Fish Tales with guest Frank Marini

A Serpent For Your Reef Tank:
A Look at Fish-Safe Eels


In filling in for Henry this month, I thought I'd present an overview of a few exceptional reef-safe fish, specifically some of the moray eels. With their unmistakable appearance and mode of locomotion, moray eels provide a unique presence in an aquarium. Many people ask if there are any reef-safe eels that will not consume their fishy tank mates. As morays have no interest in corals or other sessile invertebrates, they are technically "reef-safe." Only a few, however, are fish-safe. All morays are carnivorous and feed on either fish or moving invertebrates. However, there are a small number of them that are generally fish-safe, and these may fit into hobbyists' aquarium plans.

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Figure 1. Moray eels, such as this snowflake, can make a wonderful and unique addition to a reef tank, given some qualifications. Photo courtesy of Skip Attix.

This month I would like to highlight the moray eels that I consider to be both reef-safe and fish-safe. Additionally, I will as discuss specific husbandry issues, tank requirements, and potential compatibility conflicts. I should mention that this is not a complete listing or description of morays which could fit into a reef tank. For example, I will not discuss in detail a group of dwarf sized morays that are known fish eaters, but have mouths too small to easily consume most common reef fish. These eels, exemplified by species such as the dwarf golden moray (Gymnothorax melatremus), are rare and very expensive [Figure 2]. Rather, I wish to identify a few commonly available eels, ones you might see in your weekly excursions to the pet store, and provide background and husbandry information so you can decide if a fish-safe eel would be a candidate for your aquarium.

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Figure 2. G. melatremus-the dwarf golden moray. These rare morays are known fish eaters but have mouths too small to eat many of our common reef fish. Photo courtesy of anilabbasi.

Moray Eels

Moray eels are true eels in the taxonomic Order Anquilliformes. They are found in the family Muraendae that comprises 15 genera and approximately 200 species. Approximately 12 of these 200 species are suitable for the home aquarium, and Fenner (1995) considers only five species to be safe with other fish. These particular eels range in size from eight inches to about two feet long. Eels are readily identifiable by the lack of pelvic fins, pectoral fins, and having elongated bodies, and have a snake-like in appearance. They do not have either gill cover openings and scales. Additionally, their dorsal and anal fins are continuous around the body. This body design requires the fish to maneuver with long flowing wave-like strokes. Because morays lack scales, they protect their skin from abrasions by shedding a constant covering of mucus. Morays possess lateral line pores only on their heads, and this unique adaptation allows them to live in caves and crevices with only their head displayed, and still be able to sense movement in the water.

The vast majority of morays can be found in coral reefs in tropical waters, with a few members found in all oceans as far north as the west coast of North America. Interestingly, morays are the second-most represented fish group (after the wrasses) in the Hawaiian Islands, with more than 40 species found locally, suggesting they play an important role in the local food chains (Gonzales, 1976).

Moray eels possess a menacing appearance, with their mouths constantly agape. While an opened mouth display appears frightful, especially when coupled with pointed fangs, the eel's physiology requires this behavior. The mouth must remain open to allow water to be pumped into the oral cavity, then over the gills by muscles located in the gill cavity and attached to the mandible or lower jawbone. Juvenile morays often appear differently colored, often with more pronounced color patterns, and with different bodily proportions than are found in the adults. As the animals age, their body proportions become thicker, and the bold color patterns fade to a more diffuse coloration (Michael, 1998).

Many moray eels have limited vision and instead, they rely on a highly evolved sense of smell. This sense of smell, combined with the head-localized lateral line system to sense vibration makes these eels formidable hunters of any prey items that pass near their cave-like opening. Morays can detect sick or injured fish hiding in the reef, and as they possess wedge-shaped heads and bodies, they can squeeze into tight recesses to extract a hiding food item.

Many eels have nutritional preferences and these eels are easily divided into two groups (Axelrod, 1990). Morays with pointed teeth, the fang-toothed morays, use these fangs to hold and tear their fish prey. In my experience, almost any captive fang-toothed moray will eventually eat its fish tankmates. Even tough fish like triggers, quick swimming groupers, and puffers, may fall prey to a moray, especially at night. The fang-toothed eels will even eat cleaner wrasses. Obviously these fang-toothed morays are suitable only for tank dedicated only to them. It pains me to see any hobbyist adding Clown triggers to a tank containing a two-foot long Tessellata eel (Gymnothrax favagineus); the outcome of such a placement surely known well in advance.

