Collaborative Research: Plasticity, Selection, and Divergence in Stream Fishes of the Hawaiian Islands (RUI)

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Differences in adaptive traits often correlate with environmental variation, but can arise from two different sources: differentiation of genes, or phenotypic plasticity, the capacity of a single genotype to encode multiple phenotypes. Distinguishing between these mechanisms is a central challenge in evolutionary biology. Hawaiian stream fishes provide a superb system for directly measuring the evolutionary mechanisms leading to changes in form and function in response to environmental variation. The five fish species native to Hawaiian streams are all amphidromous: larvae hatch upstream but are quickly swept to the ocean, where they develop up to six months before returning to freshwater. For three species, returns require climbs of vertical waterfalls tens of meters tall before reaching adult habitats. However, adult habitats differ strongly between the oldest (Kaua'i) and youngest (Hawai'i) islands. Hawaiian waterfalls are close to shore, placing a premium on climbing ability to escape non-climbing predators; in contrast, Kauaian falls are far inland, placing a premium on predator evasion as fish migrate upstream. Biomechanical models predict these contrasting demands will select for divergent body shapes. This project is a collaboration among a biomechanist, a population geneticist, a morphologist, and an aquatic ecologist to test the hypothesis that natural selection has promoted adaptive divergence in stream gobies in response to differing environmental challenges between Hawai'i and Kaua'i through a combination of genetic differentiation and adaptive phenotypic plasticity. A series of complementary studies on the climbing goby Sicyopterus stimpsoni and the non-climbing goby Stenogobius hawaiiensis will be conducted. Goals are to: (1) evaluate morphological divergence in juveniles and adults from several stream subpopulations on Hawai'i and Kaua'i, and test for correlations with environmental differences in streams; (2) evaluate the strength of selection imposed on gobies by climbing and predation, and test for performance differences between populations from Hawai'i and Kaua'i; (3) evaluate neutral genetic divergence in juvenile and adult S. stimpsoni and S. hawaiiensis from several streams on Hawai'i and Kaua'i, and test for differential patterns of gene flow in these species; (4) conduct common garden predator exposure experiments to evaluate levels of genetically based local adaptation and environmentally induced phenotypic plasticity among island subpopulations. This work will provide a model for research on the interactive mechanisms promoting the evolution of organismal form and function in response to environment. This project involves collaboration of research-focused (Clemson) and teaching-centered (St. Cloud State) institutions, and will promote integrative training in genetics, morphology, and functional biology for both undergraduate and graduate students, including local interns with the Hawai'i Division of Aquatic Resources (DAR). Cooperation with the DAR will inform water management decisions so that detrimental impacts of human stream use can be limited in Hawai'i, helping to protect these endemic freshwater fishes.