ciguatoxin, which causes ciguatera fish poisoning in humans. Despite this threat, these eels are still consumed in Asia and the South Pacific (Jiang et al. 2012)*. Morays are well known for their second pharyngeal jaw, which lunges forward to grab prey after initial capture and to deliver the food to the esophagus (Mehta 2009)*. They also engage in a particularly successful form of cooperative hunting with groupers (Bshary et al 2006)*.There have been instances of land-based predation by morays (G. pictus) in areas with little human disturbance, such as the Chagos archipelago (Graham et al 2009)*.

My study had three components. A behavioral component which used an ethogram, a den fidelity component which sought to determine how likely an individual was to return to its den, mooreaand a distribution component to analyze the species composition along my transect. The map to the left shows my study sites. For the ethogram, I tried to find study sites as far away from each other as possible, to avoid any pseudoreplication. Overall, I found about 40 morays. When a moray was found, it was observed for ten minutes and its behavior was categorized using an organism-specific ethogram.

The results of this first, behavioral study clearly distinguished one species, the snowflake moray (Echidna nebulosa), from the other two, the giant (Gymnothorax javanicus) and yellow margin (Gymnothorax flavimarginatus) morays (see figure below). Aggressive behavior was defined as rapid snapping of the jaws which occurred upon an approach by the observer. The snowflake moray often exhibited this aggression, frequently remained completely exposed, and swam often. The giant and yellow margin morays, on the other hand, had calmer mouth behavior (their mouths closed or ventilating slowly), remained partially exposed except when they swam on rare occasions. Mutualists were only observed in adult giant morays.

Moray behavior across life stages followed a gradient, where juveniles exhibited the most aggressive mouth behavior, followed by young adults and then adults. Juveniles and young adults were never exposed, while adults were exposed occasionally. Juveniles never swam, young adults moved, but only within the confines of their den, and adults moved outside of their dens.


The results of the ethogram study allow us to identify camouflage, adult size, and age as important factors defining behavioral groups within morays. The snowflake morays’ light camouflage is light and well-suited for its environment, but its adult size is small, necessitating more defensive behavior. Giant and yellow margin morays are similar in behavior in that they both have dark camouflage which is not well-suited for the coral rubble environment, but have the safety provided by their large adult size, perhaps lowering their dependence on defensiveness. As far as the trends present with behavior and life stage, as age increases, aggressiveness and defensiveness decrease while activity increases. This could be a result of increased experience and size.

For the den fidelity component, I swam along a 1400 meter transect everyday. When a moray was found, its den was flagged and monitored daily for occupation. This component showed the den fidelity did in fact depend on life stage, species, and weather (see below). Juveniles had the highest den fidelity, which might be a result of their lower level of experience. Larger, darker species are less active, as asserted before, and therefore have higher den fidelity. On days with higher turbidity and inclement weather, there was lower den fidelity, suggesting perhaps that the morays were out of their dens foraging for food during this time.

den fidelity


Map marking the location of each moray den.

As I conducted the den fidelity component, I noticed that the dens lay across three very different habitats (see map on the left). The first habitat, located just outside of Cook’s Bay on the fringing reef, was deep (6-20 meters) and had the highest percent live coral (40-80&). The second, located at the mouth of the bay, was shallower (4-8 meters) and had lower percent live coral (20-40%). The third, further inside the bay and south of the Gump Station Dock, had the shallowest depths (1-4 meters) and the lowest percent live coral (0-20%).

Along this transect, and within these three habitats, there were distinct shifts in species composition which matched shifts in species composition (see below). Most notably, there was a concentration of Echidna nebulosa (snowflake moray) individuals at the mouth of the bay and many Gymnothorax javanicus (giant moray) individuals surrounding the Gump Station dock and in the bay. I think this is a result of the higher levels of human disturbance and input of discarded fish from fishermen.


To conclude, there is a ecological trade-off between camouflage and adult size in moray eels, where large adult size and light camouflage are advantageous and the opposite is true for smaller adult size and dark camouflage. Animals must compensate behaviorally for the disadvantages provided by small size or badly suited camouflage by remaining inactive or increasing defensive behavior. There is higher den fidelity in juveniles, less active species, and during periods of sunny weather. I found that there are distinct groupings of conspecifics along this particular transect, where giant morays dominate low percent live coral regions with a steadier food supply from fishermen.

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