Fish-Safe Eels: The Pebbled-Toothed Morays

The pebbled-toothed morays feed primarily on crustaceans and hard-shelled invertebrates. These morays are found in the genera Echidna, Gymnomuraena, and Siderea, and are known for having blunt molar-like teeth that are designed for crushing the shells of their prey. They tend to have small poorly-developed eyes, short head and mouth structures, and a highly developed olfactory system that compensates for their poor vision. Not surprisingly, the pebbled-tooth morays rely heavily on locating their prey based on smell, rather than by sight. Additionally, these eels have adapted their behavior to pinpoint prey based on angulations of their head. When a prey item approaches the eel's sensing perimeter, the moray will move its head from side to side to better acquire scent intensity, and to determine the signal strength based on its head orientation. To place a definitive confirmation on their location, these eels will often "bump" items with their nose before they strike. Taste and touch receptors located in the lips and nose of the eel help determine if the item is an appropriate food or not.

Species:
(listed according to most "reef safe")

Echidna nebulosa
Common name: snowflake eel, starry eel
Maximum length: 30", minimum tank size: 40 gallons

Snowflake morays are found commonly on reef slopes and reef lagoons. According to Michael (1999) this eel is common in the tidal zones and has been observed slithering out of the water at ebb tide. Snowflake eels feed primarily on shore crabs, spider crabs, and rock-dwelling xanthid crabs. Additionally, mantis shrimps and small bony fish remains have been found in the stomach content analyses of these eels and adult snowflake eels have been observed eating small cephalopods. In general, snowflake eels are excellent reef-safe morays [Figures 1, 3, 4, 5]. They adapt quickly to tank life, readily accept prepared marine-based foods, and are much less of a threat to fish than members of Gymnothorax. Snowflakes remain fairly small, under 30 inches, and it is not uncommon for a juvenile snowflake to take up to two years before adult sizes are reached. However, one word of caution is needed when keeping snowflake eels: when food is in the water, these eels may become aggressive. It is not uncommon for a medium-sized snowflake eel to attack a fish when chasing a scent trail though the water column, and it is also not uncommon for an adult snowflake eel to take a bite out of a nearby fish when food is in the water. As a way of preventing this type of feeding behavior, snowflake eels can be fed using a feeding stick, a rigid tube on which the food item is attached to the lower end and placed within inches of the eel. Stick-fed eels tend to wait for food on the stick and don't exhibit the frenzied feeding response described above. As with most morays, juvenile snowflake eels will spend the majority of their time with their head protruding from the rockwork. When they become more familiar with their surroundings, they will actively explore the tank. Snowflake eels tend not to be as shy as chainlink eels.

Figure 3. A head shot of a snowflake moray. Photo courtesy of Kpeterson of Monterey Bay Aquarium.

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Figure 4. An adult snowflake eel peering from a cave in Kona, Hawaii. Apparent in this photo is the head structure of the eel and a very unique splotchy pattern specific for snowflake morays. This dive photo courtesy of Linda Cline, www.dancing-fish.com.
Figure 5. Another dive photo of a snowflake moray peering from its hole. Notice how different the body pattern is compard to Figure 4. Each snowflake moray has a unique body pattern and coloration scheme. Photo courtesy of www.meandmephoto.com.

Gymnomuraena zebra
Common name: zebra moray
Maximum length: 3 feet, minimum tank size: 40 gallons
Must have a strong reef rock structure for these eels (Michael, 1999).

Zebra morays are found primarily in rocky and coral reefs on the east coast of Africa to the Red Sea. These passive eels have remarkable patterns of deep black to chocolate brown interspersed with vertical white stripes [Figures 6, 7, 8, 9]. Their head is designed to crush hardened crustacean shells. The primary foods of zebra morays are crabs, shrimp, and occasionally snails and urchins. Zebra morays employ a unique hunting style for finding food: they intentionally bump the prey item to first see if it moves, and second to taste the prey item, to determine its food worthiness. Small prey items are usually swallowed whole, while larger food items like crabs, are held down by their body coils as the claws and legs are broken off and eaten. A zebra moray's jaws will easily crack crab claws, and in the home aquarium, one can hear the equivalent of a nutcracker sounding off when a crab claw is cracked. Zebra morays do much of their hunting behind the reef structures and rarely venture out into the open. These fish often refuse take prepared foods, but after a lengthy starvation period will generally wean over to stick feeding. However, initially I would recommend feeding them live fiddler crabs, small blue crabs, and small shrimp to establish the fish.

