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Stony coral tissue loss disease (SCTLD) is destructive and poses a significant threat to Caribbean coral reef ecosystems. Characterized by the acute loss of coral tissue, SCTLD has impacted over 22 stony coral species across the Caribbean region, leading to visible declines in reef health. Based on the duration, lethality, host range, and spread of this disease, SCTLD is considered the most devastating coral disease outbreak ever recorded. Researchers are actively investigating the cause and transmission of SCTLD, but the exact mechanisms, triggers, and etiological agent(s) remain elusive. If left unchecked, SCTLD could have profound implications for the health and resilience of coral reefs worldwide. To summarize what is known about this disease and identify potential knowledge gaps, this review provides a holistic overview of SCTLD research, including species susceptibility, disease transmission, ecological impacts, etiology, diagnostic tools, host defense mechanisms, and treatments. Additionally, future research avenues are highlighted, which are also relevant for other coral diseases. As SCTLD continues to spread, collaborative efforts are necessary to develop effective strategies for mitigating its impacts on critical coral reef ecosystems. These collaborative efforts need to include researchers from diverse backgrounds and underrepresented groups to provide additional perspectives for a disease that requires creative and urgent solutions.

 

Upright branching sponges, such asAplysina cauliformis, provide critical three‐dimensional habitat for other organisms and assist in stabilizing coral reef substrata, but are highly susceptible to breakage during storms. Breakage can increase sponge fragmentation, contributing to population clonality and inbreeding. Conversely, storms could provide opportunities for new genotypes to enter populations via larval recruitment, resulting in greater genetic diversity in locations with frequent storms. The unprecedented occurrence of two Category 5 hurricanes in close succession during 2017 in the U.S. Virgin Islands (USVI) provided a unique opportunity to evaluate whether recolonization of newly available substrata on coral reefs was due to local (e.g. re‐growth of remnants, fragmentation, larval recruitment) or remote (e.g. larval transport and immigration) sponge genotypes. We sampledA. cauliformisadults and juveniles from four reefs around St. Thomas and two in St. Croix (USVI). Using a 2bRAD protocol, all samples were genotyped for single‐nucleotide polymorphisms (SNPs). Results showed that these major storm events favoured sponge larval recruitment but did not increase the genetic diversity ofA. cauliformispopulations. Recolonization of substratum post‐storms via clonality was lower (15%) than expected and instead was mainly due to sexual reproduction (85%) via local larval recruitment. Storms did enhance gene flow among and within reef sites located south of St. Thomas and north of St. Croix. Therefore, populations of clonal marine species with low pelagic dispersion, such asA. cauliformis, may benefit from increased frequency and magnitude of hurricanes for the maintenance of genetic diversity and to combat inbreeding, enhancing the resilience of Caribbean sponge communities to extreme storm events.

 

Gene expression profiles are correlated to disease phenotypes in resistant and susceptible corals. 

Stony coral tissue loss disease (SCTLD), one of the most pervasive and virulent coral diseases on record, affects over 22 species of reef-building coral and is decimating reefs throughout the Caribbean. To understand how different coral species and their algal symbionts (family Symbiodiniaceae) respond to this disease, we examine the gene expression profiles of colonies of five species of coral from a SCTLD transmission experiment. The included species vary in their purported susceptibilities to SCTLD, and we use this to inform gene expression analyses of both the coral animal and their Symbiodiniaceae. We identify orthologous coral genes exhibiting lineage-specific differences in expression that correlate to disease susceptibility, as well as genes that are differentially expressed in all coral species in response to SCTLD infection. We find that SCTLD infection induces increased expression ofrab7, an established marker of in situ degradation of dysfunctional Symbiodiniaceae, in all coral species accompanied by genus-level shifts in Symbiodiniaceae photosystem and metabolism gene expression. Overall, our results indicate that SCTLD infection induces symbiophagy across coral species and that the severity of disease is influenced by Symbiodiniaceae identity.

 

Stony coral tissue loss disease (SCTLD) is a widespread and deadly disease that affects nearly half of Caribbean coral species. To understand the microbial community response to this disease, we performed a disease transmission experiment on US Virgin Island (USVI) corals, exposing six species of coral with varying susceptibility to SCTLD. The microbial community of the surface mucus and tissue layers were examined separately using a small subunit ribosomal RNA gene-based sequencing approach, and data were analyzed to identify microbial community shifts following disease acquisition, potential causative pathogens, as well as compare microbiota composition to field-based corals from the USVI and Florida outbreaks. While all species displayed similar microbiome composition with disease acquisition, microbiome similarity patterns differed by both species and mucus or tissue microhabitat. Further, disease exposed but not lesioned corals harbored a mucus microbial community similar to those showing disease signs, suggesting that mucus may serve as an early warning detection for the onset of SCTLD. Like other SCTLD studies in Florida, Rhodobacteraceae, Arcobacteraceae, Desulfovibrionaceae, Peptostreptococcaceae, Fusibacter, Marinifilaceae, and Vibrionaceae dominated diseased corals. This study demonstrates the differential response of the mucus and tissue microorganisms to SCTLD and suggests that mucus microorganisms may be diagnostic for early disease exposure.

