The Oxygen Requirements of Certain Aquatic Animals and Its Bearing Upon the Source of Food Supply
Authors: Dakin W. J. , Dakin Catherine M. G. 
Published: April 2021

ABSTRACT
The original theory of the food supply of aquatic animals put forward by Ptitter, and based upon the results of certain experiments and analyses, claimed that the chief source of food of such animals was organic matter dissolved in the sea, in lakes, rivers, etc., and that this was absorbed directly and indeed often by the gills, if present. The position he took up may be emphasised by his statement regarding fishes : “There is no doubt that a nutrition without dissolved foodstuffs is possible, and it is not impossible that cases of this kind are realised in nature. But the experiments at Naples show that the fish in the Naples aquarium under approximately natural conditions obtain one-half to three-quarters or more of their food requirements by the absorption of dissolved food.” It is quite possible that small quantities of organic matter in solution in water are absorbed by aquatic animals, and in some cases (particularly amongst protozoa living under special conditions) this may be an important, perhaps the most important, source of food. It is also possible that very small quantities of organic matter in solution may eventually be found to exercise a very profound influence (acting like vitamines, for example) on the life of aquatic animals. Evidence for such is not evidence for the main thesis set up by Pütter on the results of his own experiments. The food requirements of many aquatic animals as calculated by Pütter on the basis of oxygen consumption are often remarkably high and need further investigation. It is quite possible that there is an unknown factor at work here. In connection with these calculations we consider that purely theoretical computations based upon the measurement or estimation of the active surface area of the body are not permissible, and that the application of the law of surface area in connection with metabolism must not be allowed to supplant experiment Experiments on goldfish similar to those made by Pütter, show that specimens kept in tap water without any particulate food live for varying periods (which are often of considerable duration) dependent upon the original condition of the fish, and the freedom of the experimental tanks from parasites. The addition of the organic compounds, glycerine and asparagine, makes no difference to the duration of life, and the consumption of oxygen by the fish living in tap water with these compounds does not exceed that of the control fish in tap water only. Gradual starvation takes place, and sections show that the mass of muscle tissue becomes gradually reduced. The cessation of feeding on particulate food makes the fish particularly susceptible to the attacks of parasites (Chilodon cyprini and Gyrodactylus, sp.), if there is any chance of such infection. The consumption of oxygen by plaice eggs during their development agrees fairly well with the amount computed from analyses of the composition of young eggs and eggs shortly before hatching, but the results are only approximate, although they fit in with the assumption that such floating eggs have their own food stores and absorb nothing from the sea water. Aquatic organisms are not to be grouped in one class in so far as nutrition and metabolism are concerned. It had frequently been noted that when the oxygen consumption of aquatic animals is measured, and the determination extends over several hours there is a gradual falling off during the experiment. It is necessary to look for this in every case before estimating the normal oxygen consumption over long periods. The variation may be due to handling the specimens at the beginning of the experiment, to the gradual reduction of the oxygen available, to the accumulation of waste products, or to time of feeding.
We have found that goldfish, axolotls, and Anodon used in our experiments are to a certain extent independent of the oxygen pressure, which may fall considerably (until a certain minimum is reached) before the oxygen consumption of the animals per hour is affected.