The two-step mode of growth in the scleractinian coral skeletons from the micrometre to the overall scale

It has been known since the 19th century that coral skeletons are built of aragonite crystals with taxonomy-linked arrangements, but the way by which each coral species controls this crystallization process remains an unsolved question. The problem became still more intriguing when it was shown that isotopic compositions of coral aragonite were subject to taxonomy-linked influences (the "vital effect"). On the other hand, presence of an organic component in coral skeletons is also long known, but localization of these compounds is admittedly restricted to particular structures called "centres of calcifications." Fibres, the largely predominant part of the coral skeletons, are usually considered as purely mineral units. In this paper, it is shown that in both "centres of calcification" and fibres, organic compounds are associated with the mineral material at a deep structural level. A series of variously scaled observations and localized measurements allow recognition of the presence of an organic component at the nanometre scale. Far from being a freely operating process, crystallization of coral fibres is thus permanently controlled by the polyp basal ectoderm through a cyclic two-step process acting at the micrometre-scale. The biomineralization cycle begins by secretion of a proteoglycan matrix. As the composition of these sugars-proteins assemblages has been shown taxonomy dependent, the hypothesis can be made that multiple and long recognized specificities of coral skeletons are linked to this biochemically driven crystallization process. Additionally, this new concept of the biomineralization process in coral skeletons provides us with an access to the long term evolution of the Scleractinia. Remarkably, results of a skeleton-based approach using microstructural criteria (i.e., the spatial relationships of "centres of calcification" and the three-dimensional arrangements of fibres), are consistent with a molecular phylogenetic analysis carried out on the same species. Clearly, at the overall ontogenic level, the two-step growth mode of coral skeletons is also a valuable tool to reconstruct the evolutionary history of Scleractinia.     

Uranium in scleractinian coral skeletons

Accurate determinations have been made of the distribution of uranium in fresh and diagenetically altered coral skeletons occurring both naturally and grown under a variety of experimental conditions. Whereas live coral skeletons are homogeneous in uranium distribution, dead skeletons show heterogeneities relating to lithothamnioid algal encrustations and endolithic sponges. In the analyses of over 100 live coral skeletons, no zonal uranium distributions, described by previous workers, were found. In skeletons, free from organic material, uranium was found to exchange readily with the coral skeleton and/or to be precipitated along trabecular axes and skeletal margins. Bioeroded specimens contained higher uranium concentrations than freshly formed aragonite; they were similar to fossil coral skeletons used by previous researchers for uranium scrics dating.     

How corals made rocks through the ages

Hard, or stony, corals make rocks that can, on geological time scales, lead to the formation of massive reefs in shallow tropical and subtropical seas. In both historical and contemporary oceans, reef‐building corals retain information about the marine environment in their skeletons, which is an organic–inorganic composite material. The elemental and isotopic composition of their skeletons is frequently used to reconstruct the environmental history of Earth's oceans over time, including temperature, pH, and salinity. Interpretation of this information requires knowledge of how the organisms formed their skeletons. The basic mechanism of formation of calcium carbonate skeleton in stony corals has been studied for decades. While some researchers consider coral skeletons as mainly passive recorders of ocean conditions, it has become increasingly clear that biological processes play key roles in the biomineralization mechanism. Understanding the role of the animal in living stony coral biomineralization and how it evolved has profound implications for interpreting environmental signatures in fossil corals to understand past ocean conditions. Here we review historical hypotheses and discuss the present understanding of how corals evolved and how their skeletons changed over geological time. We specifically explain how biological processes, particularly those occurring at the subcellular level, critically control the formation of calcium carbonate structures. We examine the different models that address the current debate including the tissue–skeleton interface, skeletal organic matrix, and biomineralization pathways. Finally, we consider how understanding the biological control of coral biomineralization is critical to informing future models of coral vulnerability to inevitable global change, particularly increasing ocean acidification.     

