Symbiosis regulation in a facultatively symbiotic temperate coral: zooxanthellae division and expulsion

Zooxanthellae mitotic index (MI) and expulsion rates were measured in the facultatively symbiotic scleractinian Astrangia poculata during winter and summer off the southern New England coast, USA. While MI was significantly higher in summer than in winter, mean expulsion rates were comparable between seasons. Corals therefore appear to allow increases in symbiont density when symbiosis is advantageous during the warm season, followed by a net reduction during the cold season when zooxanthellae may draw resources from the coral. Given previous reports that photosynthesis in A. poculata symbionts does not occur below approximately 6°C, considerable zooxanthellae division at 3°C and in darkness suggests that zooxanthellae are heterotrophic at low seasonal temperatures. Finally, examination of expulsion as a function of zooxanthellae density revealed that corals with very low zooxanthellae densities export a significantly greater proportion of their symbionts, apparently allowing them to persist in a stable azooxanthellate state.     

Inferred calcification rate of a temperate azooxanthellate caryophylliid coral along a wide latitudinal gradient

Correlations between environmental parameters (depth temperature and solar radiation) and growth parameters (bulk skeletal density, linear extension rate and net calcification rate) of the solitary azooxanthellate coral, Caryophyllia inornata, were investigated along an 8° latitudinal gradient on the western Italian coasts. Net calcification rate correlated positively with both bulk skeletal density and linear extension rate, showing that C. inornata allocates calcification resources evenly to thickening the skeleton and increasing linear growth. Overall, the three growth parameters did not follow gradients in the two environmental parameters, showing a different trend compared to most studies on zooxanthellate corals. However, the results are in agreement with the only previous analysis of an azooxanthellate coral, Leptopsammia pruvoti, studied along the same latitudinal gradient. In a comparison of the response to temperature of all Mediterranean species whose growth has been investigated to date, azooxanthellate corals were more tolerant to temperature increases than zooxanthellate corals.     

Large fluctuations but constant mean temperatures allow corals to persist in intertidal rock pools on the east coast of South Africa

Intertidal corals have been under-studied yet they provide scope for understanding adaptation and acclimatisation of corals to marginal conditions. Corals in intertidal rock pools along the east coast of South Africa withstand large temperature fluctuations, and marginal conditions for survival and growth. Four sites along the KwaZulu-Natal (KZN) coastline were sampled to determine latitudinal differences in coral communities, from 27°S to 31°S. Water temperature of rock pools at each site was monitored to see if temperature determined coral diversity in intertidal pools. Sixteen coral species were present in rock pools overall. Each of three sites in northern and central KZN hosted 12 coral taxa whereas only six taxa occurred at the most southern site. Anomastrea irregularis was the most abundant species at all sites, followed by Pocillopora verrucosa and P. damicornis. Unexpectedly, rock pool temperatures did not show a trend with latitude and thus cannot explain this decline in coral diversity. Temperatures in isolated rock pools showed large summer day time fluctuations of more than 10 °C at spring tide. However, temperatures drop substantially at high tide, lowering the mean rock pool temperature and possibly allowing these coral communities to persist in the marginal conditions of rock pools in South Africa.     

Gametogenesis and fecundity of <Emphasis Type="Italic">Acropora tenella</Emphasis> (Brook 1892) in a mesophotic coral ecosystem in Okinawa,Japan

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.     

Heterotrophy mitigates the response of the temperate coral Oculina arbuscula to temperature stress

Anthropogenic increases in atmospheric carbon dioxide concentration have caused global average sea surface temperature (SST) to increase by approximately 0.11°C per decade between 1971 and 2010 – a trend that is projected to continue through the 21st century. A multitude of research studies have demonstrated that increased SSTs compromise the coral holobiont (cnidarian host and its symbiotic algae) by reducing both host calcification and symbiont density, among other variables. However, we still do not fully understand the role of heterotrophy in the response of the coral holobiont to elevated temperature, particularly for temperate corals. Here, we conducted a pair of independent experiments to investigate the influence of heterotrophy on the response of the temperate scleractinian coral Oculina arbuscula to thermal stress. Colonies of O. arbuscula from Radio Island, North Carolina, were exposed to four feeding treatments (zero, low, moderate, and high concentrations of newly hatched Artemia sp. nauplii) across two independent temperature experiments (average annual SST (20°C) and average summer temperature (28°C) for the interval 2005–2012) to quantify the effects of heterotrophy on coral skeletal growth and symbiont density. Results suggest that heterotrophy mitigated both reduced skeletal growth and decreased symbiont density observed for unfed corals reared at 28°C. This study highlights the importance of heterotrophy in maintaining coral holobiont fitness under thermal stress and has important implications for the interpretation of coral response to climate change.     

