Moray eels without a doubt are among those fish you’d call oddballs if they were from freshwater. Most specimens usually hide their long bodies in narrow gaps and holes all day, but if they leave them for food or explorative purposes, they are definitely eye catchers. Not only will they catch our attention, sometimes, if given the possibility, they will also catch our fingers, hands and arms with varying success.
In the last moray related article I composed for Conscientious Aquarist, I stated: “Own experience with regard to moray bites are luckily not available.”
Well, things have changed since then and I had an unfortunate meeting with the teeth of one of my moray eel. Accordingly, after the “wound” was treated, I searched literature with regard to moray toxins and moray bites and, aside my own little experience, came across several reports of hobbyists, scientific studies with regard to moray physiology and cytology, as well as toxicological studies on substances from morays.
Therefore, an overview about moray related substances and the results of moray bites are given here.

State of knowledge
There is quite a divergence in published books for hobbyists with regard to the toxicity of moray eel.
Most interestingly there is also a divergence with regard to the language these books were written in. Searching German books (and online sources) one often stumbles over the (here translated) statements “Moray eels are poisonous. Among all moray eel five species even can give a deadly bite. One of them is the Mediterranean moray Muraena helena.”
By contrast, English sources often state: “Moray eels are not poisonous”. Comparing these statements to scientific literature, both have to be considered inaccurate.

Mucous toxins produced by moray eel themselves
Studies as well as personal reports confirm that the bite of a moray eel can be much more painful than the bite of other predatory fishes of similar size. This cannot be explained by a “pulling back effect”, which means that one automatically pulls back a bitten limb and thus increases the wound by driving the moray eel’s teeth through the flesh forcefully. Such happens with other fishes, too.
Secondary infections also fail to explain strong pain immediately after a moray bite. Such infections can likely occur if wounds are not treated properly, but will need several hours to spread.
In addition to the pain, wounds from moray bites often bleed unusually heavily.
It was suggested that bleeding and pain are related to a toxin in the slime coat of the skin and the mucous of the mouth. And truly, the mucous of moray eels was analysed and not only one, but also several toxic substances were found. One of these substances is hemagglutinin. This is a glycoprotein that makes red blood cells clump. Another yet not too well understood toxin found in the mucous coat of moray eel was shown to be haemolytic. That means the toxin destroys red blood cells. These toxins are produced by glands in the skin of the eel, and a category of toxins known as crinotoxins.
Closer examination of the Starry or Yellow-mouth moray eel Gymnothorax nudivomer, (formerly Lycodontis nudivomer) have shown that it has specific club shaped cells in its thick (up to 2 mm) skin that produce these toxins. So, in contrast to many other toxic marine animals (e.g. pufferfish) that ingest poisonous critters and store the ingested toxins in their own tissue, morays are capable of producing crinotoxins themselves.
Those club shaped cells have not only been found in the Yellow-mouth moray, but also many other species. It is likely that the majority of the approximately two hundred known species of moray eel can produce toxins, but this has not been subject to scientific research yet.
What is known is that the number and distribution of the club cells varies among species. In addition, it was suggested that moray eels with serrate teeth (e.g. G. nudivomer; G. albimarginatus; G. chlamydatus; G. ocellatus) could transfer more mucous into the wounds and thus deliver the most painful bites.
Germane to this point it the belief that the crinotoxins of morays are not particularly dangerous to man, but rather lead only to increased pain and bleeding. Even allowing for this, allergic reactions may be possible and are just one more reason why you don’t want to be bitten by a moray eel.
In short, those crinotoxins underline two general rules of tank maintenance: Don’t put your hands into the tank if you have open wounds, and don’t touch your moray eel. Both of these could make you vulnerable to crinotoxin poisoning even without the moray biting you.

Blood toxins
In common with other members of the eel order Anguilliformes, moray eels have blood that contains proteins that are toxic to humans. These proteins are usually referred to as ichthyotoxins, a word that simply means “fish poisons”.
Ichthyotoxins were among the first toxic substances identified, fisherman having long known that certain fishes have to be heated about 75˚C before they are safe to eat (high temperatures destroying the toxins). Because of these ichthyotoxins, making moray eel sushi is not a good idea!
Ichthyotoxin poisoning can lead to spasms and heavy breathing; ichthyotoxins are also haemolytic and should not touch your eyes, mouth or open wounds. Because of this bleeding moray eels should be handled carefully.