Diversity and community structure of harpacticoid copepods associated with cold-water coral substrates in the Porcupine Seabight (North-East Atlantic)

The influence of microhabitat type on the diversity and community structure of the harpacticoid copepod fauna associated with a cold-water coral degradation zone was investigated in the Porcupine Seabight (North-East Atlantic). Three substrate types were distinguished: dead fragments of the cold-water coral Lophelia pertusa, skeletons of the glass sponge Aphrocallistes bocagei and the underlying sediment. At the family level, it appears that coral fragments and underlying sediment do not harbour distinctly different assemblages, with Ectinosomatidae, Ameiridae, Pseudotachidiidae, Argestidae and Miraciidae as most abundant. Conclusions on assemblage structure and diversity of the sponge skeletons are limited as only two samples were available. Similarity analysis at species level showed a strong variation in the sediment samples, which did not harbour a distinctly different assemblage in opposition to the coral and sponge samples. Several factors (sediment infill on the hard substrates, mobility of the copepods, limited sample sizes) are proposed to explain this apparent lack of a distinct difference between the microhabitats. Coral fragments and sediment were both characterised by high species diversity and low species dominance, which might indicate that copepod diversity is not substantially influenced by hydrodynamical stress. The additive partitioning of species diversity showed that by adding locations species richness was greatly enhanced. The harpacticoid community in the cold-water coral degradation zone is highly diverse and includes 157 species, 62 genera and 19 families. Information from neighbouring soft-bottom regions is necessary to assess whether total species diversity is increased by the presence of these complex habitat-providing substrates.     

Endolithic algae: an alternative source of photoassimilates during coral bleaching

Recent reports of worldwide coral bleaching events leading to devastating coral mortality have caused alarm among scientists and resource managers. Differential survival of coral species through bleaching events has been widely documented. We suggest that among the possible factors contributing to survival of coral species during such events are endolithic algae harboured in their skeleton, providing an alternative source of energy. We studied the dynamics of photosynthetic pigment concentrations and biomass of endoliths in the skeleton of the encrusting coral Oculina patagonica throughout a bleaching event. During repeated summer bleaching events these endolithic algae receive increased photosynthetically active radiation, increase markedly in biomass, and produce increasing amounts of photoassimilates, which are translocated to the coral. Chlorophyll concentrations and biomass of endoliths were 4.6 +/- 1.57 and 1570 +/- 427 microg cm(-2) respectively, in skeletons of relatively healthy colonies (0-40% bleaching) but up to 14.8 +/- 2.5 and 4036 +/- 764 microg cm(-2) endolith chlorophyll and biomass respectively, in skeletons of bleached colonies (greater than 40% bleaching). The translocation dynamics of (14)C-labelled photoassimilates from the endoliths to bleached coral tissue showed significantly higher 14C activity of the endoliths harboured within the skeletons of bleached corals than that of the endoliths in non-bleached corals. This alternative source of energy may be vital for the survivorship of O. patagonica, allowing gradual recruitment of zooxanthellae and subsequent recovery during the following winter.     

Coral Bioerosion at Enewetak: Agents and Dynamics

Significant differences were found in the extent to which massive coral species at Enewetak are excavated by boring organisms: Goniastrea retiformis, 7.9%; Porites lutea, 2.5%; and Favia pallida, 1.2%. While polychaetes constituted the most abundant and diverse group of coral associates, clionid sponges accounted for approximately 70–80% of skeletal damage. Clionid boring rates are initially very high but burrowing ceases when a particular burrow size (˜0.6 cm) or distance from the surface (≦2 cm) is reached. Most coral skeletal excavation occurs within 2 cm of a dead surface. Therefore, bioerosional damage to corals depends primarily on the amount of skeletal surface not covered by live coral tissue. Damage to skeletons is inversely correlated with colony size but is not correlated with coral growth rates or water depth. Massive corals have a potential escape in size from catastrophic bioerosion. Models relating 1) coral growth forms to skeletal density and stability in currents, 2) resistance of coral skeletons to breakage by water movement and suspended rubble, and 3) dead surface area on coral heads to bioerosional damage and consequent probability of detachment from the substrate, are proposed.     

