Evolution of Egg Size Strategies in Fairy Shrimps

Abstract

Differences exist among fairy shrimps in the size of the eggs they develop. Large eggs that hatch into big larvae probably evolved in the relatively predictable melt-pond environment as a result of selection against small size by invertebrate predators. High density-independent mortality occurring in the absence of predation on larval stages seems to have selected for the production of increased clutch size at the expense of egg volume in the unpredictable rain-pond habitat. Differences in resource availability between the two habitats along with r- and K-selection may have aided in the evolution of ecologically relevant egg sizes. Egg size is an important measure of a female fairy shrimp's invest- ment in each of her offspring. Assuming a lower limit is established by the minimum amount of material and energy required to produce a viable larva, optimality theory predicts that realized egg dimensions will be adjusted by natural selection to those which maximize indi- vidual fitness within a given environmental context. Recently, Wil- bur, Tinkle, and Collins (1974) discussed environmental certainty, trophic level, and resource availability as significant selective factors in the evolution of life history characteristics. In this paper, I discuss how these factors seem to account for egg size difference among fairy shrimps living in difference habitats.