FishProfiles.com Message Forums

Extremophile Fishes

Most of the fishes we encounter in our aquaria tend to live within well defined water chemistry parameters. Usually, the waters of our favourite freshwater fishes depart from a neutral pH (7.0) by only modest amounts, and then usually as a result of specific biotope conditions, such as the alkaline and mineral rich waters of the Rift Lakes, or the soft, acidic waters of the Rio Negro in South America. These two biotopes tend to represent, for many aquarists, the extreme ends of the fishkeeping spectrum, which in pH terms tends to lie between pH 5.8 and pH 8.4. Likewise, the mineral content of the aquatic habitats of our aquarum fishes tend to fall into well-defined groups - the soft, mineral deficient Amazonian waters representing one end of the spectrum, and again, the Rift Lakes representing the other. Where dissolved minerals are present in some quantity, these tend to be calcium and magnesium salts, resulting from the home waters of the fishes concerned coming into contact with geological limestone formations. Sometimes, silica sand plays a part - the action over long periods of time of both chemical and biological agents upon this resulting in a small but measureable silicate presence. Without this, diatoms - single celled plant-like organisms that form silica shells - could not exist in freshwater habitats, and their presence testifies to the presence of silicate minerals in the water in which they live.

Even bigger constraints apply to familiar marine habitats. The constitution of sea water tends to be remarkably consistent throughout the world's major oceans, no doubt as a result of the transport of vast volumes of water throughout the planet via ocean currents such as the Gulf Stream, the Labrador Current and the Humboldt Current. An indication of the vast quantities of water moved by these (and the concomitant mixing that takes place ensuring the consistency of sea water mineral content) is the unit of measurement for the flow rates of ocean currents: the unit in question, the Sverdrup (symbol Sv) being defined by the relation 1 Sv = 1,000,000 cubic metres per second. If anyone wishes to visualise what a volume of one million cubic metres would look like, imagine the contents of a thousand Olympic swimming pools in one spot - that gives you an idea how much water is being moved around the Earth per second at any given time by deep ocean currents! With this kind of mixing, it's hardly surprising that sea water tends to be fairly consistent, with the exception of such locales as the Red Sea, which is largely landlocked with little movement in or out in comparison to the great ocean currents of the Atlantic or the Pacific. Even the Red Sea departs little in terms of ratios of dissolved minerals, differing principally in total concentration (hence the higher specific gravity of the Red Sea). Most marine environments, and this certainly holds true for the majority of Indo-Pacific coral reefs, are stable at a pH of 8.3, with a specific gravity varying only marginally from 1.024, principally through the action of rain upon the sea surface diluting the seawater briefly. Even a torrential downpour, however, will make relatively little long term difference when falling upon the ocean, as the volume of fresh water deposited over a given area will be a tiny fraction of the volume of the sea water upon which it falls.

While I have chosen chemical parameters for the above exposition, physical parameters such as temperature are similarly given to varying within well defined limits. The temperature range between 20 degrees Celsius and 30 degrees Celsius encompasses the vast majority of the freshwater habitats of our exotic fishes, lower temperatures being principally the seasonal province of temperate zone biotopes with fishes adapted to those variations. Reef habitats are even more stable, varying little either side of 25 degrees Celsius in the main.

However, there are habitats where such happy conditions - variability constrained within well defined limits - are replaced by something altogether different. There are places on Earth that would seem, at first glance, to be completely unsuitable for fish life, or at best, provide conditions that pose a serious challenge to anything seeking to live there. I thought, as a result of recent forays around the web, to introduce everyone here to some interesting examples of what might be termed 'extremeophile fishes'.

I have already posted, in Cichlid Central, a piece describing Alcolapia alcalicus, which is one candidate for the title of "freshwater fish that isn't". This unique Cichlid lives in Lake Natron, situated in Tanzania near the Kenyan border, in a region replete with geothermal activity. Consequently, the waters of Lake Natron, fed by volcanic hot springs, are steaming hot and rich in sodium carbonate, resulting in a pH that during the dry season reaches the dizzy heights of 10.5 - a pH level equivalent to that of a laboratory solution of pure ammonia! The carbonate hardness is way off the scale of most test kits - does your test kit record a KH of over 7,000? Plus, in the immediate vicinity of the hot springs, the temperature can hit 50 degrees Celsius. Not conditions most people would associate with fish life, but, Alcolapia alcalicus ekes out a living in this seemingly caustic environment, feeding upon the unusual aufwuchs flora of the hot springs. Find out more about this strange Cichlid in my post here.

However, there are stranger things to come. Antarctica must surely rank as one of the least hospitable places on Earth, with winter temperatures inland plummeting to a bitterly cold -80 degrees Celsius, cold enough to cause mild steel to shatter like glass if struck. Even the oceans surrounding the immediate vicinity are dangerous places to venture unless one possesses some interesting adaptations to cope with the frigid conditions. Given that the ocean temperature can reach -2 degrees Celsius (and thanks to its salt content still remain liquid), the big problem for any creature living there is preventing its internal water content - bloodstream for example - from freezing solid. Warm blooded animals circumvent this, of course, by generating their own heat, but fishes lack this advantage. Yet there are fishes that have perfected the art of living in the freezer - the Suborder Notothenioidei, known colloquially as the Ice Fishes, have made Antarctica their home. These fishes possess some weird biological features - their blood lacks haemoglobin, for example. However, one protein that IS found in their blood makes them unique - these fishes manufacture a glycoprotein that is, in effect, an 'antifreeze' that keeps the liquid water of their tissues liquid even when the ambient temperature drops below zero. Adaptive radiation has seen these fishes, in time, expand their numbers to eight Families - comprising 43 Genera and 122 species! Quite an achievement for cold-blooded animals in such an environment ...