Molecular analyses of protein components of the organic matrix in the exoskeleton of two scleractinian coral species

Biochemical and histological studies on the exoskeleton of scleractinian corals had demonstrated presence of the organic matrix containing proteins, lipids and chitin. Examination at the electron microscopic level had shown that the initial phase of calcification occurred in close association with organic substances secreted by calicoblastic cells. The possibility was thereby proposed that certain organic substances induce formation of calcium carbonate crystals, presumably functioning as templates for nucleation. In search for such a molecule, biochemical and molecular analyses were initiated on protein components of the organic matrix extracted from the calcified exoskeleton of the hermatypic coral, Galaxea fascicularis and the ahermatype, Tubastrea aurea. In SDS-PAGE analyses of the extracts, one major protein and a few other minor bands were detected in each of the two species. A cDNA encoding the major protein (named galaxin) in G. fascicularis was cloned and its primary structure was deduced. It consisted mostly of tandem repeats of a unit sequence of about 30 residues, and its sequence did not exhibit significant similarity to known proteins. Preliminary characterization of the T. aurea proteins showed that two proteins bound Ca²⁺, and suggested that the major protein of 46 kDa was not homologous to galaxin.     

Coral Skeletons Defend against Ultraviolet Radiation

Background

Many coral reef organisms are photosynthetic or have evolved in tight symbiosis with photosynthetic symbionts. As such, the tissues of reef organisms are often exposed to intense solar radiation in clear tropical waters and have adapted to trap and harness photosynthetically active radiation (PAR). High levels of ultraviolet radiation (UVR) associated with sunlight, however, represent a potential problem in terms of tissue damage.

Methodology/Principal Findings

By measuring UVR and PAR reflectance from intact and ground bare coral skeletons we show that the property of calcium carbonate skeletons to absorb downwelling UVR to a significant extent, while reflecting PAR back to the overlying tissue, has biological advantages. We placed cnidarians on top of bare skeletons and a UVR reflective substrate and showed that under ambient UVR levels, UVR transmitted through the tissues of cnidarians placed on top of bare skeletons were four times lower compared to their counterparts placed on a UVR reflective white substrate. In accordance with the lower levels of UVR measured in cnidarians on top of coral skeletons, a similar drop in UVR damage to their DNA was detected. The skeletons emitted absorbed UVR as yellow fluorescence, which allows for safe dissipation of the otherwise harmful radiation.

Conclusions/Significance

Our study presents a novel defensive role for coral skeletons and reveals that the strong UVR absorbance by the skeleton can contribute to the ability of corals, and potentially other calcifiers, to thrive under UVR levels that are detrimental to most marine life.     

The presence of multiple phenoloxidases in Caribbean reef-building corals

The melanin-synthesis pathways, phenoloxidase (PO) and laccases, are staple components of invertebrate immunity and have been shown to be vital in disease resistance. The importance of this pathway in immunity is a consequence of the release of oxygen radicals with cytotoxic effects and the production of insoluble melanin, which aids in the encapsulation of pathogens and parasites. Recently, melanization has been demonstrated as a critical immune response in several coral systems, although the biochemical components have not been thoroughly investigated. Coral diseases are posing a serious threat to coral reef survival, necessitating a full understanding of resistance mechanisms. In this study, we take a comparative approach to probe potential pathway components of melanin-synthesis in seven species from four different families of healthy Caribbean reef-building corals. Using different quinone substrates, we tested for the activity of the POs catecholase and cresolase, as well as laccase activity in each coral species. Since many invertebrate POs demonstrate some dependence on cations such as copper, calcium and magnesium, we treated the coral extracts with the chelators EDTA and EGTA to test the reliance of coral catecholase on these cations. The activity of the antioxidants peroxidase, superoxide dismutase and catalase was also tested in each coral and correlated to PO activity. All corals had demonstrable catecholase, cresolase and laccase activities, but only catecholase and cresolase activities varied significantly among species. Catecholase activity in each coral species was reduced by treatment with EDTA and EGTA, although some coral species were less affected than the others. Overall, these data show remarkable heterogeneity among the seven coral species of boulder-like reef building Caribbean coral. These differences may originate from the level of investment of each coral species into immunity and may explain disease ecology on the reef.     

Micro‐CT reconstruction reveals the colony pattern regulations of four dominant reef‐building corals

Colonies are the basic geometric building blocks of coral reefs. However, the forming regulations of both colonies and reefs are still not understood adequately. Therefore, in this study, we reconstructed 25 samples using high‐resolution micro‐computed tomography to investigate coral growth patterns and parameters. Our skeleton and canal reconstructions revealed the characteristics of different coral species, and we further visualized the growth axes and growth rings to understand the coral growth directions. We drew a skeleton grayscale map and calculated the coral skeleton void ratios to ascertain the skeletal diversity, devising a method to quantify coral growth. On the basis of the three‐dimensional (3D) reconstructions and growth parameters, we investigated the growth strategies of different coral species. This research increases the breadth of knowledge on how reef‐building corals grow their colonies, providing information on reef‐forming regulations. The data in this paper contain a large amount of coral growth information, which can be used in further research on reef‐forming patterns under different conditions. The method used in this study can also be applied to animals with porous skeletons.     

