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Brackish water fishes of South America
The brackish water fishes of Southeast Asia include such popular and widely-sold species as Monos, Scats, Archerfish and Knight Gobies. By contrast the brackish water fishes of South America are much less familiar. Or rather, while some are infrequently traded, others are simply not recognised as being brackish water fishes. This has meant that aquarists don't always realise that an aquarium representing a South American brackish water habitat can be a very viable project.
Of course, the question is why you'd want to keep such an aquarium, but that's quite an easy one to answer! Besides the various tetras and catfish South America is so famous for, it's also home to various livebearers as well, and some of these, including Guppies, Limias and especially Mollies can do rather better in slightly brackish water than plain freshwater. This is especially true if your water supply is soft or high in nitrate, both these factors stressing livebearers maintained under freshwater conditions but for largely harmless if the water is slightly saline.
In fact it's rather hard to distinguish the fish faunas of the brackish swamps and streams along the coastline of southeastern South America from equivalent habitats further north, even as far north as Florida. Indeed, some species naturally have ranges that extend along the whole Atlantic seaboard of the Americas from Brazil to the Carolinas, for example the Sleeper Goby Dormitator maculatus. In other cases fishes native to the coastal part of South America have become established outside of their natural range thanks to the aquarium industry; classic examples are things like Pike Livebearers and Brown Hoplos, both of which are now firmly entrenched in the brackish waters of Florida, among other places.
Companions for Mollies
The Brown Hoplo Hoplosternum littorale is actually a good place to start. It's a member of the Callichthyidae, the same family that contains the Corydoras catfish so popular with aquarists around the world. Compared to Corydoras it is a much bigger (6-8 inches) catfish with a more robust build and boisterous personality. It can be a bit disruptive in community tanks and does tend to uproot delicate plants, but that shouldn't stop it being selected for large (55+ gallon) aquaria alongside any number of other fish species. In the wild it is usually found in swampy habitats including brackish water marshes along the southeastern coast of South America as well as several offshore islands including Trinidad where this catfish is a highly prized food fish. Over short periods at least, it can tolerate surprisingly high salinities, and it actually does rather better in slightly brackish water than the soft, acidic water conditions favoured by Corydoras. Not in the least shy and completely omnivorous, the Brown Hoplo would make a superb companion for large livebearers that need slightly brackish water at a specific gravity of around 1.003, for example Mollies.
Another good companion for Mollies and Brown Hoplos would be the Fat Sleeper Dormitator maculatus. As noted earlier, this fish has a very wide distribution. It is sometimes found in the sea, but does seem to favour low-salinity streams and marshes. If kept in plain freshwater this species tends to be sensitive to opportunistic infections, a common trait among brackish water fishes, but kept at around SG 1.003 it is extremely hardy. In the wild some adults may be anything up to a couple of feet in length, but most specimens are much smaller, and in aquaria this holds true too, the average adult being about 12 inches in length. While omnivorous their predatory capabilities should not be underrated, and very small tankmates such as Guppies would be viewed as food. But fish too large to swallow whole are ignored, and in fact Fat Sleepers make excellent aquarium fish.
One of the most celebrated gobies in the hobby is the Violet Goby, sometimes called the Dragon Goby. Curiously, at least two species seem to be traded under this name. North American aquarists usually get the Atlantic coastline species, Gobioides broussonnetii, but European aquarists often get the Pacific coastline species, Gobioides peruanus instead. The two species are very similar, but Gobioides peruanus has much weaker violet markings on its flanks compared to the obvious violet chevron-shaped markings that run from the head all along the flanks of Gobioides broussonnetii. In any case, care is identical, and needs to reflect the peculiar environment these fish inhabit. In the wild they are usually found on mudflats and river estuaries where they live in burrows when the tide is out and only emerge when the tide comes in. So a tank with sand for digging and a few hollow tubes they can use for shelter are important. Feeding can be problematic only insofar as these big fish (potentially 18+ inches in length) and need a lot of food. They are actually rather omnivorous and naturally consume algae, plankton, and tiny animals sifted from the sediment. So offer them a mix of algae wafers, live brine shrimp and daphnia, and wet-frozen invertebrates such as krill, bloodworms and glassworms. Neither of the Violet Gobies is fussy about salinity provided there is at least some salt in the water; anything between SG 1.003 and 1.010 will suit them just fine.
