Coral Reefs最新文献

The hourly to monthly resolution Sr/Ca profiles of modern juvenile giant clam (Tridacna spp.) shells from the northern South China Sea were obtained using ICP-OES, LA-ICP-MS and NanoSIMS. The results showed the variability of Sr/Ca profiles determined by different analytical methods were consistent on monthly time scale. The hourly resolved Sr/Ca determined by NanoSIMS showed pronounced daily cycles which may be associated with the diurnal cycle of the physiology, environment and/or weather condition. By comparison of daily resolution Sr/Ca and contemporaneous instrumental environment data, no robust link was observed between environmental parameters and juvenile Tridacna Sr/Ca, and only weak correlation was found between daily outgoing longwave radiation (OLR) (r = − 0.167, p < 0.01) and Sr/Ca. However, the effective solar radiation cannot explain the variability of nightly Sr/Ca, and some other factors are more likely to influence the Sr/Ca of juvenile Tridacna. Moreover, no consistent correlation was observed between Sr/Ca and growth rate of Tridacna. The mechanism of juvenile Tridacna Sr/Ca is still mysterious, conducting artificial culture experiments seems the best way to unravel the mechanisms behind Sr²⁺ incorporation into Tridacna shells.

Photoreception is essential to coral growth, reproduction, and stress responses. Thus far, opsin-based photoreception and potential photoadaptation in Scleractinian corals remains unclear. This study used natural and light-emitting diode (LED) lighting to investigate how Acropora digitifera, which is adapted to shallow-water environments, responds to day–night conditions. We successfully cloned three opsin genes (Adopsin1, Adopsin2, and Adopsin3). Adopsin1 and Adopsin2 clustered with the Cnidopsins, whereas Adopsin3 clustered with the anthozoan-specific opsin I group. In situ hybridization showed positive signals of these genes in coral endodermal and ectodermal layers. When A. digitifera branches were reared under a day–night cycle with natural light, a day-high and night-low pattern was observed in the transcript levels of Adopsin1 and Adopsin3. Genes related to calcification [plasma membrane calcium transporting ATPase 2 (PMCA)] and oxygen homeostasis regulation [hypoxia-inducible factor 1 alpha (HIF1α)] showed similar patterns. Rearing of branches under a day–night cycle (photoperiod = 12:12, 26.5–29.3 μmol s⁻¹ m⁻²) with red (λ_max = 628 nm), but not blue (464 nm) or green (519 nm) LED lighting led to increases in transcript levels of Adopsin1 and Adopsin3 during photophase. The transcript levels of carbonic anhydrase, PMCA, HIFα, and sodium-glucose cotransporter were significantly higher during photophase than during scotophase. Furthermore, Adopsin3 upregulation occurred within 4 h of exposure to a red LED light at night. These results suggest that A. digitifera can responding to long wavelengths of light, which play a crucial role in the photophysiology of the coral host. The capacity to perceive red light provides advantages in physiological adaptation and ecological niche occupation by A. digitifera in shallow waters.

The capacity for natural coral reef recovery and the effectiveness of active restoration efforts are often contingent upon uncertain and understudied background variables, such as chronic predation for example. In the Maldives, small coral colonies (< 10 cm), primarily from the genera Pocillopora and Acropora (often found recolonising degraded reefs) are frequently predated on by the spiny cushion sea-star (Culcita schmideliana). Incidentally, these same corals (especially Acroporids) are often prioritised in active reef restoration practices. However, the level of risk these corallivores pose on restoration success has not yet been assessed. Here, we aimed to initially document the population densities of C. schmideliana on a degraded reef system in the Maldives (Kunfunadhoo, Baa Atoll). We then assessed their associated predatory effects on coral recruits and transplants, and explored the benefits of C. schmideliana removal on the survival of these corals. Population densities ranged between 1.2 and 3.3 individuals per 100 m², which resulted in high predation rates on coral recruits (4 – 20%) and transplants (11 – 43%). Culcita schmideliana predation accounted for the majority of the documented mortality (85%). Where C. schmideliana were removed, a significant increase in survival for recruits (9% higher) and transplants (24% higher) was shown. Further observations linked a cessation of C. schmideliana removal to a significant rise in predation instances for previously protected corals (up to 52%). Our study therefore highlights the severe impacts of C. schmideliana predation and shows Culcita spp. population management as a viable passive solution for effective reef restoration.

