The Changing Health of Coral Reefs

Throughout their entire global range coral reefs are in decline. Coral bleaching, macroalgal overgrowth and coral diseases — responses signaling the declining health of coral reefs — have occurred with increasing frequency and intensity in recent decades. Decreased calcification may also be affecting coral reefs over longer time scales. Declines in coral reef health have been attributed to various natural and anthropogenic processes, but assignment of causality has proved problematic. Coral bleaching has been observed during extreme climate events such as El Niño; furthermore, there are indications that exposure to UV radiation, air, infectious microbes, and elevated temperature plays a role in the dramatic increase of coral bleaching since the mid-1970s. Macroalgal overgrowth is usually ascribed to eutrophi-cation and coral diseases to weakening of the coral host resistance by anthropogenic pollution. An issue precluding a strict anthropogenic cause of coral reef decline is that both overgrowth and coral diseases are known to occur, although less frequently, on reefs remote from human development. While its causes are still being unraveled, the overall decline in coral reef health sends an unambiguous signal that the coral reef system is losing its ability to withstand sudden or persistent environmental changes.     

Using Agent-Based Models to Aid Reef Restoration: Enhancing Coral Cover and Topographic Complexity through the Spatial Arrangement of Coral Transplants

High coral cover and topographic complexity are favorable qualities of a healthy coral reef. Because coral reef restoration is expensive and coral growth is naturally slow, there is a need to strategically arrange coral transplants to maximize coral cover and topographic complexity. Similarly, it is important to understand how differences in the life history characteristics of coral transplants can influence changes in the structural attributes of coral reefs. This study utilizes agent‐based computer modeling to explore the different spatial scenarios of coral transplantation using corals with contrasting r‐ and K‐selected life histories. Spatial indexes are used to compare coral cover and topographic complexity at incremental time scales, within which disturbance events are of minor importance in spatial structuring. The outcomes of the model suggest that even‐spaced grided transplanting arrangements provide the fastest increase in coral cover and three‐dimensional habitat space (topographic complexity) across large temporal scales (<30 years) for corals with r‐selected life history strategies.     

珊瑚礁生态系统病毒研究进展

病毒对珊瑚礁生态系统中的生物进化、生物地球化学循环、珊瑚疾病等方面具有重要的生态影响。随着珊瑚礁的全球性退化,病毒在珊瑚礁生态系统中的功能与危害日益显现。综述了珊瑚礁生态系统中病毒的研究现状与进展,包括：（1）珊瑚礁病毒的多样性与分布特征（水体、宿主、核心病毒组）;（2）珊瑚礁病毒的生态功能（感染方式、促进生物进化、生物地球化学循环）;（3）珊瑚礁病毒对全球气候变化的响应（热压力、珊瑚疾病）。总体而言,珊瑚礁生态系统具有极高的病毒多样性,所发现的60个科占已知所有病毒科数量的58%。珊瑚的核心病毒组主要由双链DNA病毒、单链DNA病毒、单链逆转录病毒所组成,珊瑚黏液层对病毒具有富集作用。"Piggyback-the-Winner"（依附-胜利）是病毒在珊瑚礁中主要的生物动力学模式,其可通过水平基因迁移的方式促进礁区生物进化。病毒可通过裂解细菌与浮游藻类的途径参与珊瑚礁的生物地球化学循环,尤其是碳循环与氮循环过程。此外,病毒还具有介导珊瑚热白化与直接引发珊瑚疾病的能力,这会影响珊瑚礁生态系统应对气候变化的适应性与恢复力。基于国际上的研究进展综述,结合南海珊瑚礁生态现状提出以下研究方向,以期促进我国珊瑚礁病毒学的发展：（1）开展南海珊瑚礁中病毒多样性的识别及其时-空分布特征研究;（2）探索病毒对南海珊瑚热白化、珊瑚疾病的介导作用及其与气候变化的关系;（3）揭示病毒对南海珊瑚礁生物地球化学循环的贡献。     

Region‐wide temporal and spatial variation in Caribbean reef architecture: is coral cover the whole story?

