Gymnothorax polyuranodon, commonly called the Freshwater Moray

Posted by Aquatropic Staff on October 28, 2022

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Gymnothorax polyuranodon, commonly called the Freshwater Moray, and the Tiger Freshwater Moray, is a very interesting species. While most Moray Eels are marine (saltwater) animals, and at least some migrate to freshwater to breed and thus the juveniles can be found (and kept short term) in freshwater, the Tiger Moray can be kept throughout its adult life in Freshwater. As far as we know, it is unique in this attribute. There is some evidence to suggest that Gymnothorax polyuranodon lives most of its life in fresh or brackish water and migrates to saltwater to breed. From the angle of husbandry, we've found that they seem to do best in freshwater systems with some trace salt added, though they also do very well in full brackish environments. Gymnothorax polyuranodon prefers a slightly basic environment, with a pH over 7; in freshwater, some salts will help most aquarists achieve this.

Freshwater Tiger Morays come from all over south east Asia and northern Australia, where they are most often found in slow moving or still pools with a lot of rocks / boulders where a large amount of small natural caves are available for them to hide in. There is only one record of an adult being found in full saltwater. Juveniles are often found in full salt and brackish water, where they are a common resident among mangrove roots. There are some reports of this eel being found in brackish environments on the east coast of Africa and northern Madagascar.

When talking about husbandry for any Moray, every article should start with the following sentence: MAKE THE LID TIGHT! Honestly, this point cannot be stressed enough. Gymnothorax polyuranodon (and tbh all Moray Eels) are essentially one big muscle that pushes its way into dark caves, constantly, its whole life. They can and will get out of much smaller openings than you think possible. They constantly search boundaries and will test the top and glass on your aquarium; surprisingly small eels are capable of pushing up standard aquarium glass tops. You will need a top that is fixed down like a screw down reptile screen or similar; heavy weight or mechanical fasteners are the only ways to prevent the otherwise inevitable escape of your Moray.

Feeding the Tiger Moray is just like any other moray eel. Reports of them not eating, or not gaining weight in freshwater have been shown to be without basis. While some aquarists have had trouble getting their Morays to eat, the salinity of their environment has been scientifically demonstrated to not be a factor. Tigers can go without food for extended periods of time, with some evidence that they could live for up to 4 weeks without food, this being said, that should not be any aquarists goal for successful husbandry. We suggest feeding smaller specimens every other day, and larger ones on a weekly basis. Meals should consist of a variety of thawed seafood, cut to a size about as big as the moray-in-question's head. All moray eels have notoriously poor eyesight and hunt mostly by scent. Aquarium environments that have lots of powerheads, or lots of food in the water can be confusing for a scent hunter. Feeding with long tweezers or feeding sticks is a great way to target feed your eel. While it is possible to train morays to eat from your hands, you should not do this. They have a very toothy, painful bite, and a rather noxious mouth. Many aquarists have reported these bites to be painful for up to 24 hours and prone to infection after that. Whether or not this is the result of venom is debated, but in the real world, this seems academic. Don't hand feed eels.

What if my Gymnothorax polyuranodon doesn't want to eat? In house, these eels eat pretty well for us. But even picky fish can usually be induced to eating with live food. Mollies, guppies and feeder shrimp or crayfish should all be taken with gusto. Keeping your own cultures of these, or acquiring them from known cultures pretty much eliminates your risk of introducing unknown ailments to your system. Feeding them at consistent times will have them recognizing you as a food source in short order. In time, they should segue off live food without much issue. Attempting to feed them at night, when they would naturally eat, can also help segue them over to thawed foods.

On the topic of feeding, well fed specimens have been kept with other fish long term with mostly no issues. However, some individuals seem to consume tank mates regardless of how well they are fed. Keeping them with larger fish will reduce this risk. Avoid housing them with overly aggressive fish, as Moray Eels look all toothy and tough, but are actually very passive fish that can be picked on which often results in an eel that won't come out for meals. In our experience you can keep multiple Gymnothorax polyuranodon together as long as they are introduced to the tank together and are all a similar size. Most cleaner inverts will be eaten, but snails and hermit crabs seem to be the longest lived.

Moray eels don't move around a ton in general, and Gymnothorax polyuranodon is no exception, honestly, they may move around less. They will spend most of their day hiding in rocks with their head sticking out, and generally won't come completely out of their caves unless something very interesting is happening or food is introduced. They will come out at night and swim around the tank a few times, but that seems to be about it usually. The largest specimen on record is almost five feet long, but this is extremely uncommon, adults are generally less than three feet long. This means they don't need a massive tank, even for how big of a fish they are. Smaller specimens could easily be kept in a 40 gallon breeder tank, and 90 to 125 would be more than ample for an adult or two, with perhaps a bigger tank for those of you interested in keeping a colony of them. Like any large predator, they produce a lot of waste, and bigger tanks are always better at being able to deal with this waste. There should be a lot of smooth stable rockwork for your eel(s), to find homes in and soft sand should be the choice as Gymnothorax polyuranodon has a soft skin that could get irritated by rubbing on jagged rocks or sand.

The Freshie

For the aquarist looking for a real freshwater eel, the Tiger Eel has few challengers. This is a fish with a great temperament, that will learn to recognize its keeper. They have a ton of personality (fishality?) and adapt very well to a captive aquarium environment. While some of them may consume fish in a community setting, others will be just fine. If you're willing to deal with some potential losses to keep such an amazing animal, then Gymnothorax polyuranodon is for you. Head over to your LFS and ask them about getting you one from Aquatropic today!