Phenotypic plasticity

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.

Larval release by adult fiddler crabs occurs during the ebbing tides, but its timing
relative to the day-night and tidal amplitude cycles depends upon tidal form (e.g., shows
phenotypical plasticity). Crabs (Uca thayeri) from Florida's East Coast are exposed to
semidiurnal tides and release their larvae at night, whereas crabs from Florida's West
Coast exposed to mixed tides release their larvae during the afternoon. The purpose of
this study was to determine whether the larvae could hatch at times other than those
correlated with the tidal form at their location. Clusters of eggs at similar stages of
development, 24-72 h in advance of release, were reciprocally transferred between
females from each coast. Release ofboth the transferred larvae and maternal clutch
occurred synchronously, and at the time dictated by the female's tidal regime. These
results suggest that larvae are phenotypically plastic with respect to hatching time and
can either delay (West coast) or advance (East coast) their response to release signals
from females.