The architectural complexity of coral reefs is largely generated by reef‐building corals, yet the effects of current regional‐scale declines in coral cover on reef complexity are poorly understood. In particular, both the extent to which declines in coral cover lead to declines in complexity and the length of time it takes for reefs to collapse following coral mortality are unknown. Here we assess the extent of temporal and spatial covariation between coral cover and reef architectural complexity using a Caribbean‐wide dataset of temporally replicated estimates spanning four decades. Both coral cover and architectural complexity have declined rapidly over time, with little evidence of a time‐lag. However, annual rates of change in coral cover and complexity do not covary, and levels of complexity vary greatly among reefs with similar coral cover. These findings suggest that the stressors influencing Caribbean reefs are sufficiently severe and widespread to produce similar regional‐scale declines in coral cover and reef complexity, even though reef architectural complexity is not a direct function of coral cover at local scales. Given that architectural complexity is not a simple function of coral cover, it is important that conservation monitoring and restoration give due consideration to both architecture and coral cover. This will help ensure that the ecosystem services supported by architectural complexity, such as nutrient recycling, dissipation of wave energy, fish production and diversity, are maintained and enhanced.     

Habitat degradation drives increased gnathiid isopod ectoparasite infection rate on juvenile but not adult fish

Widespread coral mortality is leading to coral reef degradation worldwide. Many juvenile reef fishes settle on live coral, and their predator-avoidance behaviour is disrupted in seawater exposed to dead corals, ultimately increasing predation risk. Gnathiid isopods are micropredatory fish ectoparasites that occur in higher abundances in dead coral. However, the effect of seawater associated with dead coral on the susceptibility of fish to micropredators has never been investigated. We tested whether the infection rate of cultured gnathiid ectoparasites on individual damselfish, Pomacentrus amboinensis Bleeker 1868, from two different ontogenetic stages (juveniles and adults) was influenced by seawater exposed to three different treatments: dead coral, live coral, or no coral. Seawater treatments were presumed to contain different chemical properties and are meant to represent environmental changes associated with habitat degradation on coral reefs. Gnathiid infection of juvenile fish in seawater exposed to dead coral was twice as high as that of fish in live coral or no coral. Infection rates did not significantly differ between live coral and no coral treatments. In contrast to juveniles, the susceptibility of adults to gnathiids was not affected by seawater treatment. During experiments, juvenile fish mortality was relatively low, but was higher for infected fish (9.7%), compared to fish held without exposure to gnathiids (1.7%). No mortality occurred in adult fish that became infected with gnathiids. Our results suggest that chemical cues released from dead corals and/or dead coral colonisers affect the ability of juvenile, but not adult fish to avoid parasite infection. Considering increased habitat degradation on coral reefs and that gnathiids are more abundant in dead coral substrate, it is possible that wild juvenile fish may experience increased susceptibility to parasitic infection and reduced survival rate. This highlights the importance of including parasitism in ecological studies of global environmental change.

     

Macroalgae Decrease Growth and Alter Microbial Community Structure of the Reef-Building Coral, Porites astreoides

With the continued and unprecedented decline of coral reefs worldwide, evaluating the factors that contribute to coral demise is of critical importance. As coral cover declines, macroalgae are becoming more common on tropical reefs. Interactions between these macroalgae and corals may alter the coral microbiome, which is thought to play an important role in colony health and survival. Together, such changes in benthic macroalgae and in the coral microbiome may result in a feedback mechanism that contributes to additional coral cover loss. To determine if macroalgae alter the coral microbiome, we conducted a field-based experiment in which the coral Porites astreoides was placed in competition with five species of macroalgae. Macroalgal contact increased variance in the coral-associated microbial community, and two algal species significantly altered microbial community composition. All macroalgae caused the disappearance of a γ-proteobacterium previously hypothesized to be an important mutualist of P. astreoides. Macroalgal contact also triggered: 1) increases or 2) decreases in microbial taxa already present in corals, 3) establishment of new taxa to the coral microbiome, and 4) vectoring and growth of microbial taxa from the macroalgae to the coral. Furthermore, macroalgal competition decreased coral growth rates by an average of 36.8%. Overall, this study found that competition between corals and certain species of macroalgae leads to an altered coral microbiome, providing a potential mechanism by which macroalgae-coral interactions reduce coral health and lead to coral loss on impacted reefs.     