Tissue toxins
Aside from mucous and blood toxins, moray eels (like many other predatory fishes) can accumulate additional toxins in their flesh and internal organs. The two most prominent of these are ciguatoxin and maitotoxin; these toxins are not destroyed by cooking, and cause a type of food poisoning known as ciguatera. Fortunately for the aquarist, these toxins are not transferred by bites.
Ciguatoxin is not produced by the moray eel itself, but by microscopic algae dinoflagellates such as Gambierdiscus toxicus. Dinoflagellates are at the base of the food chain and are eaten by herbivores such as snails and clams. Small predators such as crabs eat these herbivores; these get eaten by bigger predators like fish; and so on up the food chain. With each step up the food chain the concentration of the toxin becomes greater because of a process known as bioaccumulation. The amount of ciguatoxin in organisms at the lower end of the food chain is small enough not to be poisonous in most instances, but the tissues of predators at the top of the food chain, such as giant morays like Gymnothorax javanicus can contain dangerously toxic amounts of ciguatoxin.
Ciguatoxin is a neurotoxin; it inhibits the sodium channels needed for signal transmission in the nerves. Symptoms are sickness, spasms, skin irritations and partial paralysis. In terms of its effects it is not unlike the pufferfish toxins tetrodotoxin and saxitoxin, but luckily it is slightly less effective and usually occurs at lower concentrations. Consequently ciguatoxin poisoning is rarely lethal, with less than 1% of those so poisoned dying. Even so, it is often recommended that moray eels larger than 1.5 m should not be eaten because the precise amount of ciguatoxin in the tissues of fish that size is unpredictable and potentially dangerous.

Secondary infections
While the physiological effects of moray eel crinotoxins are not fully understood but seemingly not usually serious, and the toxins in the blood and tissue hardly likely to cause harm if you are bitten, the secondary infections that can set in after a moray eel bite are very definitely known to be a serious threat to your health.
Moray eels don’t have pectoral fins let alone hands, but even if they could be trained to use a toothbrush, they’d have major problems cleaning the numerous spaces between they multiple rows of pointed teeth. Rotting food provides a perfect environment for bacteria to live and multiply. Among the bacteria known to inhabit moray eel jaws are species of Vibrio, known to cause septicaemia, and species of Pseudomonas.
These bacteria will be very happy to go from the jaws of a moray eel onto a nice fresh piece of meat like your hand, and a moray eel bite is the perfect way for this to happen. Although there seem to be no fatalities from secondary infections caused by moray bites, there certainly have been fatalities caused by Vibrio infections from other sources.
The most important thing when bitten by a pet moray is to clean the wound with painstaking care. Hold your hand under the tap and thoroughly wash out the wound. Disinfect the wound. Be sure to visit a doctor if you see your wound is heavily swelling or turning red. Septicaemia is extremely dangerous and requires immediate medical attention.
Larger wounds can be dangerous due to the loss of tissue and blood. They will need medical care. It’s a good idea to ask someone else to drive you there because dizziness often accompanies these types of injuries. Make an emergency call if necessary. If you are loosing lots of blood, because some larger blood vessel has been cut open, the affected limb will need to be ligatured until further help arrives.
           
Prevention
To summarise, pet moray bites are far from inevitably lethal, but there are some nasty, potentially dangerous effects nonetheless, so situations where a moray could bite you should be avoided as far as possible.
Some general rules to help you and your moray eel to get along follow; with these in mind you can enjoy your very special pet (and your moray eel will be spared the problem of digesting mammal long-fibre meat).

1. No hand feeding
It may be daring to show to friends that such a wild animal can be fed by hand, it may also seem like hand feeding could somehow improve your relationship with your pet, but hand feeding obviously puts you at a very real risk of being bitten by a moray eel.
Hand feeding is widespread among hobbyists, with many aquarists having done so without incident for years. But that is no guarantee that your moray will never decide to taste human flesh one day! Even if your specimen seems to take food very carefully out of your hand, this can change from one day to another. Especially the supposedly “tame” Snowflake eels Echidna nebulosa that are known to suddenly make up their minds when feeding, as well as being unpredictable with regard to considering their tankmates as potential food.

2. No large species
It is one thing to be bitten by a twelve-inch juvenile Snowflake eel with quite small teeth; being bitten by a 5-foot Green moray Gymnothorax funebris is an entirely different experience.
Moray eels of that size are capable of tearing out large pieces of flesh from your arms; they can rip off entire fingers and mutilate careless owners. Only people who really know what they are doing should only keep such animals; they are best left to public aquaria or perhaps insanely dedicated enthusiasts.
Among the large and potentially dangerous species seen in trade too often are the Tessellated or Honeycomb moray eel Gymnothorax favagineus; the Green moray eel Gymnothorax funebris; and the aptly named Giant moray Gymnothorax javanicus. WetWebMedia.com lists some more potentially dangerous moray eel species unsuitable for most fishkeepers.
In fact all species that exceed a body length of more than three feet (except the Zebra, Chainlink and Ribbon moray eels) should be considered potentially dangerous with regard to the wounds that they can inflict.  There are also some rather small species equipped with long and curved teeth that should also be handled carefully as well, including the various Dragon and Viper morays (Muraena spp. and Enchelycore spp.).

