PHENOTYPIC PLASTICITY AND PRIORITY RULES FOR ENERGY ALLOCATION IN A FRESH-WATER CLAM - A FIELD EXPERIMENT

We studied resource allocation among maintenance, reproduction and gro wth in the freshwater clam Anodonta piscinalis. Recent theoretical and empirical studies imply that organisms with indeterminate growth may have priority rules for energy allocation. That being so, the traits i nvolved should potentially be capable of considerable phenotypic modul ation, as a mechanism to adjust allocation. We tested this hypothesis using a 1-year reciprocal transplant experiment at six sites. Experime ntal clams were caged at higher than natural densities in order to det ect any phenotypic modulation of the traits and discover the putative priority rules in energy allocation. We recorded the survival and shel l growth of clams during the experiment, and the reproductive output, somatic mass and fat content of clams at the end of the experiment. Sh ell growth, somatic mass, and the reproductive output of females varie d more among transplant sites than among the populations of origin, su ggesting a high capacity for phenotypic modulation. However, the repro ductive investment, somatic mass and shell growth were also affected b y origin; clams from productive habitats invested more in reproduction and were heavier. In comparison to undisturbed clams, the reproductiv e output of the experimental clams was similar and their fat content w as higher, whereas their shell growth was considerably slower and thei r somatic mass lower. These results suggests that when resources are l imiting (due to high density) reproductive allocation overrides alloca tion to somatic growth. The highest mortality during the experiment co incided with the period of reproductive stress in the spring. Addition ally, the proportion of reproducing females was lower in those transpl ant groups where the survival rate was lowest, suggesting that mainten ance allocation overrides allocation to reproduction when available re sources are scarce. The results of this field experiment support theor etical predictions and results of previous laboratory experiments that suggest that there are priority rules for energy allocation in organi sms with indeterminate growth.