Recurrent partial mortality events in winter shape the dynamics of the zooxanthellate coral <Emphasis Type="Italic">Oculina patagonica</Emphasis> at high latitude in the Mediterranean

Global warming has many biological effects on corals and plays a central role in the regression of tropical coral reefs; therefore, there is an urgent need to understand how some coral species have adapted to environmental conditions at higher latitudes. We examined the effects of temperature and light on the growth of the zooxanthellate coral Oculina patagonica (Scleractinia, Oculinidae) at the northern limit of its distribution in the eastern Iberian Peninsula (western Mediterranean) by transplanting colonies onto plates and excluding them from space competition over a ~4-yr period. Each year, most of the colonies (~70%) exhibited denuded skeletons with isolated polyps persisting on approximately half of the coral surface area. These recurrent episodes of partial coral mortality occurred in winter, and their severity appeared to be related to colony exposure to cold but not to light. Although O. patagonica exhibited high resistance to stress, coral linear extension did not resume until the coenosarc regenerated. The resumption of linear extension was related to the dissociation of the polyps from the coenosarc and the outstanding regenerative capacity of this species (10.3 mm² d^?1). These biological characteristics allow the species to survive at high latitudes. However, the recurrent and severe pattern of denuded skeletons greatly affects the dynamics of the species and may constrain population growth at high latitudes in the Mediterranean.     

Latitudinal variation in coral communities in eastern Australia: a qualitative biophysical model of factors regulating coral reefs

The processes underlying the distributional limits of both corals and coral reefs can be elucidated by examining coral communities at high latitudes. Coral-dominated communities in eastern Australia cover a latitudinal range of >2,500 km, from the northern Great Barrier Reef (11°S) to South West Rocks (31.5°S). Patterns of coral species richness from 11 locations showed a clear separation between the Great Barrier Reef and subtropical sites, with a further abrupt change at around 31°S. Differences in community structure between the Great Barrier Reef and more southern sites were mainly attributable to higher cover of massive corals, branching Acropora, dead coral and coralline algae on the Great Barrier Reef, and higher cover of macroalgae and bare rock at more southern sites. The absence of some major reef-building taxa (i.e., staghorn Acropora and massive Porites) from most subtropical sites coincided with the loss of reef accretion capacity. Despite high cover of hard corals in communities at up to 31°S, only Lord Howe Island contained areas of reef accretion south of the Great Barrier Reef. Factors that have been hypothesized to account for latitudinal changes in coral community structure include water temperature, aragonite saturation, light availability, currents and larval dispersal, competition between corals and other biota including macroalgae, reduced coral growth rates, and failure of coral reproduction or recruitment. These factors do not operate independently of each other, and they interact in complex ways.     

Distribution and Abundance of Fauna on Living Tissues of Two Brazilian Hermatypic Corals (Mussismilia hispida (Verril 1902) and Siderastrea stellata Verril, 1868)

This study describes the distribution and abundance patterns of the associate fauna on the living surface of the corals Siderastrea stellata Verril, 1868 and Mussismilia hispida (Verril 1902) using a non-destructive method, on the northern coast of Rio de Janeiro State. For each coral species, infestation density and proportions of infested colonies, colonies attached and unattached to the substrate were estimated. A total of 474 colonies of S. stellata and 452 colonies of M. hispida were examined. The barnacle Ceratoconcha floridana (Pilsbry, 1931) was the dominant coral associate found, followed by gall-crabs of the family Cryptochiridae Paulson, 1875 and the bivalve Lithophaga bisulcata (d’Orbigny, 1842). Both coral species presented similar patterns of infestation dominance. S. stellata colonies were more commonly infested and showed a greater mean infestation density of 0.62 ind/cm² at Armação dos Búzios, whereas M. hispida colonies had infestation densities of only 0.20 ind/cm². Infestation density does not appear to impact negatively on corals of Armação dos Búzios. A clear negative relationship between the number of associates in the coral colony and coral size was found. Evidently abundance and frequency of occurrence of associated fauna is highly related to coral community structure and composition and the results highlight the importance of local scale studies.     

