Gymnothorax polyuranodon

Gymnothorax polyuranodon was named by Pieter Bleeker in 1853.[4] The species can reach a maximum length of 1.5 metres (4.9 feet), but it is rarely seen larger than 1 metre (3 feet). Its colouration is yellow or light tan, covered with dark brown spots and blotches. The freshwater moray has very small teeth in proportion to the size of its head, and, like all moray eels, is carnivorous. Its diet consists almost entirely of fish, and occasionally shrimp, bivalves, and worms, and it prefers to eat live food. Like many moray eels, it has poor vision but an excellent sense of smell, and it can be aggressive.[5] The species' saliva is slightly toxic. While not lethal, a bite can nevertheless be very painful, and the injury may result in infection.[5][6]

Gymnothorax polyuranodon is a shallow-water species, often dwelling in water with a depth of just ten feet (three meters) or less. It is found in shallow marine coastal waters, estuaries, river mouths, and for short distances upriver.[7]

For years, it had been the accepted wisdom that the so-called "freshwater" moray eel was actually a marine species which, unlike most other morays, is able to tolerate water with lower salinity levels and could even live in fresh water but only for short periods of time.[4][7] However, there is a growing body of evidence which suggests that Gymnothorax polyuranodon is a catadromous species of moray eel, in which the animal spends much of its life in freshwater but migrates to saltwater to spawn. The young eels are born in marine water, and then migrate into freshwater to live out most of its life. When the time comes to reproduce, the adult eels migrate back into the ocean to breed. Recently hatched eels, or elvers, are not seen in freshwater, and there is only one record of a G. polyuranodon elver found within saltwater. Juveniles are frequently encountered in brackish water, especially in mangrove forests, which they hide in for protection against larger fish. Adults are commonly found in freshwater, but it's possible that they are only short-term residents of freshwater environments.[8][9][10]

In northeastern Australia, specimens of G. polyuranodon were observed over a period of three years in their natural habitat. The majority, nearly three-quarters of the observed specimens, preferred still pools or very slow-flowing water with lots of boulders for cover.[11]

Gymnothorax polyuranodon is an uncommon species in the wild, and it has been rare in the aquarium pet trade. However, in recent years, there has been an increasing amount of interest in this species both in terms of private ownership and scientific research. It has been the subject of several scientific papers, and it is becoming more visible in the online market for tropical fish species, although still in very small numbers compared with other eel species.

Because of its full-grown size, it is recommended that the minimum tank size for this eel should be 150 gallons, and preferably larger. All moray eels have very soft skin which can become easily damaged by sharp or rough surfaces, so do not have any broken seashells or coarse gravel in your aquarium. Your aquarium should have a bed consisting of either sand or smooth rounded gravel. It should also have many large rocks with multiple openings and overhangs for shelter. The water flow should be set to a low level. Because this is a tropical species, the water should be maintained at a temperature range of 24-28 Celsius (75-82 Fahrenheit).[12][6]

Experience gained by aquarists in keeping this species in captivity in a variety of environmental conditions has resulted in certain trends being observed. Most importantly of all is the confusion regarding this species being marketed as a "freshwater" species. Because it is often marketed as such, buyers frequently assume that this fish can only survive in freshwater, which is untrue. It has been established that this eel can survive in pure saltwater, pure freshwater, and various degrees of brackish water, but its preference appears to be brackish water with medium-level or high-level salinity (specific gravity of 1.010 or higher). While this fish is able to survive for prolonged periods of time within pure freshwater, as long as two years by one account, most aquarists who have experience keeping this eel state that this species must have salt added to the water in order for the eel to have optimum health. Aquarists have noted that the lower the level of salt in the water, the less appetite the eel has. While specimens of this eel may eat heartily in medium or high-salinity brackish water, its appetite drops when in low brackish water, and it might stop eating altogether in freshwater and starve itself.[13][12][6]

Another trend noticed by aquarists is that G. polyuranodon is a highly individualistic species. Different individuals have different personalities, and are therefore to a certain degree unpredictable in terms of their behavior. Some aquarists have been able to keep multiple specimens in the same tank with little to no aggression or competition being displayed amongst them, while others were aggressive and highly territorial. Some specimens will tolerate other tankmates, while other specimens will snap at and even attack and eat any other fish in the tank.[13][6]

Due to its aggressive nature and its diet of live fish, it is unwise to keep this eel with small fish species because they will probably be eaten. It is best to keep this eel with larger fish, or as a species-only tank. If the eel is well-fed, the likelihood that it will attack and eat its tankmates is greatly reduced. One study showed that if a specimen was well-fed, it usually did not eat again for twenty-four hours. Attempts to keep this eel in aquariums with other individuals of the same species have shown that if the eels are all more-or-less the same size, they are generally tolerant of one another.[10]

While this eel prefers to eat live food, and while it may eat only live food at first when in captivity, it's possible to acclimate the eel to eating prepared foods such as pieces of shrimp, fish, and mussels.[12]

Most moray eels are nocturnal, so the best time to feed them is in the evening. Because morays are scent-based predators, adding too much food into the tank at once will confuse the eel, who will not be able to detect where exactly the scent is coming from. Therefore, it is best to do small-scale target-feeding of the eel, providing it with pieces of meat no larger than its head. Because of its poor eyesight and its toxic saliva, it is unwise to hand-feed the eel; a feeding stick is preferred.[13]

Thomas Flörkemeier has reported that a related species, Gymnothorax tile, commonly known as the Indian Mud Moray Eel, can be killed by the addition of copper-based medications. Therefore, any aquarist considering owning a specimen of G. polyuranodon must take into consideration what the negative effects of adding copper medications to their aquarium might be.[13]

If the intention of the aquarist is to breed this species, then it must be done in a marine aquarium, as breedling likely does not take place within freshwater.

Like all moray eels, this species does not like to be confined in a tank and it will try to escape. Therefore, the whole top of the aquarium must be secured with a tight-fitting lid. This eel, like others, will compress its body and squeeze through the smallest open gap.

• Fishes of Australia: Gymnothorax polyuranodon
• Australian Museum: Freshwater Moray, Gymnothorax polyuranodon (Bleeker, 1853)