Marine & Environmental Sciences Faculty Proceedings, Presentations, Speeches, Lectures


Sexual Allocation, Sexual Selection and Environmental Constraints in the Barnacle Semibalanus balanoides

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Society for Integrative and Comparative Biology 2005 Meeting, San Diego, California, January 4-8, 2005

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Barnacles have been the subject of a great deal of work on sex allocation. They are simultaneous hermaphrodites that copulate with exaggerated penises. Sexual selection should favor longer penises, capable of reaching a greater number of mates, but maximum penis length may be constrained by biomechanical factors and environmental factors (such as water movement). Sex allocation theory predicts that the relative amount of resources allotted to the ratio of male versus female function depends on the size of the mating group and local mate competition. The mating group of a focal barnacle is limited by the reach of the penis. Harsh environmental conditions, especially turbulence and wave force may make it more difficult for barnacles to seek and inseminate distant mating partners. Environmental (extrinsic) limitation of penis reach should result in reduced mating group size and, according to theory, lower overall investment into male function. If free from environmental penis limitation, barnacles may optimize penis length based on population density, intrinsically controlling the size of the mating group and thus, investment into male function. These two possibilities are compared to the null hypothesis of no significant variation of penis length and reach with respect to mating group size or environmental regime. Populations of the northern rock barnacle, Semibalanus balanoides, were sampled within Narragansett Bay (calm sites) in Rhode Island, USA and compared to similar collections from points along the Atlantic Ocean exposed coast (high energy sites). Variation in penis length to body size ratio, as well as other morphological and reproductive characters are measured. These data should demonstrate how abiotic factors may alter sex allocation and provide a contrast between the strength of sexual selection and environmental limitation on penis characters.

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