Limitations of Energy Acquisition and Energy Use in Small Poikilotherms: Evolutionary Implications

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Many fish are typical 'high-risk species' in which a high reproductive output balances the high mortality of the young (or, alternatively, high mortality is the result of high reproductive output). When the mortality of larvae and juveniles reaches 99 99% or more, even stochastic fluctuations of environmental factors may drive populations to extinction and thus determine the fate of regional fisheries (Sale, 1990). It would be of considerable practical and theoretical interest to know what makes these animals so vulnerable to environmental pressure, and how the shapes of the mortality curves of populations might respond to even slight changes in the intensity and combination of environmental factors. Here, I address this question from recent work on the energetics, development, and physiological ecology of larval and juvenile cyprinids which was designed to shed some light on this question. On the basis of morphological and physiological evidence, I suggest that the ecological problems that young fish have to face after hatching are linked to fundamental problems of energy acquisition and energy use of small metazoans. A selection is postulated that would (if sufficient genetic variability were available) tend to increase the flow of metabolic energy through animals, liberating the animals from the constraints of tight energy budgeting. Within the framework of this symposium it is of interest that a completely different hypothesis for explaining the high fecundity/mortality of bony fish has been put forward by Nellen (1986), based on life-cycle theory. According to that author, recruitment in fish is so high because the larvae serve as food for the parental generation, filling the size gap of potential prey organisms between secondary consumers (protozoa, copepods, etc.) and tertiary producers (fish). Thus, 'growing life stages of fish appear to be ideal transfer organisms for biologically bound energy up the weight pyramid' (Nellen, 1986, p. 75). The theory has a group selectionist flavour and it will be interesting to see how it fares in the hands of theoreticians, but there can be no doubt that the problems of the high fecundity and mortality of fish must have both an ecological and a physiological aspect.