Habitat associations of juvenile fish at Ningaloo Reef, Western Australia: the importance of coral and algae

Habitat specificity plays a pivotal role in forming community patterns in coral reef fishes, yet considerable uncertainty remains as to the extent of this selectivity, particularly among newly settled recruits. Here we quantified habitat specificity of juvenile coral reef fish at three ecological levels; algal meadows vs. coral reefs, live vs. dead coral and among different coral morphologies. In total, 6979 individuals from 11 families and 56 species were censused along Ningaloo Reef, Western Australia. Juvenile fishes exhibited divergence in habitat use and specialization among species and at all study scales. Despite the close proximity of coral reef and algal meadows (10's of metres) 25 species were unique to coral reef habitats, and seven to algal meadows. Of the seven unique to algal meadows, several species are known to occupy coral reef habitat as adults, suggesting possible ontogenetic shifts in habitat use. Selectivity between live and dead coral was found to be species-specific. In particular, juvenile scarids were found predominantly on the skeletons of dead coral whereas many damsel and butterfly fishes were closely associated with live coral habitat. Among the coral dependent species, coral morphology played a key role in juvenile distribution. Corymbose corals supported a disproportionate number of coral species and individuals relative to their availability, whereas less complex shapes (i.e. massive & encrusting) were rarely used by juvenile fish. Habitat specialisation by juvenile species of ecological and fisheries importance, for a variety of habitat types, argues strongly for the careful conservation and management of multiple habitat types within marine parks, and indicates that the current emphasis on planning conservation using representative habitat areas is warranted. Furthermore, the close association of many juvenile fish with corals susceptible to climate change related disturbances suggests that identifying and protecting reefs resilient to this should be a conservation priority.     

A novel mechanism for iron incorporation into coral skeletons

Intertidal corals living in seawater with high concentrations of iron incorporate the metal into their skeletons. Cross-sections of the coral skeleton reveal orange-stained banding patterns reflecting periods of high availability of iron. The mechanism of metal incorporation involves deposition of iron compounds on to skeletal spines that are exposed as a result of temporary tissue retraction during periods of extreme stress. Subsequent tissue recovery and calcification trap the iron compounds which provide a visible environmental signature in the coral skeleton. This previously unrecognised mechanism has significant implications for the reconstruction of past environments from chemical analysis of annually-banded massive coral skeletons.     

Environmental implications of skeletal micro-density and porosity variation in two scleractinian corals

The correlations between skeletal parameters (bulk density, micro-density and porosity), coral age and sea surface temperature were assessed along a latitudinal gradient in the zooxanthellate coral Balanophyllia europaea and in the azooxanthellate coral Leptopsammia pruvoti. In both coral species, the variation of bulk density was more influenced by the variation of porosity than of micro-density. With increasing polyp age, B. europaea formed denser and less porous skeletons while L. pruvoti showed the opposite trend, becoming less dense and more porous. B. europaea skeletons were generally less porous (more dense) than those of L. pruvoti, probably as a consequence of the different habitats colonized by the two species. Increasing temperature had a negative impact on the zooxanthellate species, leading to an increase of porosity. In contrast, micro-density increased with temperature in the azooxanthellate species. It is hypothesized that the increase in porosity with increasing temperatures observed in B. europaea could depend on an attenuation of calcification due to an inhibition of the photosynthetic process at elevated temperatures, while the azooxanthellate species appears more resistant to variations of temperature, highlighting possible differences in the sensitivity/tolerance of these two coral species to temperature changes in face of global climate change.     

