Phenotypic plasticity in sex allocation and body size leads to trade-offs between male function and growth in a simultaneously hermaphroditic fish
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Phenotypic plasticity in sex allocation enables organisms to maximize reproductive success in variable environments, and thus may generate different sex allocation patterns among populations that experience different mating opportunities. In this experiment, I test whether sex allocation is phenotypically plastic in Serranus tortugarum, a simultaneously hermaphroditic fish, by using reciprocal transplants among four reef study sites with populations at high and low densities and significant differences in sex allocation. Fish transplanted across different densities were predicted to alter sex allocation and body size through trade-offs in investments to somatic growth and male and/or female reproduction. As a control for effects of transplanting, I also transplanted fish across study sites with the same densities and marked and returned fish to their original study sites. As predicted, sex allocation and body size shifted significantly for fish transplanted across different densities but not for those transplanted across the same densities. Separate analyses revealed that the treatment effect on sex allocation was driven strongly by a reduction in male investment by fish transplanted from high to low density, and this reduction in male investment was accompanied by an increase in body size. Fish transplanted from low to high density did not appear to change either male or female investments, but they were smaller than transplants from low to low density. A trade-off between male and female function was not evident, but phenotypic plasticity in body size suggested a trade-off between growth and male function when sex allocation is adjusted. Large-scale empirical tests of sex allocation in the field are relatively rare, and the results of this experiment give novel insights into how animals respond to a change in mating opportunities under natural conditions. The effects of logistical problems associated with fieldwork, such as mortality of experimental animals, are considered in the discussion.
KeywordsLocal density Mating opportunities Male investment Female investment Somatic investment Sperm competition Chalk bass Serranus tortugarum
This study was conducted while the author was funded by pre-doctoral fellowship from the Smithsonian Tropical Research Institute. Additional financial support was provided by a Dissertation Year Fellowship from the University of Kentucky (UK) Graduate School, UK Graduate School Research Awards, UK Biology Ribble Research Fund Awards, an American Association of Underwater Sciences’ Kathy Johnston Fellowship, an American Society of Ichthyologists and Herpetologists’ Raney Fund Award, and an Animal Behavior Society Research Award. Andrew Kratter, Cara Lawrence, Marieke Keller, and Benjamin Williams provided assistance with diving and sample processing. Craig Sargent, Dave Westneat, and Phil Crowley helped tremendously with the experimental design and statistical analysis, as well as with manuscript development and enthusiastic support. Jane Brockmann, Chuck Fox, Kay Shenoy, and Colette St. Mary and her lab members gave helpful advice on presentations of the results and writing of the manuscript. Colette St. Mary was especially helpful with recent revisions. I thank John Endler, Martin Reichard, and two anonymous reviewers for recognizing the value of this work and for improving the clarity of the text with constructive advice.
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