Mechanisms of interaction between macroalgae and scleractinians on a coral reef in Jamaica

After several decades of disturbance, many coral reefs in the Caribbean are dominated by macroalgae. One process affecting this transition is coral-macroalgal competition, yet few studies have addressed the mechanisms involved. In this study, we investigated competition between the tall and bushy macroalga Sargassum hystrix (J. Agardh) and the branching coral Porites porites (Pallas) on a shallow reef in Jamaica. Experiments were designed to expose coral branches to different treatments to test the role of shading and abrasion by Sargassum on coral growth and polyp expansion. Corals exposed to Sargassum grew significantly more slowly (80% reduction) than controls, but this effect was absent when corals were caged to prevent physical contact with macroalgae. Light levels were reduced in both the algal and cage treatments, but shading apparently had little effect on the growth of corals in cages. Short-term measurements of integrated net water flow did not detect variation among treatments. In algal-mimic treatments, where clear plastic strips could touch but not shade the corals, growth rates were 25% lower than controls, but this effect was not statistically significant. Thus, the growth of corals in contact with Sargassum was reduced by abrasion and, to a lesser extent, by factors unique to living macroalgae. Analysis of polyp expansion showed that polyps were more frequently retracted when corals were in contact with macroalgae or algal-mimics compared to controls or cage treatment; the frequency of polyp contraction was correlated positively with growth. Together, these results suggest that abrasion-mediated polyp retraction is one of the primary mechanisms of competition utilized by tall (ca. 17 cm) macroalgae against scleractinian corals.     

Optimal thermal conditions for corals extend poleward with oceanic warming

Aim

The capacity for poleward range expansions beyond the tropics in corals hinges on ecophysiological constraints and resulting responses to climatic variability. We aimed to determine how future warming will affect coral habitat suitability at the poleward range edges of these foundational species in the Northwest Pacific.

Location

Northwest Pacific.

Methods

We generated models integrating thermal physiological constraints of corals adapted to extreme seasonality in Hong Kong, specifically the minimum annual temperature and the proportion of time annually spent at seasonal extremes. With these models, we projected habitat suitability for five coral species under current and future climatic conditions across the Northwest Pacific.

Results

Climate model projections reveal an easing of thermal constraints on the leading-edge of coral ecophysiological limits with an expansion of thermally suitable habitat poleward by 2°–7° in latitude depending on the coral species and model considered. We also highlight a potential divergence of present and future thermal regimes that may lead to a mismatch in suitability for corals currently inhabiting high latitude reefs.

Main Conclusions

Understanding the thermal constraints on coral distributions and defining the potential range of corals under climate change is critical for adaptive management that focuses on coral conservation and ensuring ecosystem function of existing subtropical and temperate ecosystems.     

Composition of the early Oligocene ocean from coral stable isotope and elemental chemistry

A sectioned and polished specimen of the coral Archohelia vicksburgensis from the early Oligocene Byram Formation (～30 Ma) near Vicksburg, Mississippi, reveals 12 prominent annual growth bands. Stable oxygen isotopic compositions of 77 growth‐band‐parallel microsamples of original aragonite exhibit well‐constrained fluctuations that range between ?2.0 and ?4.8. Variation in δ¹⁸O of coral carbonate reflects seasonal variation in temperature ranging from 12 to 24 °C about a mean of 18 °C. These values are consistent with those derived from a bivalve and a fish otolith from the same unit, each using independently derived palaeotemperature equations. Mg/Ca and Sr/Ca ratios were determined for 40 additional samples spanning five of the 12 annual bands. Palaeotemperatures calculated using elemental‐ratio thermometers calibrated on modern corals are consistently lower; mean temperature from Mg/Ca ratios are 12.5 ± 1 °C while those from Sr/Ca are 5.8 ± 2.2 °C. Assuming that δ¹⁸O‐derived temperatures are correct, relationships between temperature and elemental ratio for corals growing in today's ocean can be used to estimate Oligocene palaeoseawater Mg/Ca and Sr/Ca ratios. Calculations indicate that early Oligocene seawater Mg/Ca was ～81% (4.2 mol mol^?1) and Sr/Ca ～109% (9.9 mmol mol^?1) of modern values. Oligocene seawater with this degree of Mg depletion and Sr enrichment is in good agreement with that expected during the Palaeogene transition from ‘calcite’ to ‘aragonite’ seas. Lower Oligocene Mg/Ca probably reflects a decrease toward the present day in sea‐floor hydrothermal activity and concomitant decrease in scavenging of magnesium from seawater. Elevated Sr/Ca ratio may record lesser amounts of Oligocene aragonite precipitation and a correspondingly lower flux of strontium into the sedimentary carbonate reservoir than today.     

