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1. Coral recruitment: patterns and processes determining the dynamics of coral populations

   https://doi.org/10.1111/brv.12987  

   Edmunds, Peter J. (June 2023, Biological Reviews)

   ABSTRACT Coral recruitment describes the addition of new individuals to populations, and it is one of the most fundamental demographic processes contributing to population size. As many coral reefs around the world have experienced large declines in coral cover and abundance, there has been great interest in understanding the factors causing coral recruitment to vary and the conditions under which it can support community resilience. While progress in these areas is being facilitated by technological and scientific advances, one of the best tools to quantify recruitment remains the humble settlement tile, variants of which have been in use for over a century. Here I review the biology and ecology of coral recruits and the recruitment process, largely as resolved through the use of settlement tiles, by: (i) defining how the terms ‘recruit’ and ‘recruitment’ have been used, and explaining why loose terminology has impeded scientific advancement; (ii) describing how coral recruitment is measured and why settlement tiles have value for this purpose; (iii) summarizing previous efforts to review quantitative analyses of coral recruitment; (iv) describing advances from hypothesis‐driven studies in determining how refuges, seawater flow, and grazers can modulate coral recruitment; (v) reviewing the biology of small corals (i.e. recruits) to understand better how they respond to environmental conditions; and (vi) updating a quantitative compilation of coral recruitment studies extending from 1974 to present, thus revealing long‐term global declines in density of recruits, juxtaposed with apparent resilience to coral bleaching. Finally, I review future directions in the study of coral recruitment, and highlight the need to expand studies to deliver taxonomic resolution, and explain why time series of settlement tile deployments are likely to remain pivotal in quantifying coral recruitment. 

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2. Three-dimensional printing can provide opportunities to promote coral recruitment on disturbed reefs

   https://doi.org/10.5343/bms.2023.0131  

   Muñoz-Maravilla, J David; Edmunds, Peter J (January 2025, Bulletin of Marine Science)

   Tropical corals are undergoing population declines due to disturbances. The implications of these trends are modulated by the ability of corals to support population recovery through recruitment. Current research underscores the importance of physical features of benthic surfaces in promoting coral recruitment, which creates opportunities to enhance recruitment by engineering surfaces to replicate these features with the goal of enhancing coral settlement. This study examined the interaction between the settlement of coral larvae and three-dimensional (3D) surfaces and employed 3D printing to enhance recruitment. We tested the effects of the features of microhabitats on the settlement preference, gregariousness, and survival of the brooding coral Pocillopora acuta. Grooved microhabitats that are common in the shallow (<7 m depth) backreef of Moorea, French Polynesia, were printed onto tiles made of polylactic acid, and were favored for settlement by freshly released larvae fromP.acuta. The percent survivorship over 20 d of coral recruits that settled in grooved microhabitats was 16.4% vs none on open flat surfaces. These results underscore the importance of naturally forming benthic features in promoting coral recruitment, and they highlight the potential for duplication of these features through 3D printing to enhance coral recruitment and accelerate reef restoration following damage. 

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3. Early Life-History Dynamics of Caribbean Octocorals: The Critical Role of Larval Supply and Partial Mortality

   https://doi.org/10.3389/fmars.2021.705563  

   Martínez-Quintana, Ángela; Lasker, Howard R. (September 2021, Frontiers in Marine Science)

