Item

Abstract

I propose a mechanism of rapid phenotypic response of size at first reproduction (SFR) that may allow relatively large species of Daphnia to survive under variable fish predation. A single clone of D. magna isolated from a shallow lake with planktivorous fish is able to express the full range of SFR observed in the field (1.8-3.0 mm) when raised under controlled temperature and food conditions in the laboratory. Control of SFR is predominantly based on size differences in the eggs of the first and the following broods of Daphnia . First-brood eggs are smaller. They hatch into smaller neonates that mature at a smaller size. In the lake, fish may selectively remove large females, and this will increase the share of first-brood females in the population. Offspring of these females will mature at a much smaller size, so that the size response to predation is intensified. When predation ceases, some females can grow to larger sizes. They will contribute many large offspring to the population because they have larger clutch sizes and larger eggs. This leads to a rapid shift toward large SFR. A single clone of D. magna was hatched from isolated eggs to measure the components of maternal size dependency, which is basic to this hypothesis. Strong positive relationships were found between female size and egg size, egg size and neonate size, neonate size and SFR, size of a female and its offspring's SFR, and female size and clutch size. The predicted pattern of SFR in relation to average body size is found in the lake, but the pattern of egg size is not clear, as egg size is strongly affected by changes in food abundance. This mechanism of phenotypic response may be a fortuitous effect of selection for maximum egg numbers in the first brood, but the increase of offspring size with maternal size may also be adaptive itself.