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Mesophotic coral ecosystems (MCEs, reefs between 30 and 150 m depth) have been hypothesized to contribute to shallow reef recovery through the recruitment of larvae. However, few studies have directly examined this. Here we used mesophotic colonies of Seriatopora hystrix , a depth generalist coral, to investigate the effect of light intensity on larval behavior and settlement through ex situ experiments. We also investigated juvenile survival, growth, and physiological acclimation in situ. Bleached larvae and a significant reduction in settlement rates were found when the mesophotic larvae were exposed to light conditions corresponding to shallow depths (5 and 10 m) ex situ. The in situ experiments showed that mesophotic juveniles survived well at 20 and 40 m, with juveniles in shaded areas surviving longer than three months at 3–5 m during a year of mass bleaching in 2016. Juvenile transplants at 20 m showed a sign of physiological acclimation, which was reflected by a significant decline in maximum quantum yield. These results suggest that light is a significant factor for successful recolonization of depth-generalist corals to shallow reefs. Further, recolonization of shallow reefs may only occur in shaded habitats or potentially through multigenerational recruitments with intermediate depths acting as stepping stones.

Mesophotic coral ecosystems (MCEs, between 30 and 150 m depth) are hypothesized to contribute to the recovery of degraded shallow reefs through sexually produced larvae (referred to as Deep Reef Refuge Hypothesis). In Okinawa, Japan, the brooder coral Seriatopora hystrix was reported to be locally extinct in a shallow reef while it was found abundant at a MCE nearby. In this context, S. hystrix represents a key model to test the Deep Reef Refuge Hypothesis and to understand the potential contribution of mesophotic corals to shallow coral reef recovery. However, the reproductive biology of mesophotic S. hystrix and its potential to recolonize shallow reefs is currently unknown. This study reports for the first time, different temporal scales of reproductive periodicity and larval settlement of S. hystrix from an upper mesophotic reef (40 m depth) in Okinawa. We examined reproductive seasonality, lunar, and circadian periodicity (based on polyp dissection, histology, and ex situ planula release observations) and larval settlement rates in the laboratory. Mesophotic S. hystrix reproduced mainly in July and early August, with a small number of planulae being released at the end of May, June and August. Compared to shallow colonies in the same region, mesophotic S. hystrix has a 4-month shorter reproductive season, similar circadian periodicity, and smaller planula size. In addition, most of the planulae settled rapidly, limiting larval dispersal potential. The shorter reproductive season and smaller planula size may result from limited energy available for reproduction at deeper depths, while the similar circadian periodicity suggests that this reproductive aspect is not affected by environmental conditions differing with depth. Overall, contribution of mesophotic S. hystrix to shallow reef rapid recovery appears limited, although they may recruit to shallow reefs through a multistep process over a few generations or through random extreme mixing such as typhoons.

Distinguishing coral species is not only crucial for physiological, ecological, and evolutionary studies but also to enable effective management of threatened reef ecosystems. However, traditional hypotheses that delineate coral species based on morphological traits from the coral skeleton are frequently at odds with tree-based molecular approaches. Additionally, a dearth of species-levelmolecular markers has made species delimitation particularly challenging in speciesrich coral genera, leading to the widespread assumption that interspecific hybridization might be responsible for this apparent conundrum. Here, we used three lines of evidence—morphology, breeding trials, and molecular approaches—to identify species boundaries in a group of ecologically important tabular Acropora corals. In contrast to previous studies, our morphological analysis yielded groups thatwere congruent with experimental crosses aswell as with coalescent-based and allele sharing-based multilocus approaches to species delimitation. Our results suggest that species of the genus Acropora are reproductively isolated and independently evolving units that can be distinguished morphologically. These findings not only pave the way for a taxonomic revision of coral species but also outline an approach that can provide a solid basis to address species delimitation and provide conservation support to a wide variety of keystone organisms.