Heterotrophy in Tropical Scleractinian Corals

The dual character of corals, that they are both auto- and heterotrophs, was recognized early in the twentieth Century. It is generally accepted that the symbiotic association between corals and their endosymbiotic algae (called zooxanthellae) is fundamental to the development of coral reefs in oligotrophic tropical oceans because zooxanthellae transfer the major part of their photosynthates to the coral host (autotrophic nutrition). However, numerous studies have confirmed that many species of corals are also active heterotrophs, ingesting organisms ranging from bacteria to mesozooplankton. Heterotrophy accounts for between 0 and 66% of the fixed carbon incorporated into coral skeletons and can meet from 15 to 35% of daily metabolic requirements in healthy corals and up to 100% in bleached corals. Apart from this carbon input, feeding is likely to be important to most scleractinian corals, since nitrogen, phosphorus, and other nutrients that cannot be supplied from photosynthesis by the coral's symbiotic algae must come from zooplankton capture, particulate matter or dissolved compounds. A recent study showed that during bleaching events some coral species, by increasing their feeding rates, are able to maintain and restore energy reserves.
This review assesses the importance and effects of heterotrophy in tropical scleractinian corals. We first provide background information on the different food sources (from dissolved organic matter to meso- and macrozooplankton). We then consider the nutritional inputs of feeding. Finally, we review feeding effects on the different physiological parameters of corals (tissue composition, photosynthesis and skeletal growth).     

Microboring organisms in living stylasterid corals (Cnidaria,Hydrozoa)

Microboring or euendolithic microorganisms, which colonize and penetrate various carbonate substrates, are abundant in coral reef ecosystems and play a major role in reef carbonate dissolution. A few studies reported the presence of euendoliths in stylasterid coral skeletons but the biological identity, distribution and abundance of these microorganisms remain largely unknown. Observations of over 100 stylasterid colonies, collected in the Indo-Pacific area, revealed for the first time that the association between these corals and euendolith organisms appears to be quite common in shallow tropical waters. The most abundant euendolith was identified as a cryptic stage in the development of the rhodophyte Porphyra (Conchocelis stage). The euendoliths were observed in the skeletons of seven species of three genera (four Stylaster, two Distichopora and one Lepidotheca). The presence of euendoliths inside skeletons conferred a particular colour to the studied stylasterid corals. Distribution and abundance of microborings varied significantly among stylasterid species and among branches of a single colony and so did the colour of their skeletons. Colonization of skeletons and the associated colour distribution were almost uniform in some stylasterids, forming an upward gradually diminishing or sharply limited gradient. This study shows that patterns of euendolith colonization and growth in stylasterid skeletons may depend on the stage of the euendolith development as well as on their environmental requirements such as light exposure.     

Antimicrobial activity of the Anseriform outer eggshell and cuticle

The avian eggshell is a complex, multifunctional biomineral composed of a calcium carbonate mineral phase and an organic phase of lipids and proteins. The outermost layer of the eggshell, the eggshell cuticle, is an organic layer of variable thickness composed of polysaccharides, hydroxyapatite crystals, lipids and glycoprotein. In addition to regulating gas exchanges, the eggshell cuticle may contain antimicrobial elements. In this study, we investigated the antimicrobial activity of eggshell cuticle and outer eggshell protein extracts from four Anseriform species: wood duck (Aix sponsa), hooded merganser (Lophodytes cucullatus), Canada goose (Branta canadensis) and mute swan (Cygnus olor). Cuticle and outer eggshell protein was extracted by urea or HCl treatment of eggs. C-type lysozyme, ovotransferrin and an ovocalyxin-32-like protein were detected in all extracts. Cuticle and outer eggshell protein extracts inhibited the growth of Staphylococcus aureus, Escherichia coli D31, Pseudomonas aeruginosa and Bacillus subtilis. The presence of active antimicrobial proteins within the avian cuticle and outer eggshell suggests a role in antimicrobial defense. Protein extracts from the cavity nesting hooded merganser were especially potent. The unique environmental pressures exerted on cavity-nesting species may have led to the evolution of potent antimicrobial defenses.     

Induction of stolon settlement in the scyphopolyps ofAurelia aurita (Cnidaria,Scyphozoa, Semaeostomeae) by glycolipids of marine bacteria

The settlement of pedal stolons of scyphopolyps ofAurelia aurita Lamarck could be induced by addition of a species of bacteria from the family Micrococcaceae. After treatment of the bacteria with several organic solvents a crude lipid extract free of bacteria could be obtained which was shown to be effective in inducing stolon settlement. Crude lipids extracted from the late logarithmic growth phase had an optimal effect on stolon attachment, in contrast to previously published experiments showing that all logarithmic phases of bacteria had the same level of effectiveness. After separation of the crude lipid extracts by thin layer chromatography and subsequent bioassay of the reeluated substances, acylgalactosidyldiglyceride and monogalactosidyldiglyceride were identified as the effective substances. Monogalactosidyldiglyceride was only found in bacteria from the medium logarithmic growth phase, whereas the former was found at all stages. The effectiveness of acylgalactosidyldiglyceride was independent of the growth phase of the extracted bacteria.     

