To investigate the influence of salinity on natural variation in life history traits in Poecilia latipinna, Gambusia affinis, and Heterandria formosa, a field study was conducted on fishes collected from sites along a salinity gradient of coastal marshes. Poecilia latipinna and G. affinis collected from freshmarshes had a lower body condition index (BCI) and females had lower reproductive allotment (RA) and fecundity than those from sites with higher salinities. Female H. formosa from freshmarshes had a better BCI, similar RA, and lower fecundity than those from intermediate marshes.

To evaluate the impacts of historical (genetic) and contemporary (environmental) effects of salinity on maturity and reproduction, G. affinis and H. formosa were collected along a salinity gradient and maintained in freshwater for two years (≥ 4 generations). Both species exhibited genetic variance for size at maturity among source populations and phenotypic plasticity for sexual maturation, but no genetic variance for plasticity itself. In response to salinity stress, G. affinis males and H. formosa females matured at smaller sizes and older ages in 0 and 12 ppt rearing environments, respectively. Our results suggest that the habitats in which these fishes are most abundant do not correspond to the salinities at which they can allocate the most energy to rapid maturation at larger sizes, indicating that other environmental factors also influence their abundance along salinity gradients.

We performed an experiment on pregnant females using historical and contemporary salinity treatments to examine the effect on offspring size and number in G. affinis and H. formosa that differ in maternal provisioning strategies. In G. affinis, we found that historical salinity influenced offspring quantity while contemporary gestational salinity impacted offspring size. We detected plasticity in post-zygotic offspring provisioning under an ad libitum feeding regime. Our results suggest that G. affinis from fresh and intermediate marshes were facultatively matrotrophic while females from brackish marshes were lecithotrophic. At the highest gestational salinity levels, H. formosa produced fewer, smaller offspring (indicative of environmental stress). Both species, however, exhibited genetic variation in phenotypic plasticity and utilized maternal effects to vary the number and size of their offspring in different salinity levels.