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Figure 6. A head shot of a zebra moray. Obvious in this photo are the nostrils placed high on the head. Photo courtesy of www.exotictropicals.com.
Figure 7 (background). The full monty. A full view of a 34" adult zebra moray. Notice how thick and muscular the adult body is. Photo courtesy of J.E. Randall www.Fishbase.org. Figure 8 (insert). A zebra moray extended out of its hiding spot. Photo courtesy of Kpeterson of Monterey Bay Aquarium.

As they grow to adult size, zebra morays become relatively much thicker and more muscular, which emphasizes their apparently clumsy demeanor. Reef structures must be large and strong to withstand the body movements and contortions these eels exhibit when feeding. My only reservation in recommending these eels as a wonderful, reef-safe eel is that at adult size they are guaranteed to renovate your rockwork, toppling corals in the process. Most reef fishes are safe with a zebra moray as long as the eel is regularly fed. These eels are too large and "clumsy" to be kept with most sessile invertebrates and they definitely should not be kept in small reef systems. They will eventually eat all of the ornamental crustaceans in their tank.

Figure 9. A shot of a zebra moray in the author's tank. Photo courtesy of Frank Marini.

Echidna catenata
Common name: chainlink moray, chain moray
Maximum size: 30", minimum tank size: 40 gallons

Chainlink morays inhabit coral and rocky reefs in the warm tropical waters of Brazil (Michael, 1999). The attractive body coloration of these fish varies considerably with each individual having a unique color pattern [Figures 10, 11, 12]. Juvenile chainlink morays have attractive green banding patterns with alternating yellow stripes, however, these colors tend to fade and run together as the animal ages. These eels are extremely reclusive and hide under the rockwork. They are nocturnal hunters and feed primarily on crabs. According to Michael (1998), they will frequently pursue these crabs out of the water and onto the shore to capture them. Chainlink eels readily adapt to aquarium life if provided with plenty of hiding places and adequately sized caves. These eels remain reclusive even after an adjustment period in captivity and tend to remain this way until hungry. Fortunately, chainlink morays readily wean onto prepared food after they are initially established on live fiddler crabs and small blue crabs. After some training, these fish will learn to take food from a stick. These morays are an excellent choice for a reef safe eel, as they are moderately sized, attractive, and wean readily onto prepared foods. However, they are a secretive fish that spends much of its day out of sight.

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Figure 10. A head shot of an chainlink moray. Photo courtesy of Dick "Fishdaddy."

Figure 11. A full body shot of a chainlink moray. Obvious in the photo is how thickly muscled the body of this eel is, and the unique color pattern. Photo courtesy of Dick "Fishdaddy."
Figure 12. Shown here is a 4.5 year old captive raised chainlink moray. While the maximum published size of chainlink morays is 30", this moray is 32". Apparently, he couldn¹t read the fine print. Photo courtesy of Musipilot.

Home Care

Once you decide that a moray eel could fit into your tank situation, there are a few requirements that will ensure a perfect home for your eel. In general, you want to provide a rockwork structure in which the eels can find a number of good hiding spots and areas where the eels can peer out into the tank. The good news is that in a reef tank, this is a typical rock structure. To ensure that an eel has the appropriate home when you are designing the reef, adding a PVC pipe three inches in diameter and two feet long will provide an adequate home [Diagram 1]. Eels will often dig under structures and can topple any loose rockwork, so "eel proof" your reef structure by using heavy or tightly fitting rock arrangements. When considering a zebra moray, in particular, the lower rock structures must be large and firm, thereby preventing any movement of the lower rocks by the eels when they are feeding. Adult zebra morays are very muscular and will readily topple any weak supporting structures as they use these structures to push against while ingesting shelled foods.

Diagram 1. Elaborate tunnels for your moray to hide in can be created by simply burying PVC in the substrate. Diagram courtesy of Kevin Carroll from The Book of Coral Propagation by Anthony Calfo.

