Abstract:
Selection should favor individuals that occupy habitats in which fitness returns are the highest. In heterogeneous environments, this equates to individuals actively navigating alternative habitats and dispersing to areas that confer highest survival and reproduction potential. When habitat preferences exhibited in the laboratory do not align with conditions under which individuals are found in natural environments, it suggests that some other environmental factor must preclude optimal habitat choice, and the potential for measurable fitness consequences associated with occupying subpar conditions. Mangrove rivulus fish inhabit a wide range of salinities, and are most often collected at ~25 ppt. I demonstrated that rivulus prefer to spend time, and lay eggs, at salinities < 25 ppt. Eggs laid at lower salinities had higher hatching probabilities than eggs laid at higher salinities. To evaluate potential fitness consequences of occupying different salinities across ontogeny, fish from 36 genotypes and three life stages (hatchling, juvenile, adult) were systematically assigned, with replication within each genotype, to a salinity between 5 and 65 ppt for two weeks. Hatchling survival decreased by ~10% between 5 ppt and 25 ppt but, as salinity increased, hatchling mortality increased precipitously (~90% mortality at 65 ppt by day 12). Hatchlings also showed significantly greater mass gain at 5 and 15 ppt than 25 ppt, and considerably less as salinity increased. Juveniles did not experience significant mortality until 55 ppt and gained more mass at 5, 15 and 25 ppt than at higher salinities. Adults appeared unaffected by the various salinities with no differences in mass gained and only slight increases in mortality at 65 ppt. These results suggest that, for species inhabiting variable environments with multiple microhabitats, fitness might be maximized in habitats that deviate from conditions where the animals are commonly found in the wild.
