Mesophotic coral ecosystems (MCE) are defined as phototrophic coral habitats found deeper than 30 m. Despite being aware of these ecosystems for over 200 years, surprisingly little information is available on their ecology and biology. Recently, MCE have received renewed interest, as it appears that depth and distance from shore have the potential to buffer coral organisms from the detrimental effects of coastal development and climate change. The “deep reef refugia hypothesis” (DRRH) is an umbrella term for a collection of hypotheses concerning the role of MCE in the uncertain future of coral reefs, yet our predictions are limited by shortcomings in our understanding of some very basic effects of depth on corals and associated communities. In order to investigate the effects of depth on coral reproductive biology, sampling of Montastraea faveolata and Porites astreoides coral tissues was conducted along a depth gradient from 5 to 40 m during coral reproductive seasons in the Northern United States Virgin Islands (USVI), and observations of coral spawning and planulation were made. Samples were histologically analyzed for gamete development, reproductive activity and fecundity. Mesophotic populations of both M. faveolata and P. astreoides were reproductively active in MCE with similar gametogenic cycles to nearby shallow coral populations. There was evidence of M. faveolata split spawning in August and September at all depths, and oocyte development was delayed but more rapid in mesophotic corals. M. faveolata fecundities were significantly higher in MCE (35-40 m) than in shallow (5-10 m) sites, but the differences were not significant between mid-depth (15-22 m) and either shallow or mesophotic sites. There was no difference found in P. astreoides fecundity between mesophotic, mid-depth and shallow sites, however planulation appeared to be delayed in mesophotic colonies by 1-2 weeks. Differences in fecundity per area and coral cover between depths determine the number of propagules a unit reef will produce at different depths. In the case of M. faveolata, ova production is likely an order of magnitude greater at 35 m than at 10 m. The Connectivity Modeling System, an individual-based stochastic biophysical model of larval dispersal, parameterized with depth-specific productivity estimates and species-specific reproductive seasons and larval traits, was used to evaluate the vertical connectivity of M. faveolata and P. astreoides larvae between MCE and shallow coral habitats in the Northern USVI. Sensitivity analyses were performed to test the sensitivity of mesophotic larval subsidy into shallow habitats to depth-specific productivity, pelagic larval mortality, depth-specific fertilization rates and depth-specific post-settlement survivorship. Simulated mesophotic subsidies to shallow recruitment were found to be considerably robust, and mesophotic subsidy to shallow recruitment accounted for a greater proportion of total recruitment as shallow productivity was reduced. Even when modeled mesophotic fertilization rates and larval post-settlement survivorship were dramatically reduced, the model predicted what would likely be demographically significant mesophotic larval subsidy into shallow habitat. Mesophotic M. faveolata skeletal density, extension and calcification were estimated using micro-computed tomography. Results suggest that rates of linear extension of M. faveolata in USVI MCE may be quite fast compared to other Caribbean MCE, and that total calcification in MCE may rival shallow coral calcification. Lastly, consistencies and inconsistencies in the population connectivity of two coral and three fish constituent species in Caribbean coral reef assemblages were investigated using a nested biophysical model. Connectivity networks of coral species were more fragmented than fish, and the networks of corals and fish showed different patterns of betweenness centrality. This suggests that populations of corals and fish will likely be affected by habitat fragmentation in different ways, and that they require specific management consideration. This dissertation suggests that MCE are integral to the population connectivity of corals in the USVI and likely to wider Caribbean metapopulation connectivity as well. Further study of these highly productive ecosystems is necessary to better understand the DRRH and the role of MCE in the past, present and future of coral reefs

Depth range
5- 40 m

Mesophotic “mentions”
401 x (total of 34099 words)

Classification
* Focused on 'mesophotic' depth range
* Focused on 'mesophotic coral ecosystem'

Fields
Connectivity
Reproduction

Focusgroups
Scleractinia (Hard Corals)
Fishes

Locations
US Virgin Islands

Platforms
SCUBA (open-circuit or unspecified)
SCUBA (open-circuit or unspecified)