Date of Award


Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)


Marine Biology


Philip O. Yund

Second Committee Member

Susan H. Brawley

Third Committee Member

Peter A. Jumars


Successful fertilization in marine organisms that release sperm into the seawater was traditionally thought to be limited by the rapid dilution of gametes. More recent work has documented that egg brooders that possess mechanisms to concentrate sperm out of the water appear to be much less sperm limited than egg broadcasters. The ability of an organism to overcome dilution effects and successfully fertilize distant mates can have important consequences for both male and female reproductive success. The research presented in this dissertation demonstrates how the free-spawning colonial ascidian, Botryllus schlosseri, appears to be remarkably efficient at acquiring dilute, long-lived sperm from the water column. In part because of this enhanced sperm longevity, distant fertilizations have been recorded in the field. Past investigations of the selective pressures shaping the allocation of resources to sperm production have focused on the influence of sperm competition and local mate competition, but what we now know about sperm dispersal distances suggest that fertilization distance may be important as well. A study exploring the influence of fertilization distance on the relationship between sperm production and resulting male reproductive success demonstrates that fertilization distance may be one of the selective pressures driving the allocation of resources to sperm production in B. schlosseri. Although numerous studies have demonstrated the existence of multiple paternity in marine free-spawners, few studies have characterized the frequency of multiple paternity in natural populations or explored possible causes of this variation. A study exploring variation in multiple paternity in B. schlosseri broods from natural populations demonstrates that multiple paternity is common, but highly variable, with a few broods displaying unequal contributions. Indirect benefits from increasing the genetic diversity of broods are a possible explanation for the high level of multiple paternity in this species. A study exploring the relationship between multiple paternity and subsequent fusion-rejection interactions in B. schlosseri demonstrates how fertilization processes can subsequently affect interactions among later life history stages. In combination, these studies suggest that brooding free-spawners, with efficient sperm transmission, have very different mating dynamics than some broadcast spawning species.