Returning to more temperate climes, I alighted upon another example that must surely make everyone's eyes pop out on stalks. There is, in Mexico, a cave known as the Cueva del Azufre, complete with a water outflow called El Azufre. This water body is affected by volcanic activity, but in a manner different to that of the soda lakes of Africa - the name should give the game away to Spanish speakers who will recognise the Spanish word for sulphur. This water body contains, dissolved within it, concentrations of the gas Hydrogen Sulphide, which as any aquarist quickly learns, is lethally toxic to aquarium fishes in even tiny amounts. The reason for this is that Hydrogen Sulphide bonds to hameoglobin in the blood and converts it to a stable molecule that is useless for oxygen transport - the same mechanism that makes Cyanides so deadly. Indeed, Hydrogen Sulphide is as dangerous as Cyanide in sufficient quantities, and the quanitites present in the El Azufre watercourse - 300 micromoles per litre - are well in excess of the levels required to see off many of our favourite aquarium fishes. Yet, remarkably, there are fishes living in these stinking, toxic waters ... a population of Poecilia mexicana, all the more unusual for being cave adapted with reduced eyes (and presumably on its way to becoming a livebearer version of the Blind Cave Characin in the next million years or so) makes its home here. And, it is not alone: reports indicate that it shares its home with a swamp eel called Ophisternon aenigmaticum, and even more remarkably, with a Cichlid fish that resembles Cichlasoma salvini! More about these fishes can be found in an article from The Cichlid Room, complete with a couple of illustrations ...

American aquarists, however, do not have to travel too far to find a fish that is adapted to extremes of another sort - low or zero dissolved oxygen. Umbra limi, the Central Mudminnow (which despite its name is actually a close relative of the predatory Pikes of the Genus Esox) is a fish that possesses some remarkable features of its own. It can withstand almost ridiculous temperature extremes (and reputedly is capable of surviving in bodies of water that freeze solid in the winter), and has the ability to live in water whose oxygen content has all but disappeared. More about this small but very adaptable fish can be found here for those unfamiliar with this North American native.

Finally, how about "Eel City"? This is the name given to a strange part of the ocean floor, in the neighbourhood of an undersea volcano called Vailulu’u situated in the seas off American Samoa - and must count as one of the strangest fish habitats of all, because the small eels witnessed on a submersible camera were swimming in and out of holes in rock that had, not that long ago, been molten magma ... information about these fishes and why they choose to live there is scant, and likely to be the subject of a future research programme by the Smithsonian Institution, but, they are out there - eels that make their home in rock rubble around an active volcano, surrounded by water that at times becomes scalding hot (and at nearly 600 metres below sea level, the ambient pressure is such that water does not boil at 100 degrees Celsius - it can remain liquid at higher temperatures) and again laden with sulphurous minerals. While bacteria have long been associated with hydrothermal vents, this is the first time that a well defined fish community has been filmed in association with such a structure. This page describes the area briefly, and has clickable images of the Vailulu’u volcano that rises from 4,360 metres deep, resembling an underwater version of Japan's Mount Fuji ...

So, this Baedecker tour of unusual fishes in unusual habitats should give everyone food for thought.


Panda Catfish fan and keeper/breeder since Christmas 2002

this is a fascinating article. I love hearing facts like these about odd fish. These are some really odd adaptations

-Vincent

Cali,

Have you ever thought about submitting some of your articles to some of the fish magazines, I'm sure you have, but if not you should. \

I find them very well researched, knowledgeable and entertaining. I'm sure there are others here that would agree, and just maybe you can make a few quid.

Jim

---

Water, taken in moderation, cannot hurt anybody. -- Mark Twain

Speaking of extremophile fishes, you forgot about the infamous Gambusia affinis.

Last year I went to go visit a local geyser, and around the geyser (which erupted about every 10 minutes), there was a small pond. The air around geyser smelled strongly of sulfur (so I'm guessing the water had plenty of toxic chemicals in it) and when I felt the water, it was hotter than a typical hottub. I'm guessing the water temperature must have been at least 105 degrees Fahrenheit. There was steam coming off the surface of the pond, and when I went there it was about 90 degrees outside.

But was it devoid of life? Certainly not - there appeared to be at least a few hundred Gambusia affinis living in this "pond". There was no other life in the pond (nothing aquatic, and there were absolutely no plants growing closer than five feet from the pond edge), however, so how were the mosquitofish surviving if there was seemingly no food?

Closer inspection revealed that they were feasting on the bodies of other animals that tried to enter the pond but likely died immediately. I saw several dead frogs that probably leaped into the pond, only to find that it was not only extremely hot, but also full of toxins. And judging by the fact that the frogs were still in there, that meant they succumbed to harsh conditions before they could climb out.

And around each frog was a few dozen mosquitofish of all different sizes, trying to eat and mate at the same time.

---

I'm not your neighbor, you Bakersfield trash.