Date of Award

Spring 2019

Document Type


Degree Name

Doctor of Philosophy (PhD)


Biological Sciences


Ecological Sciences

Committee Director

Mark J. Butler, IV

Committee Member

Eric Walters

Committee Member

Dan Barshis

Committee Member

Seabird McKeon


Coral reefs have been on a trajectory of decline for nearly a century due to a variety of factors that have contributed to the shift in these communities away from dominance by reef-building corals, with commensurate changes on community composition and function. Florida’s reefs are a compelling example of a degraded system that has undergone a phase shift, and thus offered an excellent model system for my study of the effects of grazing by cryptic herbivores on community composition and their potential restoration value. I had four major objectives: (1) determine the suitability of Maguimithrax spinosissimus for manipulating grazing intensity on reefs (Chapter 2); (2) test the effect of increasing M. spinosissimus density on the abundance and distribution of macroalgae (Chapters 2 & 3); (3) test the effect of enhanced grazing and reduced algae cover on composition of the reef fish community (Chapter 2 & 3); and (4) compare the grazing of multiple species of Mithracid crabs with that of M. spinosissimus (Chapter 4).

I found that M. spinosissimus are amenable to density manipulation and, at sufficient density, their effect on benthic macroalgae is deleterious. M. spinosissimus exhibited a high degree of philopatry on patch reefs, and that crabs >30mm carapace length reach a size refuge from predation and experience lower mortality. Reef fish abundance and species richness were also greater on reefs with high crab density than on controls. Manipulated reefs had higher densities of juvenile corals than did controls. This study is uncommon in that I replicated the experiment at another location and time. Those results (Chapter 3) confirmed those described in Chapter 2 and demonstrate the broader applicability of my findings.

In Chapter 4, I describe an experiment where I discovered that M. spinosissimus consumed more algae than any of the three other species of Mithraculus tested; but, when scaled for biomass, the relationship was reversed. There was no effect of multiple individuals on algae consumption except in treatment groups that included both Mithracid genera. These studies demonstrate the potentially transformative, and often overlooked, effect of cryptic invertebrates on patterns of reef community composition and function.