Studivan, Michael

Coral reef ecosystems worldwide are facing increasing degradation due to
disease, anthropogenic damage, and climate change, particularly in the Tropical Western
Atlantic. Mesophotic coral ecosystems (MCEs) have been recently gaining attention
through increased characterization as continuations of shallow reefs below traditional
SCUBA depths (>30 m). As MCEs appear to be sheltered from many stressors affecting
shallow reefs, MCEs may act as a coral refuge and provide larvae to nearby shallow
reefs. The Deep Reef Refugia Hypothesis (DRRH) posits that shallow and mesophotic
reefs may be genetically connected and that some coral species are equally compatible in
both habitats. The research presented here addresses key questions that underlie this
theory and advances our knowledge of coral connectivity and MCE ecology using the
depth-generalist coral Montastraea cavernosa. Chapter 1 presents an overview of the
DRRH, a description of MCEs in the Gulf of Mexico (GOM), and the framework of
research questions within existing reef management infrastructure in the GOM. Through microsatellite genotyping, Chapter 2 identifies high connectivity among shallow and
mesophotic reefs in the northwest GOM and evidence for relative isolation between depth
zones in Belize and the southeast GOM. Historical migration and vertical connectivity
models estimate Gulf-wide population panmixia. Chapter 3 focuses on population
structure within the northwest GOM, identifying a lack of significant population
structure. Dominant migration patterns estimate population panmixia, suggesting
mesophotic populations currently considered for National Marine Sanctuary protection
benefit the Flower Garden Banks. Chapter 4 quantifies the level of morphological
variation between shallow and mesophotic M. cavernosa, revealing two distinct
morphotypes possibly representing adaptive tradeoffs. Chapter 5 examines the
transcriptomic mechanisms behind coral plasticity between depth zones, discovering a
consistent response to mesophotic conditions across regions. Additionally, variable
plasticity of mesophotic corals resulting from transplantation to shallow depths and
potential differences in bleaching resilience between shallow and mesophotic corals are
identified. The dissertation concludes with a synthesis of the results as they pertain to
connectivity of shallow and mesophotic corals in the Gulf of Mexico and suggests future
research that will aid in further understanding of MCE ecology and connectivity.

Coral reef ecosystems worldwide are threatened by disease, anthropogenic damage, and climate change. Recently mesophotic coral ecosystems MCEs have been discovered and characterized as continuations of shallow reefs below traditional SCUBA depths. Because MCEs appear to be sheltered from many of the stressors known to affect shallow reefs, Glynn 1996 originally proposed that deeper reefs may act as a refuge for coral species by providing larvae to nearby shallow reefs. My dissertation research will address key questions that underlie this theory and advance our knowledge of coral ecology and physiology at mesophotic depths. including: 1 How well are MCEs connected to shallow reefs, and 2 Can corals adapt their morphology and gene expression to match changing environmental stimuli? Microsatellite based analysis of genetic connectivity will be complemented by morphometric analyses of corallite phenotypic plasticity and gene expression profiling in the dominant coral species, Montastraea cavernosa. MCEs in the Flower Garden Banks and Pulley Ridge have been surveyed and sampled using both ROVs and technical diving in 2011-2013, with additional expeditions planned for 2014-2015. A reciprocal transplant experiment on Carrie Bow Cay, Belize will assess the physiological and morphological plasticity of M. cavernosa in shallow and mesophotic environments. This project will address the influences of both environment and genotype control on coral’s potentially plastic responses. Additionally, this research is designed to provide data for improved regional management of mesophotic coral reef ecosystems and to strengthen collaborative marine research through NOAA’s Cooperative Institute for Ocean Exploration, Research, and Technology CIOERT.