Moray eels are the escape artists of the fish world; they can exit through any hole in the cover of your aquarium. Adult eels can push against the top to open the cover. To prevent any Houdini-like escapes, I recommend covering your tank and weighing the cover down. A tight fitting top that is locked or clipped into place is mandatory. Just be sure to allow for the fact that if you do decide on a tight fitting top and you are using an air stone or under gravel filter, that carbon dioxide may build up in this upper air space and this buildup needs to be frequently removed. If you do happen to find your moray has escaped and has partially dried on the floor, don't give up hope, moray eels secrete a thick slime layer that will protect them for a few hours outside of water. Some juvenile morays and the dwarf morays have been known to swim down under gravel lift tubes and get beneath under gravel plates (Michael, 1998), so if you have an under gravel filtration system, it should have screens over all the intakes.

Feeding

It is critical that moray eels be fed a varied diet, one that replicates their natural prey items. In the case of the morays discussed here, crustaceans and hard-shelled invertebrates should be an integral part of the diet. A close inspection of the seafood counter at your local grocery store will yield all the food items you'll need. Fresh gulf shrimp with the shells on, whole blue crab, octopus or squid will all be eaten with great delight. Most morays must be trained to eat dead food items and this is a lengthy, albeit critical, process for survival of your moray. Many pebble-toothed morays will take live shrimp and fiddler crabs as starter foods. Juvenile snowflake eels will readily eat ghost shrimp, and small freshwater feeder fish; however, these starter foods are just that, "starter food." After a period of establishment in your tank (say, two to three weeks) you need to wean these eels off the live foods and onto these prepared meals. I find it easiest to entice the eel by using a gulf shrimp on the end of a feeding stick [Figure 13, 14]. I buy a three foot long, three quarters inch in diameter hard plastic airline tube and impale the thawed gulf shrimp onto the end. I then use the stick to gently bump the food item onto the nose and mouth area of the eel. Do this gently once or twice. Do not try feeding it repeatedly; the eel will decide within a few seconds if it wants the food. If the eel refuses, then try again later or even the next day. From experience I've found that these eels can be stubborn. It took my zebra eel eight weeks before it fed on thawed gulf shrimp. So for eight weeks this eel did not eat a thing. In fact, according to Fenner (www.wetwebmedia.com), starving this fish for extended periods replicates its natural feeding cycle. You need to tailor your eels feeding schedule to the eel. When you do feed, try to feed until the eel refuses food; this is usually a few shrimps worth or a quarter of a small blue crab. The next feeding should occur when your eel has digested this meal and is out looking for food. My eels tell me when they are hungry and this usually occurs approximately 2 times per week. So, how often should your moray be fed? Morays do not have to eat very often, and overfeeding them simply adds to water pollution and growth/size problems. Overfeeding in eels is a major health problem; overfed eels accumulate fat at an unhealthy rate, and it's not uncommon to observe a fat eel at a public aquarium where the handlers feed the eel as part of an hourly display. If your eel refuses food, consider giving the eel some time off from feeding to allow the animal to utilize some stored fats. In my experience eels stop feeding for two reasons. First, the water quality in your tank has become compromised, and doing frequent partial water exchanges easily rectifies this. Second, overfeeding will cause your eel to stop eating; it may take several weeks before your eel resumes feeding again, so be patient.

Figure 13 & 14. The feeding stick (above) with an impaled fresh gulf shrimp getting ready to feed the eels(below). Photo courtesy of Frank Marini.

Compatibility

In general, more than one pebble-toothed moray can be kept in the same aquarium, provided that you have more than one cave or PVC pipe. As juveniles, snowflake eels will cluster in the same cave for protection; however, as adults this is often not the case. If a new moray is added to a tank with an established eel, you should expect the new moray to be chased and harassed until a dominance hierarchy is arranged, especially if you are introducing the same species. In many cases with the pebble-toothed morays, they will usually attempt to defend invasion of their hole and surrounding space by head butting and pushing or by open mouth pushing the new eel away. According to Michael (1998) some morays can be cannibalistic, as he has reported seeing regurgitation from a snowflake eel containing the remains of smaller snowflake eels. The good news is, in general, smaller eels will stay out of harm's way in the presence of larger eels.