   Recruitment is a key demographic process for maintenance of local populations and recovery following disturbance. For marine invertebrates, distribution and abundances of recruits are impacted by spatiotemporal variation in larval supply, settlement rates and post-settlement survival. However, for colonial and modular organisms, differences in survival and growth between settlers and colonial recruits may also affect recruitment patterns. In the Caribbean, shifts in the benthic community structure favoring octocoral’s have been detected, and recruitment has been suggested as key for octocoral’s resilience. Hence, we studied octocoral recruitment dynamics, and evaluated the role of pre-settlement, settlement and post-settlement processes in recruit’s densities. We performed the study at two sites with different octocoral densities, on the south coast of St. John, United States Virgin Islands, and distinguished between processes occurring to recently settled polyps and to colonial recruits. At both sites, we monitored P. homomalla settlers on settlement tiles for 3 months, and colonial recruits of two of the most abundant genera ( Eunicea and Pseudoplexaura) for 3 years. In addition, we assessed whether recruits morphological traits affected recruitment and divided recruits of the genus Eunicea based on the presence of large calyces. The major contributor to both, single-polyps and colonial recruit densities was larval supply. Single-polyp densities were not limited by the availability of space, settlement cues, or early post-settlement survival. Height was the only predictor of survival and growth of colonial recruits, with potential growth rates increasing with height. However, large recruits suffered partial mortality often, distorting the relationship between recruit age and size, and causing most recruits to remain in the recruit size class (≤5 cm) longer than a year. Octocorals have been resilient to the conditions that have driven the decline of scleractinian corals throughout the Caribbean, and recruitment has been key to that success. Our results are crucial to understand early life history dynamics of Caribbean octocorals, and highlights the need to standardize the definition of recruit among colonial and modular taxa to facilitate inter-specific comparisons, and to understand future changes in coral reef community assemblages. 

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4. Recruitment hotspots and bottlenecks mediate the distribution of corals on a Caribbean reef

   https://doi.org/10.1098/rsbl.2021.0149  

   Edmunds, Peter J. (July 2021, Biology Letters)

   Recruitment hotspots are locations where organisms are added to populations at high rates. On tropical reefs where coral abundance has declined, recruitment hotspots are important because they have the potential to promote population recovery. Around St. John, US Virgin Islands, coral recruitment at five sites revealed a hotspot that has persistent for 14 years. Recruitment created a hotspot in density of juvenile corals that was 600 m southeast of the recruitment hotspot. Neither hotspot led to increased coral cover, thus revealing the stringency of the demographic bottleneck impeding progression of recruits to adult sizes and preventing population growth. Recruitment hotspots in low-density coral populations are valuable targets for conservation and sources of corals for restoration. 

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5. Ex situ spawning, larval development, and settlement in massive reef‐building corals ( Porites ) in Palau

   https://doi.org/10.1111/ivb.12447  

   Bennett, Matthew‐James; Grupstra, Carsten_G B; Da‐Anoy, Jeric; Andres, Maikani; Holstein, Daniel; Rossin, Ashley; Davies, Sarah W; Meyer‐Kaiser, Kirstin S (October 2024, Invertebrate Biology)

   Abstract Reproduction, embryological development, and settlement of corals are critical for survival of coral reefs through larval propagation. Yet, for many species of corals, a basic understanding of the early life‐history stages is lacking. In this study, we report our observations forex situreproduction in the massive reef‐building coralPoritescf.P. lobataacross 2 years. Spawning occurred in April and May, on the first day after the full moon with at least 2 h of darkness between sunset and moonrise, on a rising tide. Only a small proportion of corals observed had mature gametes or spawned (14–35%). Eggs were 185–311 μm in diameter, spherical, homogenous, and provisioned with 95–155 algal cells (family Symbiodiniaceae). Males spawned before females, andex situfertilization rates were high for the first 2 h after egg release. Larvae were elliptical, ~300 μm long, and symbiotic. Just 2 days after fertilization, many larvae swam near the bottom of culture dishes and were competent to settle. Settlers began calcification 2 days after metamorphosis, and tentacles were developed 10 days after attachment. Our observations contrast with previous studies by suggesting an abbreviated pelagic larval period inPoritescf.P. lobata, which could lead to the isolation of some populations. The high thermal tolerance and broad geographic range ofPoritescf.P. lobatasuggest that this species could locally adapt to a wide range of environmental conditions, especially if larvae are locally retained. The results of this study can inform future work on reproduction, larval biology, dispersal, and recruitment inPoritescf.P. lobata, which could have an ecological advantage over less resilient coral species under future climate change. 

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