Mesophotic communities of the insular shelf at Tutuila, American Samoa

An investigation into the insular shelf and submerged banks surrounding Tutuila, American Samoa, was conducted using a towed camera system. Surveys confirmed the presence of zooxanthellate scleractinian coral communities at mesophotic depths (30–110 m). Quantification of video data, separated into 10-m-depth intervals, yielded a vertical, landward-to-seaward and horizontal distribution of benthic assemblages. Hard substrata composed a majority of bottom cover in shallow water, whereas unconsolidated sediments dominated the deep insular shelf and outer reef slopes. Scleractinian coral cover was highest atop mid-shelf patch reefs and on the submerged bank tops in depths of 30–50 m. Macroalgal cover was highest near shore and on reef slopes approaching the bank tops at 50–60 m. Percent cover of scleractinian coral colony morphology revealed a number of trends. Encrusting corals belonging to the genus Montipora were most abundant at shallow depths with cover gradually decreasing as depth increased. Massive corals, such as Porites spp., displayed a similar trend. Percent cover values of plate-like corals formed a normal distribution, with the highest cover observed in the 60–70 m depth range. Shallow plate-like corals belonged mostly to the genus Acropora and appeared to be significantly prevalent on the northeastern and eastern banks. Deeper plate-like corals on the reef slopes were dominated by Leptoseris, Pachyseris, or Montipora genera. Branching coral cover was high in the 80–110 m depth range. Columnar and free-living corals were also occasionally observed from 40–70 m.     

Interaction between benthic algae (Lyngbya bouillonii, Dictyota dichotoma) and scleractinian coral Porites lutea in direct contact

Competition between the massive scleractinian coral Porites lutea and two benthic algal species, thin-filamentous blue-green Lyngbya bouillonii (Cyanophyta) commonly observed growing as mats and fleshy brown Dictyota dichotoma (Phaeophyta), was investigated. Experiments were designed to expose coral fragments to different treatments to test the role of abrasion, shading and allelopathy by Lyngbya and Dictyota on coral growth and physiology in direct physical contact. The growth rates of coral fragments were significantly lower in both the algal/coral and the net control (only plastic net touched the corals) treatments than in the manipulation control (contact with algae and plastic net was prevented), demonstrating the importance of abrasion in Porites-Lyngbya and Porites-Dictyota interactions. Furthermore, coral fragments exposed to Lyngbya grew significantly slowly than net controls, but this effect was not statistically significant for P. lutea maintained in contact with Dictyota. Light levels were reduced equally in the algal/coral and shading mimic (plastic net touched the corals shaded with neutral-density filters) treatments. However, there were no significant differences in the growth rates between the shading mimic and the net control treatments, suggesting that shading had no measurable effect on coral growth. Thus, the growth of P. lutea in contact with Dictyota was reduced by abrasion whereas in direct contact with L. bouillonii, abrasion was supplemented by additional factors unique to Lyngbya in mediating coral-algal competition. Physical contact with L. bouillonii induced a significant reduction in photochemical efficiency (F_v / F_m) of PSII and chlorophyll concentration of in hospite zooxanthellae of P. lutea fragments, as well as a decrease of the symbiotic dinoflagellate density. Analysis of the growth rate and F_v / F_m of the investigated algae revealed a significant reduction in both parameters for D. dichotoma but not for L. bouillonii when in direct contact with P. lutea fragments. Thus, the competitive inhibition by the coral P. lutea and the brown alga D. dichotoma was mutual, while L. bouillonii acted as a one-sided inhibitor for scleractinian corals inducing bleaching and severe damage of live coral tissue. The fact that mats-forming blue-green alga L. bouillonii acts as a poison for scleractinian corals and is able to kill live coral tissue is reported for the first time. Allelochemical mechanism of the effect on live coral tissue by this alga is suggested. Possible mechanisms of competitive interactions for substrate between the coral polyps of scleractinians and algal species investigated are discussed.     