Mesophotic coral ecosystems (MCEs) are light-dependent communities occurring at depths of 30–150 m. They have been suggested to serve as refuge against thermal stress during heat waves for some coral species. Recent studies on MCEs have revealed a high diversity of communities, some unique, and that these ecosystems are far from being immune to anthropogenic threats. However, the depths at which these ecosystems are found make their exploration and study challenging. Consequently, most suitable environments for MCEs remain unexplored. To facilitate the detection and characterization of MCEs, we improved the methodology for mesophotic scleractinian survey by environmental DNA (eDNA) metabarcoding analysis using seawater collected by underwater mini-Remote Operated Vehicle (mini-ROV). We tested this improved approach at upper mesophotic sites in Okinawa, Japan, with different corals dominating the communities (i.e., Alveopora -dominated, Seriatopora -dominated, and Acropora -dominated communities). Despite the proximity of the different sites, our eDNA metabarcoding analyses detected the dominant coral genera specific to each site. In addition, this study detected numerous other genera present at these sites, including Acropora , Pachyseris, Galaxea, Lobophyllia, Montipora, Pocillopora, Porites, and others. Therefore, this study might support a new technical gate for comprehensive survey of MCEs using eDNA samples collected by underwater mini-ROV, although further technical improvement is required for quantitative estimation.

The impact of ocean warming on coral larvae survival and dispersal is investigated using a dynamic model. The authors find that globally most reefs will experience large increases in the local retention of larvae, which make populations more responsive to local conservation efforts. However, increased larvae retention will also weaken connectivity between populations, which may affect recovery if a local population is severely disturbed. Climate change will alter many aspects of the ecology of organisms, including dispersal patterns and population connectivity 1 . Understanding these changes is essential to predict future species distributions, estimate potential for adaptation, and design effective networks of protected areas 2 . In marine environments, dispersal is often accomplished by larvae. At higher temperatures, larvae develop faster 3 , 4 , 5 , but suffer higher mortality 4 , 5 , 6 , making the effect of temperature on dispersal difficult to predict. Here, we experimentally calibrate the effect of temperature on larval survival and settlement in a dynamic model of coral dispersal. Our findings imply that most reefs globally will experience several-fold increases in local retention of larvae due to ocean warming. This increase will be particularly pronounced for reefs with mean water residence times comparable to the time required for species to become competent to settle. Higher local retention rates strengthen the link between abundance and recruitment at the reef scale, suggesting that populations will be more responsive to local conservation actions. Higher rates of local retention and mortality will weaken connectivity between populations, and thus potentially retard recovery following severe disturbances that substantially deplete local populations. Conversely, on isolated reefs that are dependent on replenishment from local broodstock, increases in local retention may hasten recovery.

Reef coral assemblages are highly dynamic and subject to repeated disturbances, which are predicted to increase in response to climate change. Consequently there is an urgent need to improve our understanding of the mechanisms underlying different recovery scenarios. Recent work has demonstrated that reef structural complexity can facilitate coral recovery, but the mechanism remains unclear. Similarly, experiments suggest that coral larvae can distinguish between the water from healthy and degraded reefs, however, whether or not they can use these cues to navigate to healthy reefs is an open question. Here, we use a meta-analytic approach to document that coral larval swimming speeds are orders of magnitude lower than measurements of water flow both on and off reefs. Therefore, the ability of coral larvae to navigate to reefs while in the open-ocean, or to settlement sites while on reefs is extremely limited. We then show experimentally that turbulence generated by fine scale structure is required to deliver larvae to the substratum even in conditions mimicking calm back-reef flow environments. We conclude that structural complexity at a number of scales assists coral recovery by facilitating both the delivery of coral larvae to the substratum and settlement.

