Doctor of Philosophy (PhD)


Biological Sciences

Document Type



Since Darwin, biologists wonder how organisms cope with environmental variation, why there are so many species, and how species form. In my dissertation, I explore how species of long-lived, clonal reef organisms originate across depth gradients. In Chapter 2, I evaluate the strength of depth to isolate populations by comparing the genes and morphologies of pairs of depth-segregated populations of the candelabrum coral Eunicea flexuosa across the Caribbean. Eunicea flexuosa is a long-lived clonal cnidarian that associates with an alga of the genus Symbiodinium. Genetic analysis revealed two depth-segregated lineages, each genetically well-mixed across the Caribbean. Survivorship data, combined with estimates of selection coefficients based on transplant experiments, suggest that selection is strong enough to segregate these two lineages. Limited recruitment to reproductive age, even under weak annual selection advantage, is sufficient to generate habitat segregation because of the cumulative selection accrued during prolonged pre-reproductive growth. I then studied in detail the genetic diversity of Symbiodinium in each Shallow and Deep E. flexuosa lineage (Chapter 3). I sampled colonies of the two ecotypes across depths at three Caribbean locations. I find that each host lineage is associated with a unique Symbiodinium variant. This relationship between host and alga is maintained when host colonies are reciprocally transplanted. Even when the clades of both partners are present at intermediate depths, the specificity between host and algal lineages remained. I then test whether the Shallow/Deep adaptive divergence occurs in natural populations, by examining the frequencies of juvenile and adults. The habitat distributions of the two lineages are more distinct when inferred from adults than from juveniles. Selection coefficients from cohort data correlates with that from transplant experiments (Chapter 4). The two lineages form a narrow hybrid zone (100 m), with coincident clines of both the host coral and its algal symbiont. Effective dispersal estimates derived from the hybrid zone are small (20 m) for a broadcast spawner with a large dispersing potential (50 km). Ecological factors associated with depth act as filters generating strong barriers to gene flow despite extensive dispersal, altering morphologies, and contributing to the potential for speciation in the sea.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Hellberg, Michael