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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 is given here.
State of knowledgeThere 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.' 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 themselvesStudies 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 hardly can 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 usually bleed 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 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. Since these toxins are related to glands in the skin of the eel, they are also called crinotoxins. The observations summarized here were made on the moray eel Gymnothorax nudivomer, also known as Starry moray or Yellow mouth moray and formerly placed in the genus Lycodontis. Closer examinations showed that the Yellow mouth morays had specific club shaped cells in their 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 well possible that the majority of all about 200 moray eel species can produce toxins, however, this has not been subject to scientific research yet. What is known so far 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 most mucous into the wounds and thus produce the most painful bites. To this point it is believed that the crinotoxins of morays are not very dangerous and only lead to increased pain and bleeding. However, allergic reactions may be possible and are just one more reason why you don't want to be bitten by a moray eel. Those crinotoxins additionally support two general rules of tank maintenance: Don't grab into a tank with wounds on your hands and don't touch your moray eel.
Blood toxinsMoray eels, as well as many other eel like fish of the order Aguilliformes have toxic proteins in their blood. They are usually referred to as ichthyotoxins, which simply means 'fish poisons'. They are among the oldest toxic substances from marine critters known to mankind. Fishermen in general are aware of the fact that certain fishes have to be heated above 75°C to destroy the toxins. Consequently, making moray eel sushi is not a good idea. Ichthyotoxin poisoning can lead to spasms and heavy breathing. These substances are also haemolytic and should not touch your eyes, mouth or open wounds. Bleeding moray eels should be handled carefully.
Tissue toxinsAside mucous and blood toxins moray eels - as many other predatory fishes -- have the possibility to store specific toxins in their flesh/organs. The two most prominent substances are ciguatoxin and maitotoxin. These toxins cannot be destroyed by heating the fish to 75°C. They can lead to a type of food poisoning known as ciguatera. It's good that these substances are not transferred by bites. Ciguatoxin is not produced by the moray eel themselves, but by dinoflagellates (e.g. Gambierdiscus toxicus). These are microscopic protozoans often capable of photosynthesis. Some of these planktonic or sessile organisms produce ciguatoxin. Dinoflagellates are at the lower end of the food chain. Sessile species are eaten e.g. by snails and floating species are filtered by all kinds of filter feeders such as clams. That way the toxin is enriched from step to step in the food chain. Especially large specimens of moray eel such as the Giant moray Gymnothorax javanicus can enrich 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. It is not unlike the saltwater puffer toxin tetrodotoxin and the freshwater puffer toxin saxitoxin. Luckily it is slightly less effective and less concentrated. Ciguatoxin poisoning is not lethal in more than 99% of all reported cases. Anyway, it is often recommended not to eat moray eels larger than 1.5 m, because the actual amount of stored poison in morays of that size is unpredictable.
Secondary infectionsWhile the effects of the moray eel crinotoxins are still being discussed and so far considered small and the blood and tissue toxins can hardly affect you when bitten, secondary infections are a serious threat to your health. Moray eels don't have pectoral fins or hands. Even if they could be trained to use a toothbrush, they'd have major problems with cleaning the numerous spaces between their multi-serial teeth. Rotting pieces of food provide a perfect environment for nasty bacteria, which would love to be transferred to a fresh piece of meat (you and me) by a bite. Among them are infamous Vibrio spp.. Those bacteria have been cultured from captive moray eel. Vibrio can cause septicemia. Although there seem to been no fatal cases reported from moray bites, there have been fatalities by Vibrio infections from other sources. Another infamous hitchhiker you can get from a moray eel is a nasty Pseudomonas sp.. 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. Septicemia is no fun and can get you into hospital soon. Larger wounds can be dangerous due to the loss of tissue and blood. They will need medical care and if you feel you can't cope with your wound of whatsoever size, get yourself to a doctor or ask someone to give you a ride, if you feel dizzy. 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 may need to be ligatured until further help arrives.
PreventionIt can be concluded that pet moray bites are far from being inevitably lethal, but they are nasty, some potentially dangerous, and should be avoided as good as possible. Some general rules can help you and your moray eel to get along, so you can truly enjoy your very special pet and the moray eel will have no problem with 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 the relationship to your pet, but in general hand feeding is the most obvious activity to be bitten at by a moray eel. Hand feeding is widely spread among hobbyists and in fact can go well for many years, but there is no guarantee you never will be tasted. Even if your specimen seems to take food very carefully out of your hand, this can change from one day to another. Especially the 'tame' Snowflake eels Echidna nebulosa are known to suddenly make up their minds with regard to feeding and also with regard to considering their tankmates as potential food.
2. No large speciesIt is one thing to be bitten by a one foot juvenile Snowflake eel, which only has quite small teeth. Being bitten by a 5 foot Green moray Gymnothorax funebris may be a different experience. Moray eels of that size are capable of tearing out large pieces of flesh from your arms, they may rip off entire fingers and mutilate their careless owners. Therefore, such animals should only be kept by people who really know, what they are doing, e.g. staff of public aquaria or insanely dedicated enthusiasts. Among those large and dangerous species seen in trade too often is the Tessellated or Honeycomb moray eel Gymnothorax favagineus as well as the Green moray eel Gymnothorax funebris. The common name 'giant moray' for Gymnothorax javanicus is self-explaining. WetWebMedia.com lists some more potentially dangerous moray eel species not suited for the average fishkeeper. All species (except Zebra, Chainlink and Ribbon moray eels) above three feet can be considered difficult with regard to the wounds they potentially can afflict. Some of the smaller species, especially the ones with long and curved teeth such as the various Dragon and Viper moray eels (Muraena spp., Enchelycore spp) also acquire specific attention.
3. Use tools for maintenanceOne of the basic rules of fishkeeping is: Keep your hands out of the tank as much as possible. Usually this refers to the possible input of non beneficial substances such as oils, fats and soap. In case of a moray eel tank it also refers to your own 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 imaginativeness. If you really need to grab into a tank with your hands, be sure the lights are on, know where your moray eels are and possibly have a net available to keep their heads away from your hands. An additional person can help you by constantly reporting what the moray eel does.
4. Never lose your attentionProbably this is the hardest rule to stick to. Maintaining tanks for months and years leads to a certain routine that permanently drains your attention. This is widely known among keepers of venomous snakes, who often fear to forget to lock their pet's quarters due to 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 short look at one of the tanks. One of the new Sarcophyton cuttings had left its place and not even half awaken I grabbed into the tank to put it back and ended up with a moray eel on my little finger. The moray released its bite quickly and hid the entire day. It was very shy for a few days, I guess this was not a nice experience for the moray eel either. So, after losing my attention, I did not use a tool and was bitten. The positive thing is that I am paying attention since and hope this will last for a while.
Final wordsIt can be summarized that moray eels in fact are poisonous in several ways and also that their bites can be poisonous, although the exact toxicity is unknown so far and empirically considered rather low. Secondary infections and massive loss of tissue and blood from the bites of large specimens probably constitute a far greater danger. However, possible bites are no reason, why moderate sized moray eel cannot be wonderful aquarium pets. They are just a reason for being careful and paying attention, two actions beneficial for any tank, be it with moray eel 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. |
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