Mesophotic coral ecosystems (below 30–40 m depth) host a large diversity of zooxanthellate coral communities and may play an important role in the ecology and conservation of coral reefs. Investigating the reproductive biology of mesophotic corals is important to understand their life history traits. Despite an increase in research on mesophotic corals in the last decade, their reproductive biology is still poorly understood. Here, gametogenesis and fecundity of the Indo-Pacific mesophotic coral , Acropora tenella , were examined in an upper mesophotic reef (40 m depth) in Okinawa, Japan for the first time. Acropora tenella is a hermaphrodite with a single annual gametogenic cycle, and both oogenesis and spermatogenesis occurring for 11–12 and 5–6 months, respectively. Timing of spawning of this species was similar to other shallow Acropora spp. in the region. However, colonies had longer gametogenic cycles and less synchronous gamete maturation compared to shallow acroporids with spawning extended over consecutive months. Both the polyp fecundity (number of eggs per polyp) and gonad index (defined as the number of eggs per square centimeter) of A. tenella were lower than most acroporids. Our findings contribute to understanding of the life history of corals on mesophotic reefs and suggest that the reproductive biology of upper mesophotic corals is similar to that of shallow-water corals.

Many corals establish symbiosis with Symbiodiniaceae cells from surrounding environments, but very few Symbiodiniaceae cells exist in the water column. Given that the N-acetyl-d-glucosamine-binding lectin ActL attracts Symbiodiniaceae cells, we hypothesized that corals must attract Symbiodiniaceae cells using ActL to acquire them. Anti-ActL antibody inhibited acquisition of Symbiodiniaceae cells, and rearing seawater for juvenile Acropora tenuis contained ActL, suggesting that juvenile A. tenuis discharge ActL to attract these cells. Among eight Symbiodiniaceae cultured strains, ActL attracted NBRC102920 (Symbiodinium tridacnidorum) most strongly followed by CS-161 (Symbiodinium tridacnidorum), CCMP2556 (Durusdinium trenchii), and CCMP1633 (Breviolum sp.); however, it did not attract GTP-A6-Sy (Symbiodinium natans), CCMP421 (Effrenium voratum), FKM0207 (Fugacium sp.), and CS-156 (Fugacium sp.). Juvenile polyps of A. tenuis acquired limited Symbiodiniaceae cell strains, and the number of acquired Symbiodiniaceae cells in a polyp also differed from each other. The number of Symbiodiniaceae cells acquired by juvenile polyps of A. tenuis was correlated with the ActL chemotactic activity. Thus, ActL could be used to attract select Symbiodiniaceae cells and help Symbiodiniaceae cell acquisition in juvenile polyps of A. tenuis, facilitating establishment of symbiosis between A. tenuis and Symbiodiniaceae cells.

Ocean warming is a major threat to coral reefs, leading to an increasing frequency and amplitude of coral bleaching events, where the coral and its algal symbiont associations breakdown. Long-term change and resilience of a symbiont community in coral juveniles is thought to be one of the most important aspects for determining thermal tolerance of the coral holobionts; however, despite its importance, they are not well documented in both under elevated temperature and even under natural condition. Here we investigated changes in symbiont communities in juveniles of the coral Acropora tenuis under controlled heat stress conditions (30 °C, 31/32 °C) and natural variations in seawater temperatures (19–30 °C) for up to four months. Compared with the ambient temperature conditions, coral survival rates were higher when exposed to 30 °C, but survival rates decreased when exposed to 31/32 °C. Symbiodinium types A3, C1, and D1-4 were detected in the juveniles under all thermal conditions; however, in higher water temperatures (31/32 °C), both the prevalence of D1-4 Symbiodinium and the number of juveniles harboring only this type of symbiont increased after two to four months later. In contrast, colonies at lower temperatures (30 °C and ambient) harbored multiple clades of symbionts over the same experimental period. These results highlight the flexibility of the coral– Symbiodinium symbiosis for juvenile A. tenuis under variable thermal conditions. In particular, the benefit of the preferential association with type D1-4 can be considered as a response when under heat-stress conditions, and that could help corals to cope with ocean warming.