Induction of gonad maturation and synchronized spawning of corals ex situ has been mostly used to propagate corals for restoration, but it also provides a unique opportunity to study the reproductive biology of species. We present, for the first time, the induction of gonad maturation and synchronous spawning of the coral Montastraea cavernosa in a laboratory. This was achieved by mimicking the annual temperature, sun and moon cycles experienced in the northern portion of Florida’s Coral Reef. Similarly to field observations, peak spawning of M. cavernosa colonies in the laboratory occurred 5–10 nights after the full moons of July, August, and/or September, 75–125 min after sunset. This coral species was known as gonochoric, meaning colonies are either females (release eggs) or males (release sperm). Yet, three consecutive years observing the same colonies ex situ revealed that these corals are capable of changing sexes annually, and they can do so in both directions. Spawning observations and histology showed corals shifting from male to female, others shifting from female to male, and several changing sex one year and reverting to their original sex in the following year. The sex change was not related to size, nor likely socially driven. A greater number of colonies was observed to shift from female to male when food provision was reduced. Further studies are required to determine if food availability drives sex change in this species.

Rubble is ubiquitous on coral reefs and can aggregate into fields, forming a significant component of the reef substrate. Rubble fields often remain unconsolidated, with the component rubble pieces subject to movement that is dependent on hydrodynamic forcing, rubble size, shape, and other factors. Settlement of corals to rubble fields has long been assumed, but the dynamic movement of rubble pieces has been presumed to deter settlement and is thought to contribute to high post-settlement mortality. Rubble often forms on coral reefs following severe disturbances, and is predicted to increase under climate change, with the potential to impact settlement and recruitment-dependent recovery processes. Through a series of laboratory and field experiments, we demonstrate that corals from broadcast spawning species on the Great Barrier Reef will settle on unstable substrates, even those in constant motion. We also observed more coral spat on settlement tiles suspended in the water column than those fixed to the reef using a common approach to censusing settlement. Sampling of natural rubble on the reef 50 days after a mass-spawning event confirmed the presence of similar numbers of coral settlers on rubble and on tiles fixed to the reef. These results suggest that rubble fields are places of significant settlement for broadcast spawning corals. Suspended tiles were also surprisingly effective in collecting coral settlers, demonstrating that a change in sampling protocol can produce significant variation in settlement data and strengthening the argument for standardisation of settlement-monitoring protocols, particularly at a time of growing need for reliable metrics. These results also suggest that movement of rubble is not precluding settlement outright, but rather post-settlement processes (i.e. competition, predation, shading or burial by shifting rubble) are limiting recruitment to rubble patches. Consequently, rubble stabilisation may increase the survival of spat that have settled in these environments.

Coral reefs are exposed to various environmental stressors that cause bleaching events, whereby endosymbiotic microalgae (Symbiodiniaceae) disassociate from coral hosts. Bleached corals are compromised and face mortality. The combination of high-light exposure and elevated seawater temperature often lead to coral bleaching. The physiological properties of the Symbiodiniaceae within the coral tissues contribute to the thermal tolerance of the holobiont (the host and all its symbionts). The present study aimed to investigate the effects of light and temperature stress on four Symbiodiniaceae species from three genera with respect to photosynthetic oxygen production and consumption. Under control conditions, the species displayed predominantly low-to-moderate light requirements for photosynthesis with increased photoinhibition at higher photon flux rates. After 30 days of heat acclimation at 32 °C, maximum photosynthetic activity declined in Effrenium voratum, doubled in Fugacium kawagutii, and remained unchanged in Breviolum psygmophilum. In subsequent acute heating assays, species-specific effects on maximum photosynthetic activity were observed. Photosynthesis in all species declined across a temperature gradient between 25 and 39 °C in the acute heating assays; full inhibition occurred at 37 °C in B. psygmophilum and E. voratum and at 39 °C in B. aenigmaticum and F. kawagutii. In contrast, respiration remained largely constant in all species across temperatures. Our data point to species-specific photophysiological traits that lead to different thermal tolerances among Symbiodiniaceae.

Hybridisation is an evolutionary process that generates genetic diversity in organisms. However, the relationship between reproductive features, such as spawning synchronisation and gamete compatibility, and the degree of introgression leading to hybridisation are poorly understood. The reef-building coral Acropora spp. have a complex evolutionary history, and the link between their ecology, life-history traits, and potential to hybridise is disputed. Here, we examined the relationship among the reproductive features involved in the intercrossing of three species, Acropora florida, Acropora gemmifera, and Acropora intermedia, at two sites: Akajima and the Sesoko islands in southern Japan. Although the examined species showed synchronous spawning and high rates of gamete compatibility, spawning synchronisation and gamete compatibility were less strongly associated with high rates of interbreeding among the three species. Model-based genetic clustering and site-pattern frequency-based tests with single nucleotide polymorphisms supported genetic admixture among the three species in each location. Demographic analyses using fastsimcoal implied that the admixture among the three species in each location might have occurred in the past (> 2,000 generations) and recently (< 50 generations). Furthermore, the recent admixture of these three species is potentially associated with heavy bleaching events and population declines. The principal component analysis, structure, and fastsimcoal showed that the extensive admixture of A. intermedia and A. gemmifera on Sesoko Island occurred recently. Therefore, gamete interactions that lead to hybridisation in the field must be clarified. Furthermore, the connectivity between the two locations needs to be identified; however, our results implied that population fluctuations could be associated with introgression.