One of the main reasons to acquire a pebble-toothed moray is to keep this eel with reef community fish, and, in fact, the majority of these pebble-toothed morays have no interest in fish at all. If it were not for the large muscular size and clumsy nature of zebra eels, I would recommend them as a perfect moray for a reef tank. The better choice is a snowflake eel as they adapt readily to tank life, prefer to feed in their holes, and only get rambunctious when food is in the water. These eels start fairly thin, reach a maximum size of 24", and will learn to take prepared foods from a feeding stick. Juvenile snowflake eels grow quite slowly, and it's not uncommon for them to achieve adult size in 2+ years.

While these crustacean feeding eels do not show much interest in fish, they may inadvertently take chunks out of them at feeding time. I have seen many a snowflake eel or zebra eel blindly thrash in the water column or actively pursue what they think is food, when it is really a tankmate that happens to be in the eel's scent trail. One thing to consider is that if you feed your eels live freshwater feeders for extended periods, it will increase the likelihood of these fish chasing and eating fish, since you have trained them to eat fish. So, again, my recommendation is to not feed these eels live feeder fish.

A Sick Moray… Now What?

Most moray eels are very durable fish. The two keys to success with morays eels are 1) excellent water quality and 2) a varied and nutritious diet. I have found that zebra eels in particular require good water quality and any decline in water conditions results in them "going off feed". This problem is easily resolved by frequent water exchanges and once the water issue is resolved, the eel is back to its normal self. According to Michael (1998), he has yet to see a moray succumb to disease or parasites and only on rare occasions has he noted external parasites or nematode infections. It is important to remember that when treating moray eels with medications is that moray eels are exceptionally sensitive to copper-based and organophosphate-based medicines. Therefore, Ich treatments should be done in hyposalinity and not copper.

Additionally, no treatments should be undertaken using medications with the tradename: Masoten, Dylox, and Dipterex. According to Fenner (1990), if your eel appears to be developing a bacterial or fungal infection, check for a decline in water quality. Should any parameter be suspect, perform water changes, or remove the eel to a quarantine tank and treat with an antibiotic. Water quality issues will also trigger a starvation response from many eels. When refusal of food is first observed, try performing water exchanges to alleviate the situation; however, if your eel continues to refuse food for more than a few weeks, attempt to find the source of the problem. This could range from bacterial infections to overfeeding. Don't fret over morays which have fasted for short periods; my eels have refused food for over 2 months with no loss of body condition or apparent health issues.

Conclusion

In conclusion, the pebble-toothed morays make an excellent tankmate in reef tank style aquariums. The snowflake eel is probably the best-suited eel for this tank as it is small, compatible with other fish species and adaptable to captivity. It is certainly one of the most passive, sociable, and desirable moray species. Just remember that snowflakes may mistakenly strike at other fish when food is in the water. Zebra eels would be the next choice, as they have no interest in fish at all, but their large size and clumsy demeanor makes them more likely to destroy reef structures. Finally, the chainlink moray are the least preferable of these eels as they get larger than zebra morays, have an equally clumsy demeanor, compounded with an exceptional shyness. If you want the coolness of an eel in your reef tank, then seriously consider a crustacean eating moray. Remember that fanged teeth equals a fish eater, pebbled-tooth equals fish safe.



If you have any questions about this article, please visit my author forum on Reef Central.

References:

Axelrod, H.R., Burgess, W.E. & R.E. Hunziker III. 1990. Atlas of Aquarium Fishes, Vol. 1 Marine Fish. T.F.H. Publ. Inc., N.J.

Fenner, Robert. 1995. Moray eels of the family Muraenidae. TFH 3/95.

Fenner, Robert. 2000. The Zebra Moray Eel, Gymnomuraena zebra. FAMA 7/00.

Gonzales, Deane. 1976. Puhi (Eel in Hawaiian). Marine Aquarist 7(7):76.

Michael, S. W. 1998. Family Scorpaenidae. Reef Fishes Volume 1. Microcosm. Shelburne. pp. 453 - 489.

Michael, S. W. 1999. Marine Fishes: 500 + Essential-To-Know Aquarium Species. Microcosm. Shelburne. pp. 63 - 69.

Additional Online Reading:

Fenner, Robert - http://www.wetwebmedia.com/morays.htm
Fenner, Robert - http://www.wetwebmedia.com/zebramor.htm




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A Serpent For Your Reef Tank: A Look at Fish-Safe Eels by Frank Marini, Ph.D. - Reefkeeping.com