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Fastwater Fishes

Posted By: Martin Thoene <martin.thoene@lakenheath.af.mil>
Date: Saturday, 19 September 1998, at 8:08 a.m.

The following is an edited version of an article written by Peter Brokenshire, which was published in March '93 in 'Aquarium' magazine a short-lived & now defunct British mag.

He was prompted to do some research and write the article after losing 6 "Hong-kong plecs" (Gastromyzon spec) and 4 Bulldog plecs ( Chaetostoma spec) over a period of 6 months.

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I decided that the answer to the death of these fishes in my tanks must lie in the difference between captive conditions and their natural environment.They require highly oxygenated water for survival, having adapted to life in fast flowing streams and were obtaining insufficient oxygen in a typical aquarium setting.But I still wondered why they cannot cope with lower oxygen levels, when other fishes can. Are the fast water catfishes and loaches really that particular and contrary, or should they be able to "get on with it" in captivity?

The answer may lie in the fact that oxygen is carried in the blood by the respitory pigment haemoglobin. Oxygen can dissolve directly into the blood, but (in humans for example ) the oxygen-carrying capacity of blood containing red cells full of haemoglobin is 70 times that of blood. The special feature about the binding of oxygen to haemoglobin is, unlike the oxidation of, say iron(II)to iron (III)(rusting), and over a narrow range of conditions, maintained by physiological regulation, the reaction is reversible. The pigment can easily pick up oxygen at the lungs or gills and give it up equally readily at the tissues where it is required for cell respiration. There is an iron ion (Fe2+) at the centre of the central haem part (a type of metalloporphyrin) of the haemoglobin molecule, which is the oxygen-binding site. The precisely controlled spatial inter-relationship of the molecules in the surrounding globin (a variety of protein) and the resultant molecular forces act to prevent ireversible binding. The special nature of the arrangement is evidenced by the fact that a haem group isolated from the globin will only bind with oxygen irreversibly.

In the gills of a fish, the thousands of secondary lamellae on the gill filaments are in contact with oxygen bearing water. Haemoglobin - carried within the millions of minute blood capillaries within the gill lamellae - loads up with oxygen when the oxygen content in the water surrounding the gills is relatively high and unloads its oxygen at the tissues throughout the body, where the oxygen content is lower. This loading and unloading is automatic, in response to the rising and falling relative pressure exerted by the dissolved oxygen (although it is a much more complicated process than this makes it sound!)

With rare exceptions, haemoglobin occurs in all vertebrates and many invertabrate groups. As far as is known, the haem parts are all identical, whereas the globin portions of the molecule vary. Herein lies the beginnings an answer why these fishes struggle in captivity. Small differences in the structure of the globin portion can have a dramatic effect on the physiological properties of haemoglobin. The ease with which a particular variety of haemoglobin takes up oxygen is termed 'the affinity of the haemoglobin for oxygen'.Haemoglobin is also sensitive to the level of dissolved Carbon Dioxide, and to temperature.

Relatively high CO2 levels reduce the haemoglobin's affinity for O2.This is but one way that Evolution has exploited the properties of biochemical substances of which living organisms are composed. CO2 is a waste product of cell respiration, so CO2 levels tend to be higher in those tissues which are doing 'work', and where, therefore O2 is needed. The high CO2 actually 'helps' the haemoglobin unload O2.

Rising temperatures reduce the affinity of Haemoglobin for O2. Fish are the same temp. as their water,so the decrease in affinity can be sufficient to prevent adequate loading of the pigment at the gills, especially if the fish has low-affinity pigment. This is compounded by the fish's increased demand for O2 at higher temperatures, and by the reduced solubility of O2 as temp. rises. Equally, falling temp. increases the haemoglobin affinity, but decreases the ease of unloading at the tissues. In other words, you can't juggle a particular species haemoglobin affinity by altering the temp of it's environment.

Some haemoglobins have greater affinity for O2, and bind easier, the benefits of which are obvious. But the downside of high affinity is that it 'costs' the organism more in terms of engineering the physical circumstances conducive to unloading the O2 at the tissues. If the O2 binds easily it is harder to "unbind". With high haemoglobin affinity a fish can inhabit low O2 level environments.Carp for example, have high affinity haemoglobin, so can inhabit still waters.

Some fishes, by contrast have adapted to waters where the O2 levels are far higher, such as fast-flowing rivers and the upper waters of the open sea.Here there is no need for a high affinity pigment, and such fish have evolved a low-affinity pigment; this has little difficulty in binding oxygen, as the concentration of dissolved O2 is so high. The benefits of low affinity pigment are in the ease with which it unloads it's O2 at the tissues, saving the costs of biochemically 'engineering' the unloading which other fish have to pay. In O2 rich waters, therefore, a low affinity haemoglobin can be more efficient in getting the O2 to the tissues than a high affinity one (this concept sounds paradoxical, but is quite logical). BUT a low affinity haemoglobin will not work properly in water with a low or only average dissolved O2 content. So these fish with low-affinity pigments are simply not found in waters that are not rich in O2.

Therefore, I strongly suspect that Chaetostoma, Gastromyzon & Psuedogastromyzon species possess these low affinity pigments, in that they are adapted to waters rich in O2. If so, then they are physiologically unable to survive in the comparitively still waters of the normal aquarium environment.

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He also touched on the fact that he believes that exporters of these fish may well be aware of their specialist nature and speculates that 99% of the fish exported may be doomed to die because they will never be kept in the conditions they get in the wild. Is this ethical (I sure don't think so), but it is 1998, 5 years since this article appeared, and they're still being imported. I've never seen any indication in any shop about any need for specialised conditions being required, so you can only speculate at the numbers that have perished over the years. I've had a Chaetostoma cat for about 3 years now who's always been kept in very well filtered tanks with reasonable water movement, no problem. I wouldn't advocate banning the import of these fascinating, specialised fish, I just wish the trade would be more responsible in the way it markets them. Experienced, dedicated aquarists (like most of us on here!), will research and set up tanks to accomodate specialised fish such as these, because we CARE. Shame is we all probably made lots of mistakes in the past, which our fish paid for, especially when we started out. Any new fishkeeper who goes to a shop and doesn't get the correct advice about a prospective purchase of a cute 'Borneo Sucker', or whatever the trade is calling them that week is often sentencing the fish to an untimely end. It's not really their fault, they trust the shop (some you can) to nurture them when they start out. Unfortunately, until people get "The Bug", and start to specialise away from the normal, multi-species, multi-continental origin community tank, into other areas, they won't be prepared to make a setup as specialised as these fishes require for OPTIMUM CARE. And of course, water movement costs money. Either in filters, powerheads or air pumps, plus the electricity to run them. And white-water movement costs more, but that amount is kinda difficult to do in a tank anyway.I figure more powerful than usual (for a given tank size) power filter, or powerhead(s)driving undergravel filtration, plus strong airation should keep these fish very happy as regards drawing breath!

Martin.
 

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