Influence of habitat degradation on fish replenishment

Temperature-induced coral bleaching is a major threat to the biodiversity of coral reef ecosystems. While reductions in species diversity and abundance of fish communities have been documented following coral bleaching, the mechanisms that underlie these changes are poorly understood. The present study examined the impacts of coral bleaching on the early life-history processes of coral reef fishes. Daily monitoring of fish settlement patterns found that ten times as many fish settled to healthy coral than sub-lethally bleached coral. Species diversity of settling fishes was least on bleached coral and greatest on dead coral, with healthy coral having intermediate levels of diversity. Laboratory experiments using light-trap caught juveniles showed that different damselfish species chose among healthy, bleached and dead coral habitats using different combinations of visual and olfactory cues. The live coral specialist, Pomacentrus moluccensis, preferred live coral and avoided bleached and dead coral, using mostly visual cues to inform their habitat choice. The habitat generalist, Pomacentrus amboinensis, also preferred live coral and avoided bleached and dead coral but selected these habitats using both visual and olfactory cues. Trials with another habitat generalist, Dischistodus sp., suggested that vision played a significant role. A 20 days field experiment that manipulated densities of P. moluccensis on healthy and bleached coral heads found an influence of fish density on juvenile weight and growth, but no significant influence of habitat quality. These results suggests that coral bleaching will affect settlement patterns and species distributions by influencing the visual and olfactory cues that reef fish larvae use to make settlement choices. Furthermore, increased fish density within the remaining healthy coral habitats could play an important role in influencing population dynamics.     

Effects of corallivory and coral colony density on coral growth and survival

A suite of processes drive variation in coral populations in space and time, yet our understanding of how variation in coral density affects coral performance is limited. Theory predicts that reductions in density can send coral populations into a predator pit, where concentrated corallivory maintains corals at low densities. In reality, how variation in coral density alters corallivory rates is poorly resolved. Here, we experimentally quantified the effects of corallivory and coral density on growth and survival of small colonies of the staghorn coral Acropora pulchra. Our findings suggest that coral density and corallivory have strong but independent effects on coral performance. In the presence of corallivores, corals suffered high but density-independent mortality. When corallivores were excluded, however, vertical extension rates of colonies increased with increasing densities. While we found no evidence for a predator pit, our results suggest that spatio-temporal variation in corallivore and coral densities can fundamentally alter population dynamics via strong effects on juvenile corals.

     

The urgent need for robust coral disease diagnostics

Coral disease has emerged over recent decades as a significant threat to coral reef ecosystems, with declines in coral cover and diversity of Caribbean reefs providing an example of the potential impacts of disease at regional scales. If similar trends are to be mitigated or avoided on reefs worldwide, a deeper understanding of the factors underlying the origin and spread of coral diseases and the steps that can be taken to prevent, control, or reduce their impacts is required. In recent years, an increased focus on coral microbiology and the application of classic culture techniques and emerging molecular technologies has revealed several coral pathogens that could serve as targets for novel coral disease diagnostic tools. The ability to detect and quantify microbial agents identified as indicators of coral disease will aid in the elucidation of disease causation and facilitate coral disease detection and diagnosis, pathogen monitoring in individuals and ecosystems, and identification of pathogen sources, vectors, and reservoirs. This information will advance the field of coral disease research and contribute knowledge necessary for effective coral reef management. This paper establishes the need for sensitive and specific molecular-based coral pathogen detection, outlines the emerging technologies that could serve as the basis of a new generation of coral disease diagnostic assays, and addresses the unique challenges inherent to the application of these techniques to environmentally derived coral samples.     

