Larval precompetency and settlement behaviour in 25 Indo-Pacific coral species

Knowledge of coral larval precompetency periods and maximum competency windows is fundamental to understanding coral population dynamics, informing biogeography and connectivity patterns, and predicting reef recovery following disturbances. Yet for many species, estimates of these early-life history metrics are scarce and vary widely. Furthermore, settlement cues for many taxa are not known despite consequences to habitat selection. Here we performed a comprehensive experimental time-series investigation of larval settlement behaviour, for 25 Indo-Pacific broadcast-spawning species. To investigate the duration of precompetency, improve predictions of the competency windows, and compare settlement responses within and amongst species, we completed replicated and repeated 24-hour assays that exposed larvae to five common settlement cues. Our study revealed that larval competency in some broadcast-spawning species begins as early as two days post fertilization, but that the precompetency period varies within and between species from about two to six days, with consequences for local retention and population connectivity. We also found that larvae of some species are competent to settle beyond 70 days old and display complex temporal settlement behaviour, challenging the assumption that competency gradually wanes over time and adding to the evidence that larval longevity can support genetic connectivity and long-distance dispersal. Using these data, we grouped coral taxa by short, mid and long precompetency periods, and identified their preferred settlement cues. Taken together, these results inform our understanding of larval dynamics across a broad range of coral species and can be applied to investigations of population dynamics, connectivity, and reef recovery.

The population sizes and global extinction risk of reef-building coral species at biogeographic scales

Knowledge of a species' abundance is critically important for assessing its risk of extinction, but for the vast majority of wild animal and plant species such data are scarce at biogeographic scales. Here, we estimate the total number of reef-building corals and the population sizes of more than 300 individual species on reefs spanning the Pacific Ocean biodiversity gradient, from Indonesia to French Polynesia. Our analysis suggests that approximately half a trillion corals (0.3 × 10¹²-0.8 × 10¹²) inhabit these coral reefs, similar to the number of trees in the Amazon. Two-thirds of the examined species have population sizes exceeding 100 million colonies, and one-fifth of the species even have population sizes greater than 1 billion colonies. Our findings suggest that, while local depletions pose imminent threats that can have ecologically devastating impacts to coral reefs, the global extinction risk of most coral species is lower than previously estimated.

Thermal performance of scleractinian corals along a latitudinal gradient on the Great Barrier Reef

Species have evolved different mechanisms to cope with spatial and temporal temperature variability. Species with broad geographical distributions may be thermal generalists that perform well across a broad range of temperatures, or they might contain subpopulations of locally adapted thermal specialists. We quantified the variation in thermal performance of two coral species, Porites cylindrica and Acropora spp., along a latitudinal gradient over which temperature varies by approximately 6°C. Photosynthesis rates, respiration rates, maximum quantum yield and maximum electron transport rates were measured on coral fragments exposed to an acute temperature increase and decrease up to 5°C above and below the local average temperature. Results showed geographical variation in the performance curves of both species at holobiont and symbiont level, but this did not lead to an alignment of the optimal temperature for performance with the average temperature of the local environment, suggesting suboptimal coral performance of these coral populations in summer. Furthermore, symbiont thermal performance generally had an optimum closer to the average environmental temperature than holobiont performance, suggesting that symbionts have a higher capacity for acclimatization than the coral host, and can aid the coral host when temperatures are unfavourable. This article is part of the theme issue 'Physiological diversity, biodiversity patterns and global climate change: testing key hypotheses involving temperature and oxygen'.

Light limitation selects for depth generalists in urbanised reef coral communities

Depth range is an important species trait for coral reef organisms, yet it remains to be quantified and analysed adequately among tropical coral species. Filling this knowledge gap is crucial as the depth limits of corals are related to important environmental factors such as light and temperature. Furthermore, the health and survivorship of corals may be threatened due to warming-induced sea-level rise, particularly for colonies living at the deeper limits of species depth ranges. Here we collected benthic and environmental data along the reef profile to characterise the depth ranges of coral species, and analysed species diversity and community structure in relation to possible depth-related biophysical parameters on the sediment-stressed reefs of Singapore. The results reveal clear environmental covariations with depth, expectedly with light availability showing the most marked decline as depth increases. Live coral cover, species richness and diversity are associated positively and significantly with light, which also structures coral communities along the reef profile more strongly than temperature or sediment levels. Relatedly, we detect species-specific depth distributions with two main strategies observed among coral species: shallow specialists and depth generalists. We suggest that corals in Singapore are unlikely to be impacted by light limitation specifically as sea level rises due to the wider depth range of the deeper species. Our data will inform conservation efforts especially in the selection of sites and depths for coral transplantation.

