MEPS 256:111-121 (2003)  -  doi:10.3354/meps256111

ABSTRACT: Fecundity-time models have been widely used to analyze the evolution of larval strategies in marine benthic invertebrates. To further explore the behavior of this influential class of models, we examined the effect of fluctuating food availability on the expected duration of the planktonic larval period and the number of offspring that survive to metamorphosis in marine invertebrates with planktotrophic larvae. Food concentrations were allowed to fluctuate randomly on a daily basis between specified upper and lower bounds. Variation in food levels generally had a much stronger effect on development time and reproductive success when the level of egg provisioning was low (small-egg strategies). When food was abundant, smaller eggs were favored. Fluctuations in planktonic food concentrations affected small-egg strategies more strongly than large-egg strategies, but the variation in fitness was small relative to fitness differences across egg sizes. There should be consistently strong directional selection to minimize egg size whenever food is abundant, even if the concentrations fluctuate widely. However, when larvae were strongly food-limited, larger eggs were favored and fluctuations in planktonic food supply led to variation in fitness that was large relative to fitness differences among strategies. There was no clear peak on the fitness curve, due to overlap of fitness distributions across reproductive strategies. This leads to the prediction that there should be a range of intermediate- to large-egg strategies, rather than a single optimal egg size. With facultative feeding by planktotrophic larvae, there were intermediate egg sizes above which variation in food level had only negligible effects on development and survival. When the magnitude of environmentally caused variation in reproductive success exceeds the fitness differences among reproductive strategies, this should flatten out the adaptive landscape, reduce the intensity of disruptive or directional selection, and facilitate evolutionary transitions between planktotrophy and lecithotrophy or vice versa.

KEY WORDS: Fecundity-time model · Life history · Facultative feeding · Egg size · Reproductive strategy · Larva