Related FAQs: Brackish Water Fishes, Brackish Invertebrates, Brackish Plants, Related Articles:  Brackish Livestock, Brackish Fishes By Groups: (see below for links to family, species accounts), Brackish Invertebrates, Brackish Plants Why do the oddballs all seem to need brackish water?
by Neale Monks

Visit your local tropical fish store or flip through any encyclopaedia of freshwater aquarium fish, and you'll soon notice that virtually all of the commonly kept species come from just a handful of taxonomic groups. The cyprinids and characins top the list, but not far behind are the cichlids, catfish, and livebearers. A bit less diverse, but still important, are the gouramis, loaches, and killifish. Whether you are a specialist who likes to keep and breed challenging species or a beginner with a mixed community tank, the odds are pretty good that the fish you have come from one or more of these eight groups. 

But there are exceptions. For a start there are those small families of freshwater fish that aren't very diverse to begin with, like the spiny eels, mormyrids (elephant noses), bonytongues (Arowanas), and garpike. Because there are less than a dozen species of garpike, for example, it isn't really all that surprising that only one or two are traded with any regularity. 

And then there are the oddballs. The classic oddball is a species from a big, diverse family of fish that for whatever reason hasn't become popular with aquarists. No species sums this up better than the bumblebee goby. Practically every aquarium store sells these fish, and many aquarists have tried (with mixed results) to keep them in their community tanks. They are certainly colourful and amusing to watch, even if they are a bit tricky to look after. But the odd things is this: the goby family is one of the biggest known to science, with thousands of species described and many more left to discover, and yet only this species, and perhaps the knight goby as well, have left any sort of mark on the freshwater aquarium hobby. Why should this be?

 

Strange families 

It isn't just the goby family that fits into this oddball pattern, there are plenty of others. There are hundreds of pufferfish, and yet less than ten are imported with any regularity, and really only two, the figure eight puffer and the green spotted puffer, are at all commonly seen. Similarly, the atherinids are another big group of fish, but only two, the Madagascar rainbowfish and the Celebes rainbowfish, are kept by hobbyists. You will find much the same pattern with a whole host of other families, including the moray eels (one commonly traded species, and a couple more occasionally seen); scorpionfish (only one species, the Bullrout, is regularly traded); flatfish (two flounders and two soles are kept, but traded only very rarely); pipefish (as many as three species have been kept by hobbyists, but none commonly); snappers (only one species, and that one very rarely); and Snooks (the glassfish, one quite common, two or three more less frequently). 

So what's the story here? Are aquarists somehow ignoring some perfectly decent aquarium fish? Or is there a cartel of tropical fish traders deliberately limiting the selection of species they offer? No, the simple fact is that virtually all of the oddball species that you're likely to see in your local fish store -- from the tiny gobies to giant snappers -- are rare freshwater, or more usually brackish water, interlopers from otherwise marine families. Indeed, one of the most common reasons people set up a brackish water aquarium is because they want to try out one or more of these oddball species. In my experience, pufferfish and shark catfish are among the most eloquent of these 'persuaders'. Few fish are as comical or seemingly intelligent as the pufferfish, and so despite the fact that they do need rather particular care to do well, in the right tank they are among the most entertaining fish in the hobby. Shark catfish, on the other hand, win their devotees thanks to their good looks and impressive size. Among the most handsome catfish traded, they are also peaceful (though predatory) and, unusually for a catfish, active by day.

 

So why brackish water? 

So we accept that puffers and shark cats, not to mention gobies and sleepers, glassfish and flatfish, usually prefer brackish water; but why? The answer is in their physiology. Freshwater fish families, such as cichlids and loaches, live in an environment where the saltiness of the water outside them (the river or lake they are swimming in) is lower than the saltiness of their blood and tissue fluids. Because of this salinity difference, they tend to absorb water from, but lose salt to, their environment. To counter this, they spend energy getting rid of excess water while gathering up salts to make up the losses. You probably remember from biology at school how humans, like other warm blooded animals, have specific physiological processes to allow them to keep their temperature constant, such as shivering to increase temperature and sweating to lower temperature; that's thermoregulation. This process whereby fish control the salts and water in a careful balancing act is similar, and called osmoregulation. 