  

3. Use tools for maintenance
One of the basic rules of marine fishkeeping is to keep your hands out of the tank as much as possible. Usually this refers to the possible transferral of non-beneficial substances such as oils, fats and soap into the water, but in case of a moray eel aquarium it also refers to your safety.
Numerous long tools are available for almost anything that needs to be done in a tank. Feeding sticks, long pliers, tweezers and other items are widely available in the shops. This arsenal can be extended with basic DIY abilities and a little imagination.
If you really need to work in a tank with your hands, be sure the lights are on, know where your moray eels are in the tank, and have a net available to keep their heads away from your hands. Ideally, have another person with you to keep watching the moray and reporting what its doing and where it is.

4. Keep your attention focused
Probably this is the hardest rule to stick to. Maintaining tanks for months and years leads to a certain complacency that inevitably drains your attention. This is widely known among keepers of venomous snakes, who often have a fear of forgetting to lock their pet’s quarters once they’ve become too used to the routine.
My own moray eel biting experience was also caused by a lack of attention. I got up 6.30 AM and just had a quick look at one of the tanks. One of the new Sarcophyton cuttings had left its place and not even half awake I put my hand into the tank to put the Sarcophyton where it should have been and ended up with a moray eel attached to my little finger!
The moray released its bite quickly and hid for the rest of the day. It was very shy for the next few days as well, so I guess this was not a nice experience for the moray eel any more than it was for me.
By not being attentive and not using the right tool for the job, I ended up being bitten. Of course the positive thing for me is that I’m now a lot more careful about paying attention, and hope this painful reminder will have this effect for quite a while yet!

Final words
In summary, moray eels are poisonous in several ways. Their bites can be poisonous, although the exact toxicity is so far unknown and considered to be rather low. However, secondary infections following any moray bite, as well as the loss of tissue and blood from large moray bites, both constitute a more probable and potentially more serious health risk.
With this said, the fact that morays can bite is no reason why aquarists should not keep small or medium sized species and enjoy them as the wonderful aquarium pets that they are. What moray bites do constitute is a good reason for being careful when handling them and paying attention to any work in the aquarium, two attitudes that make for better fishkeeping anyway, whether you are keeping moray eels or not.

Literature
Böhlke, E.B. & Randall, J.E. (2000): A review of the moray eel (Anguilliformes: Muraenidae) of the Hawaiian Islands, with descriptions of two new species. Proc. Acad. Nat. Sci. Philad. 150, p. 203-278
Erickson, T.; Vanden Hoek, T.L.; Kuritza, A.; Leiken, J.B. (1992): The emergency management of moray eel bites. Ann. Emerg. Med. 21, p. 212-216.
Fenner, R.M.: The Moray Eels, Family Muraenidae, pts. 1 & 2 - The Diversity of Aquatic Life Series. published at http://www.wetwebmedia.com/morays.htm
Lewis, R.J.; Sellin, M.; Poli, M.A.; Norton, R.S.; MacLeod, J.K.; Sheil, M.M. (1991): Purification and characterization of Ciguatoxin from moray eel (Lycodontis javanicus, Muraenidae). Toxicon 29(9), p. 1115-1127.
Mebs, D. (1989): Gifte im Riff: Toxikologie und Biochemie eines Lebensraumes. 120 p., Wissenschaftliche Verlagsgesellschaft Stuttgart. (In German)
Randall, J.E. ; Aida, K.; Oshima, Y.; Hori, K.; Hashimoto, Y. (1981): Occurrence of a crinotoxin and hemagglutinin in the skin mucus of the moray eel Lycodontis nudivomer. Marine Biology 62(2-3), p. 179-184.
Randall, J.E.; Earle, J.L.; Pyle, R.L.; Parrish, J.D., Hayes, T. (1993): Annotated checklist of the fishes of the Midway Atoll, Northwestern Hawaiian Islands. Pacific Science 47(4), p. 356-400; p. 461.
Riordan, C.; Hussain, M.; McCann, J. (2004): Moray eel attack in the tropics: a case report and review of the literature. Wilderness and Environmental Medicine 15, p. 194-197.
Schäfer, F. (2005): Brackish water fishes. Aqualog Verlag.
Suzuki Y. & Kaneko T. (1986): Demonstration of the mucous hemagglutinin in the club cells of eel skin. Dev. Comp. Immunol. 10(4), p. 509-518.

  

1. The aggressor: an almost adult Gymnothorax tile (© Marco Lichtenberger)
2. The victim after cleaning the “wound”. Two similar scratches from the lower jaw were on the other side of the finger (© Marco Lichtenberger)
   

1. Intraoperative photo showing segmental tissue loss from a bite of a large Green Moray Gymnothorax funebris (Photo with kind permission of Dr. Colin Riordan)


 

2. The same wound one year after the attack (Photo with kind permission of Dr. Colin Riordan)This baby Green Moray Gymnothorax funebris is only 7 cm/2.75 inches now but can reach a length of 2.5 m/8 feet; quite clearly a species not suited to average home aquarium (© Marco Lichtenberger)

    

   
   
   
3. The Tessellated Moray Gymnothorax favagineus is potentially dangerous, so unless you’re a clearer shrimp keep your limps out of the tank! (© Marco Lichtenberger)