238-5 Costs of Cooperation? Testing Competitive Advantages of Division of Labor in Bryozoans

Abstract:

Competition is often invoked as a powerful evolutionary driver, influencing how organisms survive and reproduce. Identifying traits associated with competitive outcomes offers one way to evaluate how individual ecological interactions scale to evolutionary change. In bryozoans, division of labor among polymorphic zooids is associated with greater taxonomic diversity, with improved colony performance during direct overgrowth competition as a proposed mechanism. Here, we tested whether colonies involved in overgrowth interactions differed in zooid types and morphologies from colonies without visible interactions and whether, among interacting colonies, these traits were associated with winning or losing. We quantified colony-level polymorphism and morphology of 100 shallow-water, modern bryozoan colonies collected from coastal reefs in Curaçao and Panama. We used logistic mixed-effects models to test whether a colony’s interaction status or its competitive outcome was associated with colony size, zooid size, and the number of distinct polymorphs and their relative frequencies, accounting for genus identity as a random effect. 

Across genera, none of the measured traits were significantly associated with whether a colony was interacting or not, and among interacting colonies, whether it won or lost. Within Stylopoma, however, zooid size was larger in interacting colonies, but this factor did not predict competition outcomes. Larger zooids might help colonies persist during interactions long enough to reproduce before being overgrown; alternatively, zooid size might be tied to higher growth rates, which would be an important factor to consider in future models of competition. Thus, these results do not support a simple link between the colony traits measured here and competitive interaction and its outcomes, leaving the ecological basis of the association between polymorphism and diversity unresolved. Future studies incorporating growth rate and energetic efficiency may improve our understanding of how competition shapes trait evolution in bryozoans.

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