Marine fish face a different set of problems. Their bodies are less salty than the sea, and so they tend to lose water but gain salt. To compensate for this, their osmoregulatory mechanisms work in the opposite way to freshwater fish, and get rid of salt while conserving water. What should be immediately obvious is that what works in the sea would be deadly in freshwater, and vice versa. A marine fish dumped in fresh water would not be able to get rid of the water it was absorbing, and have no way to collect the salts it would be losing. Unsurprisingly then, marine fish cannot survive in rivers and lakes, and freshwater fish cannot live in the sea. This is why the important families of fish kept by marine aquarists, like angels, butterflyfish, and surgeonfish, are completely different ones to those kept by freshwater aquarists. 

Brackish water provides a middle ground. A few freshwater fish families include species that are able to adapt to brackish water, the cichlids being among the best known to aquarists at least. But much more common are the representatives of marine families that have successfully adapted to the reduced salinities of brackish water habitats, among which various species of flatfish, goby, sleeper, and pufferfish are some of the most common.  Brackish water is close to the saltiness to that on the inside of the fish. In strongly brackish water, the basic marine physiology works fine so long as it is toned down a little because it does not need to put right the gained salt and lost water all that rapidly. Evolution doesn't actually need to do all that much for a fish to be able to adjust it osmoregulatory system so that it has a 'high' and a 'low' setting, and it is no wonder that lots of marine groups can be found in strongly brackish water, including such familiar species as batfish, snappers, and porgies. 

A marine fish that goes into the slightly brackish waters does have a bit more to do though. Simply turning down its standard issue, marine-adapted, osmoregulatory system isn't enough, because now the problem is correcting for the loss of salt and the gain of water. But because slightly brackish water is only a bit less saline that the tissues inside the fish, the rate at which it loses salts and gains water is slower than that for a fish living in fresh water. Evolution only needs to come up with some thing adequate for this, and even then, only for use in the short term. These fish keep the option of returning to the sea when they need to, perhaps for breeding, but more often just simply adopting a lifestyle that means that they swim between brackish and marine waters on a regular basis. For these fish, their freshwater osmoregulatory system is what an aqualung is to us: it does the job on a temporary basis, but they would no more want to be stuck in freshwater than we would stay under water and breath from an aqualung for the rest of our lives. No wonder then that when we try and keep these sorts of marine species -- like scats and monos -- in freshwater constantly, they tend to get sick and die.

 

Was brackish water the ancestral home of the bony fish? 

I've mentioned that freshwater fish are saltier than fresh waters, and that marine fish are less salty than the sea, but here's the really interesting thing: the saltiness of the blood and tissues of marine and freshwater fish is more or less the same. Why is this strange? The blood and tissues of sharks, as opposed to the bony fish we've been discussing here, have the same 'osmotic pressure' as the sea they live in. This isn't quite the same thing as saltiness, since they are using a nitrogenous by-product called urea instead of mineral salts, but the upshot is the same: there is no overall movement of water in or out of sharks. The urea is what gives shark meat its distinctive smell, and it means that unlike a bony fish, it doesn't need to spend energy drinking water and excreting the excess salt. Why don't marine species of bony fish do this? 

The clue to the answer is the 'saltiness' of the fluids inside bony fish. We know that both freshwater and marine bony fish are related and shared a common ancestor around 400 million years ago, and the fact that their blood and tissue fluids are so similar seems to confirm this. The fossils of early bony fish don't come from the open sea or from fresh water sediments, but rather from inshore, often brackish water, ones. In short, the first bony fish were brackish water fish! At the time, their blood and tissue fluids were just as salty as the brackish waters they lived in, and although they have no invaded marine and freshwater habitats from the deepest sea trench to mountain streams, their bodies still retain this echo of their original habitat. In contrast, sharks evolved in the open sea, and their bodies reflect this, and it is probably the fact that sharks would have needed to make a much bigger change to go from marine to freshwater conditions than the early bony fish had to make going from brackish to freshwater that explains why it is the bony fish, and not the sharks, that dominate the rivers and lakes of the world. So when you think about brackish waters next, don't just see them as the backdoor through which a few marine species have managed to creep into a freshwater aquaria, but remember that they were also the springboard from which the bony fish leapt to their present dominance of practically every conceivable aquatic habitat.