Metamorphosis is a critical aspect of coral reef fish ecology. This developmental milestone marks changes in form and function that permit successful transition of pelagic larvae to the demersal lifestyle on coral reefs. However, we know very little about the physiological changes that occur during this period, specifically potential changes in energetics associated with swimming. This is critical, as swimming is the mechanism by which pelagic larvae find a suitable reef on which to settle. Coral grouper larvae (Serranidae: Plectropomas leopardus) were collected at night as they came into the vicinity of a fringing reef to settle, and their physiological metamorphosis was characterized. Larvae and 24 h-settled juveniles were exposed to an endurance swimming test at ecologically relevant swimming speeds, and oxygen uptake rates were measured during activity. To describe how aerobic and anaerobic properties of tissues change during metamorphosis, we also measured whole body citrate synthase and lactate dehydrogenase activity, respectively, as well as mitochondrial density in the trunk and pectoral fins. Our approach accurately measures the oxygen uptake rates these life stages need during the recruitment process, with larvae having a 74% higher mass-specific oxygen uptake rate (ṀO₂) than settled juveniles despite swimming at speeds that are only 1.5 body-lengths per second (BLs⁻¹) faster. Citrate synthase activity significantly decreased upon settlement; as larvae had 3.7 times higher activities than juveniles, suggesting that rapid changes in aerobic metabolism of tissues may be an important process during metamorphosis in this species. In contrast, lactate dehydrogenase did not significantly differ upon settlement. These findings highlight some physiological modifications that pelagic coral grouper larvae undertake within 24 h that contribute to successfully settling onto a coral reef.

Mesophotic coral ecosystems are characterised by the presence of photosynthetic scleractinian corals despite the decreasing amounts of light available with depth. To better understand physiological strategies across a broad depth gradient, we studied the biological trait responses of Pocillopora cf. verrucosa from 6 to 60 m depth and Pachyseris “speciosa” spp. from 20 to 90 m depth at four islands of French Polynesia. Specifically, we characterised associated Symbiodiniaceae communities, photophysiological traits (Symbiodiniaceae density and chlorophyll concentrations), micro-morphology and trophic plasticity (autotrophy vs heterotrophy inferred from stable isotopes). Our results showed that both taxa can live at mesophotic depths without significant genetic structuring in their generic Symbiodiniaceae communities, mainly composed of Cladocopium and Durusdinium. Yet, the prevalence of Symbiodiniaceae ITS2 profiles revealed location-based variations that sometimes interact with depth and highlight putative shallow- or depth-tolerant taxa. For both taxa, symbiont density and chlorophyll pigment concentrations increased with increasing depth. We also found a change in their skeletal micro-morphology with an increase in the inter-corallite distance for Pocillopora cf. verrucosa and a decrease in the height of septa for Pachyseris “speciosa” spp. with depth. Finally, we found no isotopic evidence of switching to a more heterotrophic diet as their primary energy source, although host–tissue δ¹³C ratios became more negative with depth in both corals. Overall, our findings show similarity (across the two species) and species-specific strategies (biological trait patterns with increasing depth) underlying the capacity of symbiotic scleractinian corals to live in low-light environments.

When coral cover declines, numeric responses of parrotfish can facilitate top-down control of algae and help reefs recover. Yet many parrotfish are facultative corallivores and we know surprisingly little about how their numeric or functional responses to coral decline modify their interactions with coral prey to shape their impacts on surviving corals. Here, we use benthic and fish surveys conducted in the Florida Keys more than a decade apart to assess how coral communities have changed, and how these changes have impacted parrotfish and their predation rates on corals. We found that disturbances and disease have continued to drive declines in coral cover and changes in coral community composition, but that the parrotfish abundance has not changed. In turn, while parrotfish corallivory has remained relatively constant or even declined for some coral taxa, predation on preferred branching Porites species increased 10% in frequency and, when normalized to live tissue area, > 50% in intensity. These coral-mediated shifts in predation correlated with declines in conspecific cover and are indicative of depensatory predation, which can destabilize trophic interactions and drive prey to low densities or even extinction. While coral reefs cannot recover from disturbances without robust parrotfish populations, our study suggests that parrotfish corallivory has important ramifications for coral community structure and, after prolonged degradation, the ability of some diminished coral populations to recover or persist. In a world where corals bleach annually, understanding the functional responses of corallivorous parrotfish to changes in resource abundance will be increasingly important for effective ecosystem-based management.