Date of Award

1-1-2025

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Biological and Environmental Sciences

Specialization

Ecology and Ecosystem Science

Department

Biological Sciences

First Advisor

Carlos Prada

Abstract

Light is critical for the survival of most shallow water scleractinian corals. Corals are mixotrophs, acquiring energy via both feeding on water column plankton and photosynthesis via a symbiotic relationship with dinoflagellates living within their tissues. Corals rely heavily on photosynthetically derived metabolites through this symbiotic relationship, intrinsically linking their biology to light availability. Corals are known to modulate their biology to maximize light capture. However, there is a limited understanding of the interaction between morphology, physiology, and energetic strategy, as well as how light may affect these relationships. In Chapter 1, we quantified the morphology, physiology and energetic strategy of six Caribbean corals along a light gradient in Puerto Rico. We found that each species utilizes one main energetic strategy regardless of light availability. Morphology, but not physiology, shifted along the light gradient in all six species, and we found interactions between skeletal morphology, energetic strategy and phylogenetic clade. To build on naturally occurring changes in coral biology along light gradients, we conducted a transplant experiment on three lineages of Orbicella in Chapter 2. O. faveolate has two genetically distinct cryptic lineages in the shallow and deep reefs of Puerto Rico, and its congener O. franksi is typically found in deep environments. When transplanted away from its home environment into high light, O. franksi exhibited improved survival, growth and asexual reproduction, as well as physiological and morphological plasticity. Conversely, the two lineages of O. faveolate each performed better in their original environment, with shallow O. faveolate experiencing higher mortality, negative growth rates, lower asexual reproduction and patterns of physiology consistent with a decline in health when transplanted into darker environments. We are among the first to document cryptic specialization in a coral with a known cryptic lineage, which is critical to understanding the functional diversity of scleractinian corals.

The temperate coral Astrangia poculata has evolved facultative symbiosis, or the ability to survive with varying degrees of symbiont density. This enables high- and low- symbiont density ecotypes to survive in cold, turbid waters, often with limited light availability. A. poculata has historically been used as a model system for Cnidarian symbiosis, but our understanding of its distribution, ecological role and trophic ecology is limited. In Chapter 3, we used random quadrat sampling to document the distribution of A. poculata along a depth gradient in Narragansett Bay, Rhode Island. Macroalgae, light availability and depth control the ecotype distribution and abundance of A. poculata. Finally, we documented the effects of light and depth on the trophic ecology, morphology, and physiology of A. poculata in Chapter 4. After a four-month in situ shading experiment, corals exhibited photophysiological plasticity by rapidly acclimating their symbiont and chlorophyll density to their new light environment. Compound-specific stable isotope analysis of amino acids revealed distinct trophic niches between the coral host and its symbionts, as well as high trophic positions in both. These findings and its physiological plasticity indicate that A. poculata primarily relies on heterotrophy for energy, regardless of symbiont density or light availability. The coral host sustains symbiont populations in high-light conditions by exchanging metabolites bidirectionally, even though the symbionts provide minimal energetic benefits.

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Available for download on Thursday, November 27, 2025

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