Some Observations on the Respiration of Certain Tropical Fresh-Water Fishes

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Abstract
1. The blood of the fish of the rivers and swamps of British Guiana is not all equally affected by CO2. A regular series can be shown to exist between the paku (Myleus setiger Müll and Troschel), an active river fish whose blood is considerably affected, and the electric eel (Electrophorus electricus (L.)), which is an air-breather, frequenting relatively stagnant and poorly oxygenated water, and whose blood is almost independent of the presence of CO2. 2. The blood of all the fish examined was found to be acid, generally about pH 6·7. 3. The CO2 combining power of the blood of the fish examined was found to be very much lower than in other animals, but higher than might be expected on theoretical grounds. The possibility of another type of buffering is suggested. 4. In the one fish examined (the bom-bom, Pterodoras granulosus (Val.)) the CO2 contained in reduced blood was found to be no higher than in oxygenated blood. 5. In spite of the low oxygen content of some of the waters, no fish examined showed a haemoglobin with any markedly increased affinity for oxygen, such as might enable it to be saturated more easily at low pressures. 6. The yarrow (Erythrinus erythrinus (Schn.) or Hoplerythrinus unitaeniatus (Spix.)) was found to be a fish which could either use its gills or take in bubbles of air into its air-bladder as a means of respiration. This fish has been studied in detail and the conditions controlling its respiration are described. Three factors are concerned, pH, oxygen content of the water, and CO2 content of the water. Of the three the last is most often the limiting factor. 7. Carbonic anhydrase was found in the blood of all the fish examined for its presence, which include practically all those mentioned in this paper. 8. The bearing of these results on certain problems of general biological interest is discussed.