Sugar enrichment provides evidence for a role of nitrogen fixation in coral bleaching

The disruption of the coral–algae symbiosis (coral bleaching) due to rising sea surface temperatures has become an unprecedented global threat to coral reefs. Despite decades of research, our ability to manage mass bleaching events remains hampered by an incomplete mechanistic understanding of the processes involved. In this study, we induced a coral bleaching phenotype in the absence of heat and light stress by adding sugars. The sugar addition resulted in coral symbiotic breakdown accompanied by a fourfold increase of coral‐associated microbial nitrogen fixation. Concomitantly, increased N:P ratios by the coral host and algal symbionts suggest excess availability of nitrogen and a disruption of the nitrogen limitation within the coral holobiont. As nitrogen fixation is similarly stimulated in ocean warming scenarios, here we propose a refined coral bleaching model integrating the cascading effects of stimulated microbial nitrogen fixation. This model highlights the putative role of nitrogen‐fixing microbes in coral holobiont functioning and breakdown.     

珊瑚礁区碳循环研究进展

珊瑚礁是海洋中生产力水平最高的生态系统之一,其碳循环受到有机碳代谢(光合作用/呼吸作用)和无机碳代谢(钙化/溶解)两大代谢过程的共同作用,过程十分复杂.珊瑚礁植物的光合作用保证了有机碳的有效补充,动物摄食及微生物降解等生物过程驱动了珊瑚礁区有机碳高效循环,只有不超过7％的有机碳进入沉积物,而向大洋区水平输出的有机碳通量变化幅度较大,主要受到水动力条件的影响.珊瑚礁区碳酸盐沉积(无机碳代谢)是全球碳酸盐库的重要组成部分,年累积量达到全球CaCO3年累积量的23%～26%,是影响大气CO2浓度的重要组成;珊瑚礁是大气CO2源或汇则取决于净有机生产力与净无机生产力的比值(ROI),当ROI <0.6时,珊瑚礁区是大气CO2的源,反之,则是大气CO2的汇.     

不同改良剂对珊瑚砂理化性质和氮素固持的影响

高碱环境和土壤养分的匮乏严重限制了我国热带珊瑚岛土壤环境下植物的生长适应能力,因此,珊瑚砂改良对促进珊瑚岛植被恢复,维持珊瑚岛生态环境可持续健康发展具有重要意义。该研究通过室内土柱模拟试验,对比了同一梯度下4种常用土壤改良材料(蛭石、珍珠岩、生物炭和钙基膨润土)对珊瑚砂理化性质、氮素淋溶以及总氮含量的影响。结果表明,与对照(CK)相比,施用生物炭使珊瑚砂pH显著降低1.4%,钙基膨润土、生物炭和蛭石能够显著提高珊瑚砂的阳离子交换量至CK的24.21、10.43和9.43倍。同时,施用生物炭、钙基膨润土和蛭石并不能降低以硝态氮形式为主的氮素淋失,但是能显著减少其他途径的氮素损失,从而达到促进珊瑚砂氮素固持的效果,3种改良剂施用下的珊瑚砂总氮损失相较于CK分别降低了40.92%、27.32%和25.09%。因此,施用生物炭、钙基膨润土和蛭石均能有效提高珊瑚砂土壤质量,对改良珊瑚砂和热带珊瑚岛植被恢复具有重要意义,其中生物炭的改良效果最为显著,是改良珊瑚砂的理想材料。     

Live coral repels a common reef fish ectoparasite

Coral reefs are undergoing rapid changes as living corals give way to dead coral on which other benthic organisms grow. This decline in live coral could influence habitat availability for fish parasites with benthic life stages. Gnathiid isopod larvae live in the substratum and are common blood-feeding parasites of reef fishes. We examined substrate associations and preferences of a common Caribbean gnathiid, Gnathia marleyi. Emergence traps set over predominantly live coral substrata captured significantly fewer gnathiids than traps set over dead coral substrata. In laboratory experiments, gnathiids preferred dead coral and sponge and tended to avoid contact with live coral. When live gnathiids were added to containers with dead or live coral, significantly fewer were recovered from the latter after 24 h. Our data therefore suggest that live coral is not suitable microhabitat for parasitic gnathiid isopods and that a decrease in live coral cover increases available habitat for gnathiids.     