A corrosive concoction: The combined effects of ocean warming and acidification on the early growth of a stony coral are multiplicative

Survival of coral planulae, and the successful settlement and healthy growth of primary polyps are critical for the dispersal of scleractinian corals and hence the recovery of degraded coral reefs. It is therefore important to explore how the warmer and more acidic oceanic conditions predicted for the future could affect these processes. This study used controlled culture to investigate the effects of a 1 °C increase in temperature and a 0.2-0.25 unit decrease in pH on the settlement and survival of planulae and the growth of primary polyps in the Tropical Eastern Pacific coral Porites panamensis. We found that primary polyp growth was reduced only marginally by more acidic seawater but the combined effect of high temperature and lowered pH caused a significant reduction in growth of primary polyps by almost a third. Elevated temperature was found to significantly reduce the amount of zooxanthellae in primary polyps, and when combined with lowered pH resulted in a significant reduction in biomass of primary polyps. However, survival and settlement of planula larvae were unaffected by increased temperature, lowered acidity or the combination of both. These results indicate that in future scenarios of increased temperature and oceanic acidity coral planulae will be able to disperse and settle successfully but primary polyp growth may be hampered. The recovery of reefs may therefore be impeded by global change even if local stressors are curbed and sufficient sources of planulae are available.     

Temperature effects on calcification rate and skeletal deposition in the temperate coral, Plesiastrea versipora (Lamarck)

The geographic range of the coral, Plesiastrea versipora (Lamarck, 1816), extends into temperate waters outside the southern limit for hermatypic corals. In the present study, calcification in Plesiastrea collected from Port Phillip Bay, Victoria was examined over the coral's normal annual temperature range (10-21 °C), which is well below the normal optimum for coral calcification in tropical corals (25-28 °C). Calcification rate in Plesiastrea was considerably lower than in reef corals, but showed a similar pattern in temperature responses, with a trend towards higher rates at ∼18 °C. The light/dark calcification ratio was markedly lower than that in tropical corals. Autoradiography showed that calcification occurred primarily by deposition of calcium carbonate at the upper surfaces of the septo-costae. Scanning electron microscopy (SEM) showed that skeletal deposition in Plesiastrea had a temperature-dependent diel pattern. In the light, calcium carbonate was deposited as small spheroidal crystals and, at higher temperatures, small needle-shaped crystals. In the dark, calcium carbonate deposition appeared to be in the form of an amorphous sheet-like cementation. Compared with other scleractinian corals, calcification rate in Plesiastrea was relatively slow and showed different patterns of skeletal deposition.     

A Novel Method for Coral Explant Culture and Micropropagation

We describe here a method for the micropropagation of coral that creates progeny from tissue explants derived from a single polyp or colonial corals. Coral tissue explants of various sizes (0.5?C2.5?mm in diameter) were manually microdissected from the solitary coral Fungia granulosa. Explants could be maintained in an undeveloped state or induced to develop into polyps by manipulating environmental parameters such as light and temperature regimes, as well as substrate type. Fully developed polyps were able to be maintained for a long-term in a closed sea water system. Further, we demonstrate that mature explants are also amenable to this technique with the micropropagation of second-generation explants and their development into mature polyps. We thereby experimentally have established coral clonal lines that maintain their ability to differentiate without the need for chemical induction or genetic manipulation. The versatility of this method is also demonstrated through its application to two other coral species, the colonial corals Oculina patigonica and Favia favus.     

Importance of live coral habitat for reef fishes

Live corals are the key habitat forming organisms on coral reefs, contributing to both biological and physical structure. Understanding the importance of corals for reef fishes is, however, restricted to a few key families of fishes, whereas it is likely that a vast number of fish species will be adversely affected by the loss of live corals. This study used data from published literature together with independent field based surveys to quantify the range of reef fish species that use live coral habitats. A total of 320 species from 39 families use live coral habitats, accounting for approximately 8 % of all reef fishes. Many of the fishes reported to use live corals are from the families Pomacentridae (68 spp.) and Gobiidae (44 spp.) and most (66 %) are either planktivores or omnivores. 126 species of fish associate with corals as juveniles, although many of these fishes have no apparent affiliation with coral as adults, suggesting an ontogenetic shift in coral reliance. Collectively, reef fishes have been reported to use at least 93 species of coral, mainly from the genus Acropora and Porities and associate predominantly with branching growth forms. Some fish associate with a single coral species, whilst others can be found on more than 20 different species of coral indicating there is considerable variation in habitat specialisation among coral associated fish species. The large number of fishes that rely on coral highlights that habitat degradation and coral loss will have significant consequences for biodiversity and productivity of reef fish assemblages.     

