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A ciliate belonging to theDiadema antillarumscuticociliatosis (DaSc)-associatedPhilasterclade (DaScPc) caused catastrophic long-spined urchin mass mortality in spring and summer of 2022. The ciliate can be grown in culture in both the presence and absence ofD. antillarumtissues, suggesting that it may persist outside its host by consuming microorganisms or dissolved organic nutrients. We hypothesized that DaScPc was present outside its host during and after mass mortality and absent prior to 2022. We examined DaScPc in DNA extracted from 500 swabs of sympatric metazoa and abiotic surfaces, and plankton samples, collected at 35 sites in the Caribbean in 2022 and 2023. DaScPc was detected on corals, turf algae, and a sponge, predominantly at sites with active or prior DaSc. We examined whether it was present prior to 2022 by surveying extracted DNA from Caribbean corals and water collected near corals by PCR and by mining publicly available transcriptomes and metagenomes for DaScPc rRNAs. These efforts yielded no DaScPc genes. We further hypothesized that DaScPc may recruit to the specific corals detected in field surveys, and that these may then infect naïve hosts. A mesocosm experiment to test DaScPc recruitment suggested that, while it recruited to corals, it did so inconsistently between coral species. Incubation of corals that recruited DaScPc with naïve urchins yielded inconclusive results since urchins died without characteristic DaSc signs. Overall, our results suggest that DaScPc may occur outside its urchin host, and that it may have been absent in the region prior to 2022. 

Caribbean coral reefs are experiencing a shift to algal dominance at the expense of stony corals. Determining the factors leading to algal phase shifts is crucial for assuring the survival of Caribbean coral reefs. In this study, factors controlling the growth of the abundant brown macroalgae Dictyota spp. were investigated by varying herbivory pressure (caging) and nutrients (fertilizer addition) on coral reefs near St. Thomas (US Virgin Islands). Experiment 1 measured Dictyota heights and percent cover at 3 sites (11-20 m depth) and showed no growth response to nutrient addition and a weak negative response to herbivory. To confirm results of Experiment 1, a caging and nutrient manipulation (Experiment 2) was conducted at one site (14 m depth) using the dependent variable Dictyota biomass. A strong negative response of growth to nutrient addition was shown, presumably because of nutrient inhibition, and an equally negative response to herbivory (loss of ~50% biomass over 21 d). The inhibitory effect of fertilization on growth was confirmed in a third experiment that showed increasing biomass loss over 4 treatment levels of increasing fertilizer addition (0 [ambient], 5, 10, 20 g). Overall, Dictyota was not nutrient limited at any sites, and was weakly controlled by herbivore populations. Factors responsible for Dictyota abundance on Caribbean reefs may reflect decreased herbivory caused by overfishing and reductions in coral cover and do not appear to be affected by recent changes in nitrogen or phosphorus load. This study reinforces the need for conservation and management of herbivores in coral reef ecosystems, to mitigate the effects from anthropogenic stressors. 

Competition for limited space is an important driver of benthic community structure on coral reefs. Studies of coral-algae and coral-sponge interactions often show competitive dominance of algae and sponges over corals, but little is known about the outcomes when these groups compete in a multispecies context. Multispecies competition is increasingly common on Caribbean coral reefs as environmental degradation drives loss of reef-building corals and proliferation of alternative organisms such as algae and sponges. New methods are needed to understand multispecies competition, whose outcomes can differ widely from pairwise competition and range from coexistence to exclusion. In this study, we used 3D photogrammetry and image analyses to compare pairwise and multispecies competition on reefs in the US Virgin Islands. Sponges ( Desmapsamma anchorata, Aplysina cauliformis ) and macroalgae ( Lobophora variegata ) were attached to coral ( Porites astreoides ) and arranged to simulate multispecies (coral-sponge-algae) and pairwise (coral-sponge, coral-algae) competition. Photogrammetric 3D models were produced to measure surface area change of coral and sponges, and photographs were analyzed to measure sponge-coral, algae-coral, and algae-sponge overgrowth. Coral lost more surface area and was overgrown more rapidly by the sponge D. anchorata in multispecies treatments, when the sponge was also in contact with algae. Algae contact may confer a competitive advantage to the sponge D. anchorata, but not to A. cauliformis , underscoring the species-specificity of these interactions. This first application of photogrammetry to study competition showed meaningful losses of living coral that, combined with significant overgrowths by competitors detected from image analyses, exposed a novel outcome of multispecies competition.