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.

This book summarizes what is known about mesophotic coral ecosystems (MCEs) geographically and by major taxa. MCEs are characterized by light-dependent corals and associated communities typically found at depths ranging from 30-40 m. and extending to over 150 m. in tropical and subtropical ecosystems. They are populated with organisms typically associated with shallow coral reefs, such as macroalgae, corals, sponges, and fishes, as well as specialist species unique to mesophotic depths. During the past decade, there has been an increasing scientific and management interest in MCEs expressed by the exponential increase in the number of publications studying this unique environment. Despite their close proximity to well-studied shallow reefs, and the growing evidence of their importance, our scientific knowledge of MCEs is still in its early stages. The topics covered in the book include: regional variation in MCEs; similarities and differences between mesophotic and shallow reef taxa, biotic and abiotic conditions, biodiversity, ecology, geomorphology, and geology; potential connectivity between MCEs and shallow reefs; MCE disturbances, conservation, and management challenges; and new technologies, key research questions/knowledge gaps, priorities, and future directions in MCE research.

Every year, 10 outstanding Research Topics are selected as finalists of the Frontiers Spotlight Award. These shortlisted article collections each address a globally important field of research with the potential to drastically impact our future. They bring together the latest, cutting-edge research to advance their fields, present new solutions and foster essential, large-scale collaborations across multiple disciplines and research groups worldwide. This international research prize recognizes the most innovative and impactful topics and the winning team of editors receives $100,000 to organize an international scientific conference on the theme of their successful collection.

The extent to which reefs are effectively connected to one another, and their potential to serve as sources of larval replenishment following disturbance, are topics of considerable interest in contemporary reef science. To date, most assessments of reef connectivity have emphasized long-distance horizontal dispersal of propagules from one shallow reef to another. The extent of short-distance vertical connectivity, however, has been largely unquantified. To fill this gap in knowledge, I developed DNA microsatellite loci for two Caribbean depth-generalist coral species with different life-history reproductive strategies (Montastraea cavernosa and Porites astreoides), and assessed connectivity in >1,200 coral samples collected from 3 depth zones (=10 m, 15-20 m and =25 m) at sites in Florida (within the Upper Keys, Lower Keys and Dry Tortugas), Bermuda and the U.S. Virgin Islands (USVI). I also tested whether depth zonation in algal symbionts (Symbiodinium spp.) could limit effective vertical connectivity (Chapters 2 and 3). Finally, in Chapter 4, I led a collaborative seascape genetics effort to examine coral connectivity between the Flower Garden Banks (FGB) and the Florida Reef Tract at different depth intervals. This is a timely and important investigation because the FGB are located close to many oil and gas platforms in the Gulf of Mexico (GOM), including the Deepwater Horizon oil rig which exploded in 2010. Overall, Chapters 2 and 3 revealed significant genetic differentiation by depth in Florida (but not in Bermuda or the USVI) for both species, despite high levels of horizontal connectivity between all three geographic locations (M. cavernosa), or between Florida and the USVI (P. astreoides) at shallow depths. However, at all sites, and regardless of the extent of vertical connectivity, migration always occurred asymmetrically, with greater downward migration from shallow to deep habitats. Finally, whether or not M. cavernosa or P. astreoides exhibited depth zonation in algal symbionts did not appear to limit effective connectivity. Together, these findings suggest that: (1) depth is an important population structuring factor for corals, (2) the extent of vertical connectivity varies among and within geographic locations, likely as a consequence of local hydrology, (3) reproductive mode does not necessarily correlate with realized dispersal ability, and (4) shallow reefs are more likely to rely on distant (unimpacted) shallow reefs, rather than nearby deep reefs, to provide a viable source of new recruits following disturbance. Finally, Chapter 4 revealed high levels of gene flow between the FGB and the shallow Florida population of M. cavernosa, but not P. astreoides, suggesting limited gene flow among these regions. Results from biophysical modeling were in general agreement, suggesting that differences in reproductive mode and season might be important drivers of reef coral connectivity within the GOM region. Together, these findings suggest that FGB, despite its deep depth, might be an important larval source for shallow coral populations of broadcast spawning taxa in Florida. Furthermore, findings suggest that an oil spill originating in the GOM: (1) has the potential to impact coral communities in Florida by reducing recruitment from the FGB, (2) is more likely to affect broadcast spawning taxa like M. cavernosa, due to high levels of gene flow between FGB and Florida, and (3) regardless of coral reproductive mode, these impacts are more likely to affect shallow habitats, likely sinks for coral larvae produced at FGB. While deep coral populations in Florida may constitute refugia due to their partial isolation from the shallow population (see Chapters 2 and 3), they too might eventually be impacted if shallow populations were slow, or unable to recover.

This book comprehensively introduces recent important studies on coral reefs from various research fields including biology, ecology, chemistry, the earth sciences, and conservation studies. Coral reef is one of the important ecosystems characterized by high biodiversity and the beauty. Coral reefs around Japan are located at the northern limit, composed by mainly fringing reefs along archipelago, and easily impacted by human activities. Thus, coral reef studies around Japan have provided important knowledge on basic sciences and conservation studies regarding coral reef ecosystem. This book would contribute to systematic understanding of vulnerable coral reef ecosystems due to human activities in the Indo-Pacific and Caribbean regions. The conservation efforts provide good reference to graduate and undergraduate students, and researchers in marine sciences, as well as those who are involved in coral reef studies.

Examination of corals and reef-associated organisms which endure in extreme coral reef environments is challenging our understanding of the conditions that organisms can survive under. By studying individuals naturally adapted to unfavorable conditions, we begin to better understand the important traits required to survive rapid environmental and climate change. This Research Topic, comprising reviews, and original research articles, demonstrates the current state of knowledge regarding the diversity of extreme coral habitats, the species that have been studied, and the knowledge to-date on the mechanisms, traits and trade-offs that have facilitated survival.

This volume presents a broad panorama of the current status of research of invertebrate animals considered belonging to the phylum Cnidaria, such as hydra, jellyfish, sea anemone, and coral. In this book the Cnidarians are traced from the Earth’s primordial oceans, to their response to the warming and acidifying oceans. Due to the role of corals in the carbon and calcium cycles, various aspects of cnidarian calcification are discussed. The relation of the Cnidaria with Mankind is approached, in accordance with the Editors’ philosophy of bridging the artificial schism between science, arts and Humanities. Cnidarians' encounters with humans result in a broad spectrum of medical emergencies that are reviewed. The final section of the volume is devoted to the role of Hydra and Medusa in mythology and art.

The Philippines is located in a region called the Coral Triangle; an area of exceptional importance as it is recognized as the global center for marine biodiversity. Within this region, the Philippines is also known as the center of this center. The marine resources are made up of coral reefs, seagrass beds, seaweeds, invertebrates, fisheries, and many others. Unfortunately, the Philippines is facing challenges to conserve its marine biodiversity due to overfishing, destructive fishing practices, and mariculture-derived eutrophication and coastal acidification, among others. These human activities coupled with climate change pose serious threats and possible decimation of its marine biodiversity.