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Abstract

Both theory and the published literature support the importance of demographic selection for differences in life history traits between populations as well as between related species. Much attention in this area has been paid to species in the livebearing fish family Poeciliidae. I studied populations of three species of poeciliid fishes of the genus Brachyrhaphis (episcopi, roswithae, terrabensis) found from low to high elevation in western and central Panamá for reproductive life history phenotypes. To determine how reproductive life history phenotypic characters subject to both selection and environmental influence may vary between these species, field collections were made at the end of the wet and dry seasons over several years. Work with other poeciliid species suggested that these three stream species, found in different environments largely defined along an elevational gradient, may have evolved different responses in their reproductive timing and effort in response to differences in abiotic and biotic environmental factors. Differences were found between female wet and dry season reproductive output in all three species. Brachyrhaphis episcopi and B. roswithae showed similar responses to seasonal changes, with larger reproductive allotment (RA) and brood size but smaller embryo size in the wet season and a higher percentage of gravid females found in the wet season compared to the dry, 86% to 49% for B. episcopi and 71% to 45% for B. roswithae. Female B. terrabensis showed different responses to seasonal changes. RA, brood size and embryo size were all larger in the wet season than in the dry season. The percentage of gravid females among those of reproductive size was 37% in the wet season compared to 10% in the dry season. Patterns of male size varied between the three species, with more muted seasonal differences than was found with females of all three species. Brachyrhaphis terrabensis males showed a much wider size range, possibly indicative of two male reproductive strategies in the species. There is a gradient in several measures from lowland to highland species such as highland reproduction apparently stopping entirely in the dry season.

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