 

Coral communities in the Caribbean face a new and deadly threat in the form of the highly virulent multi-host stony coral tissue loss disease (SCTLD). In late January of 2019, a disease with signs and characteristics matching that of SCTLD was found affecting a reef off the coast of St. Thomas in the U.S. Virgin Islands (USVI). Identification of its emergence in the USVI provided the opportunity to document the initial evolution of its spatial distribution, coral species susceptibility characteristics, and its comparative impact on coral cover at affected and unaffected coral reef locations. Re-assessments at sentinel sites and long-term monitoring locations were used to track the spread of the disease, assess species affected, and quantify its impact. The disease was initially limited to the southwest of St. Thomas for several months, then spread around the island and to the neighboring island of St. John to the east. Differences in disease prevalence among species were similar to reports of SCTLD from other regions. Highly affected species included Colpophyllia natans, Eusmilia fastigiata, Montastraea cavernosa, Orbicella spp., and Pseudodiploria strigosa. Dendrogyra cylindrus and Meandrina meandrites were also highly affected but showed more variability in disease prevalence, likely due to initial low abundances and the rapid loss of colonies due to disease. Siderastrea spp. were less affected and showed lower prevalence. Species previously reported as unaffected or data deficient that were found to be affected by SCTLD included Agaricia spp., Madracis spp., and Mycetophyllia spp. We also observed multi-focal lesions at SCTLD-affected sites on colonies of Porites astreoides, despite that poritids have previously been considered low or not susceptible to SCTLD. Loss of coral cover due to acute tissue loss diseases, which were predominantly SCTLD, was significant at several monitoring locations and was more impactful than previous mass bleaching events at some sites. There are no signs that the USVI SCTLD outbreak is abating, therefore it is likely that this disease will become widespread across the U.S. Caribbean and British Virgin Islands in the near future. 

Stony coral tissue loss disease (SCTLD) was first observed in the United States Virgin Islands in January 2019 on a reef at Flat Cay off the island of St. Thomas. A year after its emergence, the disease had spread to several reefs around St. Thomas causing significant declines in overall coral cover. Rates of tissue loss are an important metric in the study of coral disease ecology, as they can inform many aspects of etiology such as disease susceptibility and resistance among species, and provide critical parameters for modeling the effects of disease among heterogenous reef communities. The present study quantified tissue loss rates attributed to SCTLD among six abundant reef building species ( Colpophyllia natans, Montastraea cavernosa, Diploria labyrinthiformis, Pseudodiploria strigosa, Orbicella annularis , and Porites astreoides ). Field-based 3D models of diseased corals, taken approximately weekly, indicated that the absolute rates of tissue loss from SCTLD slowed through time, corresponding with the accumulation of thermal stress that led to mass bleaching. Absolute tissue loss rates were comparable among species prior to the bleaching event but diverged during and remained different after the bleaching event. Proportional tissue loss rates did not vary among species or through time, but there was considerable variability among M. cavernosa colonies. SCTLD poses a significant threat to reefs across the Caribbean due to its persistence through time, wide range of susceptible coral species, and unprecedented tissue loss rates. Intervention and management efforts should be increased during and immediately following thermal stress events in order maximize resource distribution when disease prevalence is decreased. 

Stony coral tissue loss disease (SCTLD) was initially documented in Florida in 2014 and outbreaks with similar characteristics have since appeared in disparate areas throughout the northern Caribbean, causing significant declines in coral communities. SCTLD is characterized by focal or multifocal lesions of denuded skeleton caused by rapid tissue loss and affects at least 22 reef-building species of Caribbean corals. A tissue-loss disease consistent with the case definition of SCTLD was first observed in the U.S. Virgin Islands (USVI) in January of 2019 off the south shore of St. Thomas at Flat Cay. The objective of the present study was to characterize species susceptibility to the disease present in St. Thomas in a controlled laboratory transmission experiment. Fragments of six species of corals ( Colpophyllia natans , Montastraea cavernosa , Orbicella annularis , Porites astreoides , Pseudodiploria strigosa , and Siderastrea siderea ) were simultaneously incubated with (but did not physically contact) SCTLD-affected colonies of Diploria labyrinthiformis and monitored for lesion appearance over an 8 day experimental period. Paired fragments from each corresponding coral genotype were equivalently exposed to apparently healthy colonies of D. labyrinthiformis to serve as controls; none of these fragments developed lesions throughout the experiment. When tissue-loss lesions appeared and progressed in a disease treatment, the affected coral fragment, and its corresponding control genet, were removed and preserved for future analysis. Based on measures including disease prevalence and incidence, relative risk of lesion development, and lesion progression rates, O. annularis, C. natans , and S. siderea showed the greatest susceptibility to SCTLD in the USVI. These species exhibited earlier average development of lesions, higher relative risk of lesion development, greater lesion prevalence, and faster lesion progression rates compared with the other species, some of which are considered to be more susceptible based on field observations (e.g., P. strigosa ). The average transmission rate in the present study was comparable to tank studies in Florida, even though disease donor species differed. Our findings suggest that the tissue loss disease affecting reefs of the USVI has a similar epizootiology to that observed in other regions, particularly Florida.