Gymnothorax polyuranodon

The freshwater moray eel (Gymnothorax polyuranodon) is a species of moray eel that is native to the Indo-Pacific region, including Sri Lanka, the Philippines, Indonesia, Papua New Guinea, the northern coastline of Australia, and various islands in the western Pacific. Other common names include the many-toothed moray, spotted freshwater moray, blackspotted moray, and tiger freshwater moray. Gymnothorax polyuranodon was named by Pieter Bleeker in 1853. 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. The species' saliva is slightly toxic. While not lethal, a bite can nevertheless be very painful, and the injury may result in infection. 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. 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. 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. 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. 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). 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.