The origin and evolution of coral species richness in a marine biodiversity hotspot

The Coral Triangle (CT) region of the Indo-Pacific realm harbors an extraordinary number of species, with richness decreasing away from this biodiversity hotspot. Despite multiple competing hypotheses, the dynamics underlying this regional diversity pattern remain poorly understood. Here, we use a time-calibrated evolutionary tree of living reef coral species, their current geographic ranges, and model-based estimates of regional rates of speciation, extinction, and geographic range shifts to show that origination rates within the CT are lower than in surrounding regions, a result inconsistent with the long-standing center of origin hypothesis. Furthermore, endemism of coral species in the CT is low, and the CT endemics are older than relatives found outside this region. Overall, our model results suggest that the high diversity of reef corals in the CT is largely due to range expansions into this region of species that evolved elsewhere. These findings strongly support the notion that geographic range shifts play a critical role in generating species diversity gradients. They also show that preserving the processes that gave rise to the striking diversity of corals in the CT requires protecting not just reefs within the hotspot, but also those in the surrounding areas.

The Coral Trait Database, a curated database of trait information for coral species from the global oceans

Trait-based approaches advance ecological and evolutionary research because traits provide a strong link to an organism's function and fitness. Trait-based research might lead to a deeper understanding of the functions of, and services provided by, ecosystems, thereby improving management, which is vital in the current era of rapid environmental change. Coral reef scientists have long collected trait data for corals; however, these are difficult to access and often under-utilized in addressing large-scale questions. We present the Coral Trait Database initiative that aims to bring together physiological, morphological, ecological, phylogenetic and biogeographic trait information into a single repository. The database houses species- and individual-level data from published field and experimental studies alongside contextual data that provide important framing for analyses. In this data descriptor, we release data for 56 traits for 1547 species, and present a collaborative platform on which other trait data are being actively federated. Our overall goal is for the Coral Trait Database to become an open-source, community-led data clearinghouse that accelerates coral reef research.

The future of evolutionary diversity in reef corals

One-third of the world's reef-building corals are facing heightened extinction risk from climate change and other anthropogenic impacts. Previous studies have shown that such threats are not distributed randomly across the coral tree of life, and future extinctions have the potential to disproportionately reduce the phylogenetic diversity of this group on a global scale. However, the impact of such losses on a regional scale remains poorly known. In this study, we use phylogenetic metrics in conjunction with geographical distributions of living reef coral species to model how extinctions are likely to affect evolutionary diversity across different ecoregions. Based on two measures-phylogenetic diversity and phylogenetic species variability-we highlight regions with the largest losses of evolutionary diversity and hence of potential conservation interest. Notably, the projected loss of evolutionary diversity is relatively low in the most species-rich areas such as the Coral Triangle, while many regions with fewer species stand to lose much larger shares of their diversity. We also suggest that for complex ecosystems like coral reefs it is important to consider changes in phylogenetic species variability; areas with disproportionate declines in this measure should be of concern even if phylogenetic diversity is not as impacted. These findings underscore the importance of integrating evolutionary history into conservation planning for safeguarding the future diversity of coral reefs.

Faunal breaks and species composition of Indo-Pacific corals: the role of plate tectonics, environment and habitat distribution

Species richness gradients are ubiquitous in nature, but the mechanisms that generate and maintain these patterns at macroecological scales remain unresolved. We use a new approach that focuses on overlapping geographical ranges of species to reveal that Indo-Pacific corals are assembled within 11 distinct faunal provinces. Province limits are characterized by co-occurrence of multiple species range boundaries. Unexpectedly, these faunal breaks are poorly predicted by contemporary environmental conditions and the present-day distribution of habitat. Instead, faunal breaks show striking concordance with geological features (tectonic plates and mantle plume tracks). The depth range over which a species occurs, its larval development rate and genus age are important determinants of the likelihood that species will straddle faunal breaks. Our findings indicate that historical processes, habitat heterogeneity and species colonization ability account for more of the present-day biogeographical patterns of corals than explanations based on the contemporary distribution of reefs or environmental conditions.