Abstract

Determining growth rate is essential in population ecology. Like all crustaceans, crayfish can only increase in size by moulting. This produces a stepwise size curve with rapid increases in size at moult, while actual growth or production of tissue primarily occurs between moults. To be able to assess relative growth in studies of too short duration to include a moult, we have applied a biochemical technique where the RNA concentration in tail muscle tissue is used as a measure of instantaneous growth rate. In this study we investigated the effect of age and size on instantaneous growth. Signal crayfish of three age classes, 0+, 1+ and 2+, were kept in compartments, with identical conditions regarding temperature, light and food. There was substantial overlap in size between the year classes. The RNA concentration was then deter­mined for 12 individuals from each year class. We found that instantaneous growth rate was highest for small and young individuals and that growth decreased, both with increasing size and for the older crayfish. Age explained more (40%) of the variation in growth than did size (32%). Furthermore, within age classes, there was no significant effect of size on growth. Thus, current growth rate in crayfish was more an effect of age than of body size.

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