Variation in the haemoglobin content of Daphnia.

Abstract

It is known that the concentration of haemoglobin inDaphniavaries inversely with the amount of oxygen dissolved in the water in which the animal lives, and that this variation is influenced by temperature and certain dietary factors. The individual variation in haemoglobin concentration found in populations ofDaphniaspecies has now been analyzed. One reason for individual variation was already known, namely, the stage of the instar in which an individual happens to be, for at the end of an instar haemoglobin passes from the blood into the eggs before they are laid. The haemoglobin concentration in the blood is gradually restored during the following instar. It is now shown that, with a large enough number of eggs, the passage of haemoglobin into the eggs is sufficient to account for the observed rate of loss of haemoglobin from the blood of redDaphniawhen in well aerated water. The concentration of haemoglobin is found to be higher in immature than in mature females, and males have a higher concentration than females under the same conditions. Fluctuations in haemoglobin and total haem concentrations in a population ofD. magnahave been followed for a period of three months.Daphniadoes not appear to store haem. The amount of haem passed into parthenogenetic eggs varies with the number of eggs per brood. Species ofDaphniadiffer in their ability to synthesize haemoglobin. It was found that when two pairs of pond-dwelling species are compared, the smaller species in each pair produces a higher concentration of haemoglobin than the larger one at the same low oxygen concentration. Racial and clonal differences in ability to synthesize haemoglobin were found within a single species ofDaphnia. It is concluded that much of the individual variation in haemoglobin concentration at any one oxygen concentration can be explained by individual differences in metabolic rate, which in turn can be related to the age, size and sex of the individual. A high metabolic rate lowers the internal oxygen concentration and so intensifies the stimulus to synthesize haemoglobin.