Changes in coral assemblages during an outbreak of Acanthaster planci at Lizard Island, northern Great Barrier Reef (1995–1999)

Population outbreaks of crown-of-thorns starfish (Acanthaster planci L.) represent one of the most significant biological disturbances on tropical coral reefs and have the potential to devastate coral communities, thereby altering the biological and physical structure of reef habitats. This study reports on changes in area cover, species diversity and taxonomic composition of corals during an outbreak of A. planci at Lizard Island, in the northern Great Barrier Reef, Australia. Mean coral cover declined by 28.8% across ten locations studied. However, densities of A. planci, and their effects on local coral assemblages, were very patchy. Declines in coral cover were mostly due to the selective removal of certain coral taxa (mainly Acropora and Pocilloporidae corals); such that the greatest coral loss occurred at locations with highest initial cover of preferred coral prey. Most notably, coral assemblages in back-reef locations were transformed from topographically complex staghorn Acropora-dominated habitats, to relatively depauperate assemblages dominated by alcyonacean soft corals. Although coral loss was greatest among formerly dominant taxa (especially Acropora), effects were sufficiently widespread across different coral taxa, such that overall coral diversity tended to decline. Clearly, moderate outbreaks of A. planci have the potential to greatly alter community structure of coral communities even if they do not devastate live corals. Recovery in this instance is expected to be very rapid given that all coral taxa persisted, and effects were greatest among fast growing corals.     

Coral reef crisis in deep and shallow reefs: 30 years of constancy and change in reefs of Curacao and Bonaire

Coral reefs are thought to be in worldwide decline but available data are practically limited to reefs shallower than 25 m. Zooxanthellate coral communities in deep reefs (30–40 m) are relatively unstudied. Our question is: what is happening in deep reefs in terms of coral cover and coral mortality? We compare changes in species composition, coral mortality, and coral cover at Caribbean (Curacao and Bonaire) deep (30–40 m) and shallow reefs (10–20 m) using long-term (1973–2002) data from permanent photo quadrats. About 20 zooxanthellate coral species are common in the deep-reef communities, dominated by Agaricia sp., with coral cover up to 60%. In contrast with shallow reefs, there is no decrease in coral cover or number of coral colonies in deep reefs over the last 30 years. In deep reefs, non-agaricid species are decreasing but agaricid domination will be interrupted by natural catastrophic mortality such as deep coral bleaching and storms. Temperature is a vastly fluctuating variable in the deep-reef environment with extremely low temperatures possibly related to deep-reef bleaching. An erratum to this article can be found at     

珊瑚及共生藻在白化过程中的适应机制研究进展

珊瑚礁生态系统具有非常重要的生态学功能。但是随着全球气候变暖和CO2浓度的升高,珊瑚白化事件越来越频繁,珊瑚礁生态系统面临严重的危机。影响珊瑚白化的重要因子主要有海水温度的异常（过高或过低）,太阳辐射与紫外线辐射,海水盐度的偏离,珊瑚疾病,海洋污染,长棘海星的爆发,人类的过度捕鱼和全球CO2浓度升高等。其中,海洋表面水体温度（SST）的异常升高为珊瑚白化的主要因素。珊瑚主要是通过珊瑚与共生藻的生理适应机制以及更换共生藻基因型机制两种方式来适应环境胁迫的。生理适应机制主要通过叶黄素循环、珊瑚色素荧光（热）、活性氧清除系统（自由基）、分泌紫外线吸收物质MAAs（紫外光）、产生热休克蛋白HspS（热）来实现的。珊瑚共生藻基因型更换适应机制是指珊瑚的适应性白化假说。珊瑚的适应性白化假说还有很多争议,还需要更多的实验证据提供支持。未来的研究重点将在珊瑚白化过程中共生藻-珊瑚共生功能体作为整体性的研究,尤其是珊瑚宿主在白化过程中对共生功能体作出贡献的研究。     

Persistence and Change in Community Composition of Reef Corals through Present,Past, and Future Climates

