Coral diversity and the severity of disease outbreaks: a cross-regional comparison of Acropora White Syndrome in a species-rich region (American Samoa) with a species-poor region (Northwestern Hawaiian Islands)

The dynamics of the coral disease, Acropora white syndrome (AWS), was directly compared on reefs in the species-poor region of the Northwestern Hawaiian Islands (NWHI) and the species-rich region of American Samoa (AS) with results suggesting that biodiversity, which can affect the abundance of susceptible hosts, is important in influencing the impacts of coral disease outbreaks. The diversity-disease hypothesis predicts that decreased host species diversity should result in increased disease severity of specialist pathogens. We found that AWS was more prevalent and had a higher incidence within the NWHI as compared to AS. Individual Acropora colonies affected by AWS showed high mortality in both regions, but case fatality rate and disease severity was higher in the NWHI. The site within the NWHI had a monospecific stand of A. cytherea; a species that is highly susceptible to AWS. Once AWS entered the site, it spread easily amongst the abundant susceptible hosts. The site within AS contained numerous Acropora species, which differed in their apparent susceptibility to infection and disease severity, which in turn reduced disease spread. Manipulative studies showed AWS was transmissible through direct contact in three Acropora species. These results will help managers predict and respond to disease outbreaks

Ocean warming and acidification have complex interactive effects on the dynamics of a marine fungal disease

Diseases threaten the structure and function of marine ecosystems and are contributing to the global decline of coral reefs. We currently lack an understanding of how climate change stressors, such as ocean acidification (OA) and warming, may simultaneously affect coral reef disease dynamics, particularly diseases threatening key reef-building organisms, for example crustose coralline algae (CCA). Here, we use coralline fungal disease (CFD), a previously described CCA disease from the Pacific, to examine these simultaneous effects using both field observations and experimental manipulations. We identify the associated fungus as belonging to the subphylum Ustilaginomycetes and show linear lesion expansion rates on individual hosts can reach 6.5 mm per day. Further, we demonstrate for the first time, to our knowledge, that ocean-warming events could increase the frequency of CFD outbreaks on coral reefs, but that OA-induced lowering of pH may ameliorate outbreaks by slowing lesion expansion rates on individual hosts. Lowered pH may still reduce overall host survivorship, however, by reducing calcification and facilitating fungal bio-erosion. Such complex, interactive effects between simultaneous extrinsic environmental stressors on disease dynamics are important to consider if we are to accurately predict the response of coral reef communities to future climate change

Disease dynamics of Porites bleaching with tissue loss: prevalence, virulence, transmission, and environmental drivers

The prevalence, number of species affected, and geographical extent of coral diseases have been increasing worldwide. We present ecological data on the coral disease Porites bleaching with tissue loss (PBTL) from Kaneohe Bay, Oahu (Hawaii, USA), affecting P. compressa. This disease is prevalent throughout the year, although it shows spatio-temporal variability with peak prevalence during the warmer summer months. Temporal variability in disease prevalence showed a strong positive relationship with elevated water temperature. Spatially, PBTL prevalence peaked in clearer waters (lower turbidity) with higher water flow and higher densities of parrotfish, together explaining approximately 26% of the spatial variability in PBTL prevalence. However, the relatively poor performance of the spatial model suggests that other, unmeasured factors may be more important in driving spatial prevalence. PBTL was not transmissible through direct contact or the water column in controlled aquaria experiments, suggesting that this disease may not be caused by a pathogen, is not highly infectious, or perhaps requires a vector for transmission. In general, PBTL results in partial tissue mortality of affected colonies; on average, one-third of the tissue is lost. This disease can affect the same colonies repeatedly, suggesting a potential for progressive damage which could cause increased tissue loss over time. P. compressa is the main framework-building species in Kaneohe Bay; PBTL therefore has the potential to negatively impact the structure of the reefs at this location

Influence of local environmental variables on the viral consortia associated with the coral Montipora capitata from Kaneohe Bay, Hawaii, USA

Coral-associated viruses are a component of the coral holobiont that have received attention only relatively recently. Given the global increase in the prevalence of coral disease, and the lack of positively identified etiological agents for many diseases, these virus consortia require increased investigation. Little is known about the viruses that are naturally associated with coral reefs and how they are affected by the local environment. In the present study, a short-term analysis of viral consortia associated with the coral Montipora capitata in Kaneohe Bay, Hawaii, USA, was carried out to determine the environmental factors influencing their composition. Coral surface microlayer (CSM) and seawater samples collected at 4 sites with a range of environmental characteristics were analyzed using transmission electron microscopy (TEM), and relative abundances of virus-like particle (VLP) morphotypes were correlated with environmental measurements. Relative proportions of several CSM-associated VLP types, including phages and filamentous VLPs, were correlated with water temperature, turbidity and chlorophyll a levels. In seawater samples, turbidity and temperature showed the strongest correlation, altering the proportion of Podoviridae-like, Geminiviridae-like and putative Archaeal viruses, among others. Overall VLP consortium composition differed significantly between the CSM and seawater only at the more degraded sites, suggesting that human activity may be affecting coral reef-associated virus consortia