Seasonality predicts variation in life history phenotypes in the livebearing fish Priapichthys annectens

Livebearing fishes (Poeciliidae) have been used as models to test several aspects of life history theory. Previous work in these fishes showed that traits such as age and size at maturity, reproductive allocation, and number and size of offspring can each be shaped by natural selection. Life history traits can also be influenced by the environment, and in some cases, such phenotypic plasticity can also be adaptive. Resource availability in the environment can affect life history trait expression with increased resources expected to result in increased investment in reproduction. Here, we test this hypothesis in the Costa Rican livebearing fish Priapichthys annectens by quantifying life history traits and comparing trait values between populations collected during the dry season versus the wet season, periods when resource availability varies. We found that life history traits related to reproductive investment, but not the timing of reproductive events, differed between seasons. During the wet season, female body condition increased, and females invested less in reproduction, having fewer but larger offspring. Irrespective of the season, larger females gave birth to larger offspring. Finally, we found that this species utilizes a lecithotrophic strategy of provisioning eggs with nutrients being deposited in the ova prior to fertilization, and that females only carry one developing brood at a time (no superfetation). This is the first study documenting the life history of P. annectens, and it adds to a growing body of work suggesting that seasonal environmental differences can induce changes in the expression of some—but not all—life history traits.