Title

Experimental Tests of Sex Allocation Theory in Two Species of Simultaneously Hermaphroditic Acorn Barnacles

Event Name/Location

Society for Integrative and Comparative Biology 2010 Meeting, Seattle, Washington, January 3-7, 2010

Document Type

Conference Proceeding

Publication Date

1-4-2010

Abstract

Sex allocation theory predicts that for simultaneous hermaphrodites, allocation to male-specific function should increase as competition for mates increases. Eric Charnov developed a local mate competition model (LMC) that makes specific predictions of relative allocation to the sex roles (specifically the proportion of reproductive resources allocated to male function) for simultaneously hermaphroditic acorn barnacles competing for mates (1980). We tested these predictions using field experiments with two acorn barnacle species, Semibalanus balanoides on Long Island, NY, and Balanus glandula on San Juan Island, WA. In Charnov’s model, barnacles are predicted to allocate resources according to the number of individuals in the mating group. Our previous results show that barnacles alter the morphology of their penises in response to aggregation density- that is, how close neighbors sit to each other, independent of their numbers. We used a full-factorial design to separate the effects of the number of potential mates within the reach of the penis and the density at which those barnacles are crowded. For both species, we did not observe the relative sex allocation (proportion male) predicted by the LMC model. However, when each sex role was looked at separately, we saw that absolute allocation to male function did increase with increasing aggregation density, but not number of individuals in the mating group. Female function was tightly associated with body size and did not appear to tradeoff with investments in male function. Overall, our results suggest that allocation to the male role does increase with increased competition for mates (perceived by the barnacles from density of crowding) and allocation to female function is limited by the ability to produce and brood eggs.

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