The reduction in coral cover on many contemporary tropical reefs suggests a different set of coral community assemblages will dominate future reefs. To evaluate the capacity of reef corals to persist over various time scales, we examined coral community dynamics in contemporary, fossil, and simulated future coral reef ecosystems. Based on studies between 1987 and 2012 at two locations in the Caribbean, and between 1981 and 2013 at five locations in the Indo-Pacific, we show that many coral genera declined in abundance, some showed no change in abundance, and a few coral genera increased in abundance. Whether the abundance of a genus declined, increased, or was conserved, was independent of coral family. An analysis of fossil-reef communities in the Caribbean revealed changes in numerical dominance and relative abundances of coral genera, and demonstrated that neither dominance nor taxon was associated with persistence. As coral family was a poor predictor of performance on contemporary reefs, a trait-based, dynamic, multi-patch model was developed to explore the phenotypic basis of ecological performance in a warmer future. Sensitivity analyses revealed that upon exposure to thermal stress, thermal tolerance, growth rate, and longevity were the most important predictors of coral persistence. Together, our results underscore the high variation in the rates and direction of change in coral abundances on contemporary and fossil reefs. Given this variation, it remains possible that coral reefs will be populated by a subset of the present coral fauna in a future that is warmer than the recent past.     

珊瑚共生体碳代谢特征研究进展

珊瑚礁作为一种典型的海洋生态系统,具有巨大的固碳和储碳潜力。然而,目前对于珊瑚礁的净碳能力(碳释放与碳吸收)仍存在争议,主要归因于珊瑚共生体碳代谢的多样性和复杂性。珊瑚礁在生物钙化、呼吸过程中向大气释放二氧化碳(CO₂);但在生物合成和沉积过程中却可以将碳进行固定与埋藏;为此,珊瑚礁的碳源碳汇身份还有待明确。现有部分研究表明,共生体通过碳代谢可以促进珊瑚礁吸收大气中的CO₂。此外,珊瑚礁和海岸带蓝碳生态系统通常表现出很强的连通性,珊瑚共生体碳代谢能有效提高海岸带盐沼植被、海草床、海洋浮游植物等生物的碳汇功能。为了加深对珊瑚礁碳源-碳汇功能的理解,综述了珊瑚共生体的碳代谢特征,梳理了共生体中碳的关键生态过程(有机碳的迁移、无机碳的转化、两者的赋存状态),总结了细菌-虫黄藻-病毒在共生体碳代谢中的作用,评述了珊瑚礁碳源-碳汇特征及影响因子。旨在阐明珊瑚共生体碳代谢的关键过程,并基于此寻求有效的珊瑚礁碳增汇技术,形成以碳增量为主的珊瑚保护与修复技术,提升珊瑚礁在蓝碳生态系统中的贡献。     

An intertidal Rhabdopleura (Hemichordata, Pterobranchia) from Fiji

Rhabdopleura has been discovered living in coral rubble on reefs in Fiji. The habitat is unusual, being the underside of coral boulders in the intertidal zone. Some features of the environment and the fauna associated with the Rhabdopleura are briefly described. The Rhabdopleura zooids exhibit other modes of tube building besides the regular cylindrical horizontal and erect tubes. The colony ramifies through interstices in the dead coral and the zooids can line larger cavities within the coral with coenecial tissue. It is probably these cavities, together with the overall porosity of coral, that retain enough water to keep the zooids alive while the intertidal reef flat is exposed to the air. Within the depths of the coral the black stolons are frequently naked. The species is probably R. normani Allman.     

Status and progress in coral reef disease research

Recent findings on the ecology, etiology and pathology of coral pathogens, host resistance mechanisms, previously unknown disease/syndromes and the global nature of coral reef diseases have increased our concern about the health and future of coral reef communities. Much of what has been discovered in the past 4 years is presented in this special issue. Among the significant findings, the role that various Vibrio species play in coral disease and health, the composition of the 'normal microbiota' of corals, and the possible role of viruses in the disease process are important additions to our knowledge. New information concerning disease resistance and vectors, variation in pathogen composition for both fungal diseases of gorgonians and black band disease across oceans, environmental effects on disease susceptibility and resistance, and temporal and spatial disease variations among different coral species is presented in a number of papers. While the Caribbean may still be the 'disease hot spot' for coral reefs, it is now clear that diseases of coral reef organisms have become a global threat to coral reefs and a major cause of reef deterioration.