Growth Dynamics of the Threatened Caribbean Staghorn Coral Acropora cervicornis: Influence of Host Genotype,Symbiont Identity,Colony Size,and Environmental Setting

Background

The drastic decline in the abundance of Caribbean acroporid corals (Acropora cervicornis, A. palmata) has prompted the listing of this genus as threatened as well as the development of a regional propagation and restoration program. Using in situ underwater nurseries, we documented the influence of coral genotype and symbiont identity, colony size, and propagation method on the growth and branching patterns of staghorn corals in Florida and the Dominican Republic.

Methodology/Principal Findings

Individual tracking of> 1700 nursery-grown staghorn fragments and colonies from 37 distinct genotypes (identified using microsatellites) in Florida and the Dominican Republic revealed a significant positive relationship between size and growth, but a decreasing rate of productivity with increasing size. Pruning vigor (enhanced growth after fragmentation) was documented even in colonies that lost 95% of their coral tissue/skeleton, indicating that high productivity can be maintained within nurseries by sequentially fragmenting corals. A significant effect of coral genotype was documented for corals grown in a common-garden setting, with fast-growing genotypes growing up to an order of magnitude faster than slow-growing genotypes. Algal-symbiont identity established using qPCR techniques showed that clade A (likely Symbiodinium A3) was the dominant symbiont type for all coral genotypes, except for one coral genotype in the DR and two in Florida that were dominated by clade C, with A- and C-dominated genotypes having similar growth rates.

Conclusion/Significance

The threatened Caribbean staghorn coral is capable of extremely fast growth, with annual productivity rates exceeding 5 cm of new coral produced for every cm of existing coral. This species benefits from high fragment survivorship coupled by the pruning vigor experienced by the parent colonies after fragmentation. These life-history characteristics make A. cervicornis a successful candidate nursery species and provide optimism for the potential role that active propagation can play in the recovery of this keystone species.     

Calcification process dynamics in coral primary polyps as observed using a calcein incubation method

Calcification processes are largely unknown in scleractinian corals. In this study, live confocal imaging was used to elucidate the spatiotemporal dynamics of the calcification process in aposymbiotic primary polyps of the coral species Acropora digitifera. The fluorophore calcein was used as a calcium deposition marker and a visible indicator of extracellular fluid distribution at the tissue-skeleton interface (subcalicoblastic medium, SCM) in primary polyp tissues. Under continuous incubation in calcein-containing seawater, initial crystallization and skeletal growth were visualized among the calicoblastic cells in live primary polyp tissues. Additionally, the distribution of calcein-stained SCM and contraction movements of the pockets of SCM were captured at intervals of a few minutes. Our experimental system provided several new insights into coral calcification, particularly as a first step in monitoring the relationship between cellular dynamics and calcification in vivo. Our study suggests that coral calcification initiates at intercellular spaces, a finding that may contribute to the general understanding of coral calcification processes.     

Thermal refugia against coral bleaching throughout the northern Red Sea

Tropical reefs have been impacted by thermal anomalies caused by global warming that induced coral bleaching and mortality events globally. However, there have only been very few recordings of bleaching within the Red Sea despite covering a latitudinal range of 15° and consequently it has been considered a region that is less sensitive to thermal anomalies. We therefore examined historical patterns of sea surface temperature (SST) and associated anomalies (1982–2012) and compared warming trends with a unique compilation of corresponding coral bleaching records from throughout the region. These data indicated that the northern Red Sea has not experienced mass bleaching despite intensive Degree Heating Weeks (DHW) of >15°C‐weeks. Severe bleaching was restricted to the central and southern Red Sea where DHWs have been more frequent, but far less intense (DHWs <4°C‐weeks). A similar pattern was observed during the 2015–2016 El Niño event during which time corals in the northern Red Sea did not bleach despite high thermal stress (i.e. DHWs >8°C‐weeks), and bleaching was restricted to the central and southern Red Sea despite the lower thermal stress (DHWs < 8°C‐weeks). Heat stress assays carried out in the northern (Hurghada) and central (Thuwal) Red Sea on four key reef‐building species confirmed different regional thermal susceptibility, and that central Red Sea corals are more sensitive to thermal anomalies as compared to those from the north. Together, our data demonstrate that corals in the northern Red Sea have a much higher heat tolerance than their prevailing temperature regime would suggest. In contrast, corals from the central Red Sea are close to their thermal limits, which closely match the maximum annual water temperatures. The northern Red Sea harbours reef‐building corals that live well below their bleaching thresholds and thus we propose that the region represents a thermal refuge of global importance.