[Discussion] Moray Eels and Temperature? Rambling about Moray eels...

[Nevadensis]

We know Moray eels (Family: Muraenidae) are ectothermal poikilotherms. This, put simply, means their core body temperatures are reliant on external temperature sources. Presumably, like most ectotherms, they will move from warmer waters to cooler waters in order to thermoregulate as necessary. This thermoregulation behavior has a direct effect on the eel's metabolic rate, food and nutritional intake frequency, digestion efficiency, immune system efficiency, activity levels, and so on. All of these aspects are necessary to maintain a healthy Moray eel, and better yet, acclimate Moray eels to captivity. The idea of temperature and its significant effects is further supported by this study done on Anguilla eels. The metabolic rate is slowed by lower temperatures (some even becoming torpid), and increased significantly with higher temperatures. There are also many other internal effects that occur when varying temperatures are applied to the eels in question, such as oxygen consumption levels.

What does this mean for our captive Morays? In theory, our captive Moray eels should grow comparatively quickly with access to higher temperatures, and should have a higher intake of food, than captive Morays kept at cooler temperatures, thus food intake is lower. I have Morays here that I have been raising up, and they will take food every day, if not multiple times a day. I do keep them warmer (aprox. 74-77° degrees) and they appear to eat, process, digest and produce waste within 12-24/hrs. I have read that liver disease - I assume hepatic lipidosis (fatty liver disease) - is common in captive Moray eels, and its often copped up to feeding too often. I don't know how much I truly believe that notion. Its true overfeeding can result in fatty liver in vertebrates, but it is also seen in animals that undergo starvation (anorexia) and/or 'food strikes' as often reported by aquarists. Lipids (fat) will enter the bloodstream in large quantities and will rush to the liver. It can also be the result of poor temperature gradients, thus inefficiency of digestion and utilization of fats.

Why aren't wide thermalgradients offered to Moray eels, specifically species that are known to emerge from deeper water and near the shore? Take a look at this video of a group of chainlink eels (Echidna catenata) swimming among the rocky shores. The surrounding waters and surface rock temperatures must be significantly higher than the deeper waters they emerged from. These eels must use various temperature sources for all kinds of physiological reasons.

 

[Jay Hemdal]

Welcome to Reef2Reef!

A couple of things - most species of moray are site-specific, that is, they tend to stay in a certain area of a reef. That limits their ability to thermoregulate by relocating. In this way, they are no different than other species of reef fishes. Green morays do have an extended depth range, and can naturally be found in very deep, cold water, but they don't migrate much.

Here is an excerpt from my upcoming fish disease book on fatty liver disease:

Fatty liver disease (Hepatic lipidosis)
A major chronic health problem facing most long-term captive fishes is “fatty liver disease,” or liver degeneration. Fish do not assimilate fats well, so often, if a fish's diet is too high in fat, it will then be deposited in various tissues, especially the liver. Unlike with mammals, this fat is not readily usable during times of starvation. Even if food is withheld from a fat-laden fish, very little of the fat is ever reconverted into usable energy. Some fish do not show outward signs of obesity, especially sedentary species such as groupers and lionfish. In these, fatty liver disease may only be diagnosed after death, when sections of the liver will show oil droplets and pieces of the liver itself will float in sea water. With many animals, a period of acute anorexia can cause liver damage.

Fatty liver disease is most common in adult fish whose growth rate has slowed considerably. Younger fish tend to grow fast enough to minimize fat deposition—excess calories are turned into muscle tissue instead. With long-term captive fish, fatty liver disease is the leading cause of (or contributor to) death. Virtually every lionfish held in captivity longer than three years will show evidence of fatty liver disease upon necropsy.
What techniques are available to solve this problem? Nothing can be done for a fish once fatty liver disease has become evident. Prevention is the only course of action an aquarist can take. Simply putting the fish on a "diet" may not work, in that this may result in increased aggression towards other fish in the aquarium. The best solution is to avoid feeding herbivores diets high in meat proteins and to train carnivores to feed on low-protein, low-fat diets such as squid or gelatin diet. Also, avoid feeding any saturated fats to fish. Overall dietary fat levels should be less than 16% in actively growing fish and less than 9% in adult fish.

There has also been a theory that feeding freshwater fish to predatory marine fish will cause malnutrition and fatty liver disease. Most of the rationale behind this has to do with slightly different fat profiles between marine and freshwater prey fish species. Credence is given to the theory when so many marine aquarists report that they lost their lionfish to liver degeneration after feeding them live freshwater goldfish (See the next section for an alternative hypothesis regarding issues with feeder goldfish and thiaminase). In reality, they would have run into the same trouble had they fed any single fish species as a diet for their lionfish. Feeding carnivores too frequently can also cause fatty liver disease. The general recommendation is to feed larger carnivores only twice a week.


Jay