At the other end of the size range is a surprisingly addition to any roster of brackish water fishes, a tetra! The X-Ray Tetra Pristella maxillaris has been appreciated by aquarists for decades because it is so tolerant of hard, alkaline water, something most tetras despise. The reason it does so well in the "wrong" water chemistry for tetras is that this exceptional species occurs along the South American coastline too, in slightly brackish streams. While this species isn't recommended for the average brackish water community, it's a handy species for use in tanks where very slight salinity can be useful, as with Guppies and Limias.
Speaking of livebearers, a couple of Micropoecilia species have started to turn up at aquarium shops with some regularity. One of them, Micropoecilia picta, has been around in the hobby for years, though tends to get traded at fish clubs and auctions rather than through retailers. It has sometimes been known as the Swamp Guppy and is native to Guyana, Brazil and offshore islands including Trinidad. The other species, Micropoecilia parae, comes from Guyana and Brazil and used to be rare in the hobby, but the so-called "Red Melanzona" in particular is now available through the better retailers. Both species are extremely variable and occur in numerous colour morphs. The Swamp Guppy is somewhat like a Guppy in shape, though smaller, and the males have bright patches of colour on their dorsal and tail fins. The Red Melanzona is similar in shape and size but males have longitudinal red and black markings on the flanks and tail fins. In both cases the females are plain greenish-silver. Maintenance of both species is much like that of Guppies, in other words, they do best in clean, spacious tanks with lots of plants. Also like Guppies, they need to be kept in groups where females outnumber males by at least two to one, otherwise aggression can be a problem. Water chemistry is a matter of some debate, and both species do naturally occur in plain freshwater as well as brackish water habitats, but under aquarium conditions slightly brackish water does seem to result in consistently better health and more frequent breeding. The exact salinity doesn't matter much, and anything from SG 1.002 to 1.005 is acceptable.
And finally, what about catfish for the South American brackish water community? Well, there are a few suckermouth catfish that naturally inhabit slightly brackish water, including Hypostomus ventromaculatus and Hypostomus watwata, but these catfish aren't traded much, if at all. At least one other species, Hypostomus plecostomus, has become established in slightly brackish lagoons and canals along the Florida coastline, and it certainly can tolerate low-end brackish conditions in aquaria, up to around SG 1.002-1.003, without coming to any harm. In fact the two catfish families most strongly associated with brackish water habitats in South America are the Aspredinidae (the Banjo Catfishes) and the Ariidae (the Sea Catfishes).
Of the Banjo Cats, it's the bigger species like Aspredo aspredo and Platystacus cotylephorus that migrate between fresh and brackish water, but these are only occasionally traded. Platystacus cotylephorus for example gets to 12 inches in length and has a strongly flattened body that allows it to hide in the sand much like a flatfish or flounder. After mating the females carry the eggs around attached to their underbellies; supposedly this is because the fish mate in brackish water but the fry need to be released in freshwater. Salinity isn't a key issue with these catfish and they can do well in both hard freshwater and moderately brackish water conditions to about SG 1.010. These catfish feed mostly on worms and insect larvae, and would make great companions for Mollies and other medium-sized livebearers.
The Colombian Shark Catfish Sciades seemanni is by far the most regularly traded brackish water catfish from South America. It has a large distribution in the wild, from Mexico as far south as Peru. Adult length in captivity is typically 12-15 inches, but though they are well-armed predators, they are also very peaceful schooling fish. In fact they do best kept in groups of at least three specimens alongside peaceful tankmates of similar size. Juveniles may be kept in freshwater for many weeks, even months, without harm, but contrary to what some retailers suggest, these catfish cannot be kept in freshwater indefinitely. Once they get to about 3-4 inches in length they should be kept in brackish water that has a specific gravity of at least 1.003, and adults will need at least 1.010 to remain in good health (adults also do very well in marine tanks).
So that rounds up this quick look at the brackish water fishes of South America. While some are of interest as additions to general brackish water communities alongside Southeast Asian and African species, as is certainly the case with the Colombian Shark Catfish, others could play a useful role in livebearer communities where only slight salinity would be needed.
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