Walking with worms: coral‐associated epifaunal nematodes

Aims To study the community structure and habitat preferences of the Epsilonematidae and Draconematidae in coral degradation zones. To assess the contribution of different localities and microhabitats to meiobenthic diversity in such ecosystems. To discuss dispersive capacities and the occurrence of cryptic species in meiobenthic organisms. Location Porcupine Seabight (north‐east Atlantic Ocean; continental slope) and a transect along the Kenyan coast (Indian Ocean; shallow lagoon). Methods In the north‐east Atlantic, dead coral fragments, sponge skeletons and sediment were collected with a boxcorer. Along the Kenyan coast, dead coral fragments and coral gravel were collected during snorkelling and skin diving. Only nematodes belonging to the families Epsilonematidae and Draconematidae were considered. Community structure was analysed using multivariate techniques. Biodiversity was represented via rarefaction curves. Additive partitioning of species diversity was conducted. Turnover between microhabitats within locations and between locations within microhabitats were compared in a ternary plot. Results Twelve epsilonematid and five draconematid species were found in the Porcupine Seabight. In Kenya, 39 epsilonematid and 20 draconematid species were distinguished. Three species were found at both sampling locations. A table with the known distribution of all currently described species encountered in our study area is provided. At both sampling locations, the communities on coral fragments were significantly different from those in the other microhabitats, and were most diverse. In Kenya, species richness was mainly determined by local diversity and by turnover between localities. The contribution of β‐diversity decreased when abundance data were analysed. Turnover between microhabitats and between coral samples from different localities was higher than turnover between locations for gravel samples. Main conclusions Coral fragments were recognized as favourable substrata for typically epifaunal nematodes. Species‐specific habitat preferences were explained by finely tuned morphological adaptations. Our results suggest that cosmopolitan species could well be cryptic species, and this explanation for the existence of morphologically identical nematodes in geographically distant areas is weighed up against other plausible explanations. Coral degradation zones are an important source for new species of Epsilonematidae and Draconematidae. The addition of sampling locations contributed to the total number of species, although the added species were generally rare.     

Allelochemicals produced by Caribbean macroalgae and cyanobacteria have species-specific effects on reef coral microorganisms

Coral populations have precipitously declined on Caribbean reefs while algal abundance has increased, leading to enhanced competitive damage to corals, which likely is mediated by the potent allelochemicals produced by both macroalgae and benthic cyanobacteria. Allelochemicals may affect the composition and abundance of coral-associated microorganisms that control host responses and adaptations to environmental change, including susceptibility to bacterial diseases. Here, we demonstrate that extracts of six Caribbean macroalgae and two benthic cyanobacteria have both inhibitory and stimulatory effects on bacterial taxa cultured from the surfaces of Caribbean corals, macroalgae, and corals exposed to macroalgal extracts. The growth of 54 bacterial isolates was monitored in the presence of lipophilic and hydrophilic crude extracts derived from Caribbean macroalgae and cyanobacteria using 96-well plate bioassays. All 54 bacterial cultures were identified by ribotyping. Lipophilic extracts from two species of Dictyota brown algae inhibited >50% of the reef coral bacteria assayed, and hydrophilic compounds from Dictyota menstrualis particularly inhibited Vibrio bacteria, a genus associated with several coral diseases. In contrast, both lipo- and hydrophilic extracts from 2 species of Lyngbya cyanobacteria strongly stimulated bacterial growth. The brown alga Lobophora variegata produced hydrophilic compounds with broad-spectrum antibacterial effects, which inhibited 93% of the bacterial cultures. Furthermore, bacteria cultured from different locations (corals vs. macroalgae vs. coral surfaces exposed to macroalgal extracts) responded differently to algal extracts. These results reveal that extracts from macroalgae and cyanobacteria have species-specific effects on the composition of coral-microbial assemblages, which in turn may increase coral host susceptibility to disease and result in coral mortality.