Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


J. Emmett Duffy

Committee Member

Jan McDowell

Committee Member

Mark Patterson

Committee Member

Jeffrey D. Shields

Committee Member

Dustin R. Rubenstein


The diversity of animal social strategies has interested evolutionary biologists since the time of Darwin. Eusociality—the apex of animal sociality—traditionally characterized by cooperative offspring care, overlapping generations and reproductive division of labor, was until recently known only in insects and a few vertebrate species. The independent evolution of eusociality in shrimps in the genus Synalpheus offers a unique opportunity to test the generality of social evolution theories that are based mainly on insects and social vertebrates. The genus Synalpheus is particularly ideal for comparative analysis because their social organizations are highly diverse, yet they share very similar ecology of being sponge dwellers. Further, their close associations with sponges, in which many are considered microbial fermenters, allow one to test the ecological drivers of species diversity in Synalpheus. In this dissertation, I first explored the nature and consequences of reproductive altruism in eusocial species. Chapter 1 showed that workers in eusocial Synalpheus retain reproductive capability, but reproduction of female workers is suppressed by the queen. Chapter 2 showed further that such reproductive inequity among females within a colony leads to potentially strong competition among females for reproductive opportunities, and is associated with reduced sexual dimorphism in eusocial Synalpheus species. Second, I examined the evolutionary trajectories between and ecological advantages associated with different social organizations in Synalpheus. Chapter 3 shows that the two demographically distinct social organizations found in Synalpheus—communality and eusociality—have evolved via separate evolutionary trajectories and represent alternative social strategies. Chapter 4 further shows that these social strategies are associated with different aspects of ecological advantages conferred on Synalpheus living together. Finally, the intimate association with host sponges constrains the lifestyle of Synalpheus and may be one factor that has predisposed their evolution of eusociality. In Chapter 5, I examined the association pattern of Synalpheus with their host sponges and found that the symbiotic microorganisms in sponges, rather than the phylogenetic histories of the host sponges, are a better predictor and potential driver of the host association pattern. This dissertation has sought to test, and ended up challenging, several paradigms in ecology and evolution. My results suggest that 1) polymorphic reproductive soldiers may represent a natural transition towards eusociality, 2) reproductive monopolization can modulate the pattern of sexual dimorphism in social species, 3) communality and eusociality evolved from distinct trajectories and have different ecological advantages, and 4) symbiotic microorganisms may mediate biological interactions between their hosts and other organisms.




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