Phenotypic plasticity of life-history traits in clonal and sexual fish (Poeciliopsis) at high and low densities.

Models of resource allocation strategies predict an array of life-history responses of individuals living in resource-stressed versus non-stressed environments. I tested a number of these predictions using three fish strains (a sexual and two clonal strains) in high and low density treatments. To examine the plasticity of life-history traits in females raised in these two environments, I measured survival, growth, egg production, egg size, and proportion mature at 10 weeks of age. Survival was not affected by density treatment. However, both growth and overall egg production were lower in females from the high density treatments, and reproductive maturity was significantly delayed at the high density for all strains. Egg production per unit size was not affected by density in any strain, signifying that differences in the numbers of eggs produced was merely a reflection of the differences in size of fish in the two density treatments. Egg size was also unaffected by density in all strains. These results are related to models of resource allocation in stressful environments. There was a consistent pattern of increased reproductive investment in the sexual strain relative to the two clonal strains. The sexual strain matured earlier, produced more eggs per unit body weight, and had larger eggs than either clone at both densities. These results are interpreted by considering the predicted adaptive responses of these three strains to the long-term environmental differences in their natural habitats.