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By Neale Monks
Order Pleuronectiformes, Family Soleidae
Common names: Some species are known as pan or hogchoker soles, but usually all simply referred to as freshwater flounders or freshwater soles
Freshwater soles are all pretty similar in appearance. They are obviously very flat, but do notice that the compression is different to that of a freshwater stingray, for example. Stingrays are flattened from top to bottom, and the left and right sides of the body remain symmetrical just as they are in a regular fish. In contrast, flounders and soles are compressed laterally, like an angelfish or discus, except they swim with one side (called the blind side) towards the bottom of the tank. As a result, the left and right sides of their bodies, at least from the perspective of someone looking at them from the top, are not symmetrical. In fact, their whole body morphology is really weird: the fins on the edges of the body are enormously elongated dorsal and anal fins; the mouth opens on one side of the head; and the eye that should be on the blind side of the body is on the top side, putting both eyes next to one gill opening! While there are many unusual fish known to scientist and aquarist alike, from the seahorse to the pufferfish, surely the flatfish must take the prize as being far and away the most bizarre.
The freshwater soles all belong to the sole family, the Soleidae, which puts them alongside species more familiar to us as items on a restaurant menu, like the Dover sole. They are known as ‘right-sided flatfish’ because the eyes are on the anatomically right-hand side of the fish (though in life the right side is actually the dorsal surface of the fish for all practical purposes). Soles also tend to be rather rounded or tongue-shaped compared to the other major family of flatfish, the Pleuronectidae, such as the plaice, which tend towards a more rhomboid or diamond-shaped body. Only two species of freshwater sole are at all commonly imported, Trinectes maculatus, from North America, and Brachirus panoides, from South East Asia. Brachirus pan, from India and Bangladesh, may also be seen, but more commonly its name is wrongly applied to specimens of Brachirus panoides. Telling these species apart isn’t difficult. Trinectes maculatus (sometimes called the hogchoker sole) is rounded with a distinct tail fin; in contrast, species of Brachirus (often called pan soles) have round heads but long, tapering bodies that merge into the tail. Of the two species of Brachirus, Brachirus panoides is less elongate that Brachirus pan, which can be about five times longer than it is wide.
All freshwater soles are nocturnal animals, and most of the time they will be hidden in the sand at the bottom of the tank with only their eyes and gills showing. They are not particularly predatory, although they can and will eat very small fish such as livebearer fry, and can be kept safely with any quiet species too big to be eaten. Gobies, glassfish halfbeaks, and pipefish would all make suitable companions for a freshwater sole. More active fish that stayed at the top of the aquarium, such as hatchetfish, rainbowfish, killifish, and certain livebearers, would also be work well. Placid bottom dwellers, like banjo catfish, that wouldn’t compete with them for food are another option. On the other hand, anything that lives on the bottom and insists on digging up the sand or chasing away anything else in the lower part of the aquarium, as is the case with many cichlids and catfish, wouldn’t work well.
Brachirus panoides and Trinectes maculatus both occur in fresh and brackish water, and Trinectes maculatus can also be found in the sea. Slightly brackish water would perhaps be the ideal, with the specific gravity maintained around the 1.005 mark. Of the two, Brachirus panoides is the better bet for long-term maintenance in hard, alkaline freshwater, but both species are noted for their overall hardiness provided extremes of pH and alkalinity are avoided. One issue that is important is proper oxygenation of the water; like most brackish water species that are interlopers from the sea, these fish do not tolerate low oxygen concentrations as well as many other freshwater fish.
The only other demand these fish place on their owners is the need for soft sand to burrow into. This needs to be stated boldly: these fish do not like plain gravel substrates! Like spiny eels and dragon gobies, a sole that cannot burrow is an unhappy soul (if you’ll pardon the pun), and unhappy fish eventually become sick fish. Silica sand is ideal because it is chemically neutral as well as nice and smooth, and can be inexpensively bought from a garden centre. Since these fish don’t mind an alkaline pH, beach or even coral sand can also be used if necessary. Though these fish love sand, undergravel filters don’t, so an aquarium set up for freshwater soles is best filtered using an internal or external canister filter of some sort.
Freshwater soles aren’t easy to feed because they are nocturnal predators. They will not take food offered by day, and at night they cannot compete with more active scavengers like large catfish. It’s best to ensure that the other fish in the tank don’t feed at night, so that the freshwater sole can feed at its leisure.
In terms of variety, these fish are fairly accommodating. Frozen bloodworms, Mysis shrimps, brine shrimp, and Tubifex all work well will all be taken. Some freshwater soles also develop a taste for catfish pellets, but take care to choose the pellets made for things like Corydoras because these fish have quite small mouths. Freshwater flounders will also eat some green algae as well.
Unknown in captivity; these fish probably spawn at sea or at least in estuaries, but as yet our knowledge of their reproductive biology is limited.
Diseases and disorders
As noted earlier, these fish are sensitive to low oxygen concentrations, but they are otherwise quite robust and long-lived, if kept well. But mortality in home aquaria is quite high because many aquarists fail to ensure that they get enough to eat. It is entirely possible that specimens in display tanks at your local retailer have not had a good feed in weeks, so always check to see what food the staff in the shop are offering these fish.
Freshwater soles are interesting fish that will add an accent to any quiet community of hard water or brackish community fish. They are a bit tricky for absolute beginners, and will not do well in a mixed community tank with robust fish likely to snatch up all the food before the soles even get a bite, but they otherwise have a great deal to recommend them.
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