An Explanation of the High Strain Diversity of a Self‐Fertilizing Hermaphroditic Fish

The tropical and subtropical estuarine fish Rivulus marmoratus is known for its synchronous, self—fertilizing hermaphroditism. It matures within 3 mo and can produce eggs throughout the year. These features would allow a particular strain to quickly colonize refuge such as crab holes in the high intertidal zone habitat. However, a high strain diversity has been found in the field. In order to explain this observation, we examined the phenotypic responses of R. marmoratus to variation in biotic and abiotic environmental factors over two consecutive generation and employed a modified Euler—Lotka equation to estimate and compare the net reproductive rate, R, among strains. We included five strains of R. marmoratus and manipulated levels of temperature, salinity, and food for the parental generation and levels of food only for the second generation. Low temperature (19°C) inhibited sexual maturity, precluding any egg production during the entire 6—mo study. For individuals kept at high temperature (26°C), salinity and food had a interactive effect on time to maturity, body size at maturity , and number of eggs produced. Using multivariate analysis of variance, we found a significant two—way interactive effect (foods levels for the parental and the second generations) on the five reproductive traits studied in the second generation. When each trait was analyzed separately, we found significant three—way interactions for time to maturity and total length and maturity. We found that the fitness values for each strain varied in different environmental combinations (3 salinites X 3 incubation salinities for eggs X 2 food levels). However, four of five strains had similar overall fitness. Despite a lack of genetic variation within each strain, R. marmoratus demonstrates considerable phenotypic variation in response to environmental changes. This feature and genetic variation among strains may explain how this species can survive and even prosper in an extremely variable and stressful environment.