Opportunistic consumption of coral spawn by the ruby brittle star (Ophioderma rubicundum).

Many reef invertebrates reproduce through simultaneous broadcast spawning, with an apparent advantage of overwhelming potential predators and maximizing propagule survival. Although reef fish have been observed to consume coral gamete bundles during spawning events, there are few records of such predation by benthic invertebrates. Here, we document several instances of the ruby brittle star, Ophioderma rubicundum, capturing and consuming egg-sperm bundles of the mountainous star coral, Orbicella faveolata, and the symmetrical brain coral, Pseudodiploria strigosa, during spawning events in the Cayman Islands in 2012 and the Florida Keys in 2022. These observations are widely separated in space and time (>600 km, 10 years), suggesting that this behavior may be prevalent on western Atlantic reefs. Since O. rubicundum spawns on the same or subsequent nights as these coral species, we hypothesize that this opportunistic feeding behavior takes advantage of lipid-rich coral gamete bundles to recover energy reserves expended by the brittle star during gametogenesis. The consumption of coral gametes by adult brittle stars suggests an underexplored trophic link between reef invertebrates and also provides evidence that ophiuroid-coral symbioses may oscillate between commensalism and parasitism depending on the ontogeny and reproductive status of both animals. Our observations provide insights into the nuanced, dynamic associations between coral reef invertebrates and may have implications for coral reproductive success and resilience.

Positive interactions between corals and damselfish increase coral resistance to temperature stress.

By the century's end, many tropical seas will reach temperatures exceeding most coral species' thermal tolerance on an annual basis. The persistence of corals in these regions will, therefore, depend on their abilities to tolerate recurrent thermal stress. Although ecologists have long recognized that positive interspecific interactions can ameliorate environmental stress to expand the realized niche of plants and animals, coral bleaching studies have largely overlooked how interactions with community members outside of the coral holobiont shape the bleaching response. Here, we subjected a common coral, Pocillopora grandis, to 10 days of thermal stress in aquaria with and without the damselfish Dascyllus flavicaudus (yellowtail dascyllus), which commonly shelter within these corals, to examine how interactions with damselfish impacted coral thermal tolerance. Corals often benefit from nutrients excreted by animals they interact with and prior to thermal stress, corals grown with damselfish showed improved photophysiology (Fv /Fm ) and developed larger endosymbiont populations. When exposed to thermal stress, corals with fish performed as well as control corals maintained at ambient temperatures without fish. In contrast, corals exposed to thermal stress without fish experienced photophysiological impairment, a more than 50% decline in endosymbiont density, and a 36% decrease in tissue protein content. At the end of the experiment, thermal stress caused average calcification rates to decrease by over 80% when damselfish were absent but increase nearly 25% when damselfish were present. Our study indicates that damselfish-derived nutrients can increase coral thermal tolerance and are consistent with the Stress Gradient Hypothesis, which predicts that positive interactions become increasingly important for structuring communities as environmental stress increases. Because warming of just a few degrees can exceed corals' temperature tolerance to trigger bleaching and mortality, positive interactions could play a critical role in maintaining some coral species in warming regions until climate change is aggressively addressed.

Priority effects in coral-macroalgae interactions can drive alternate community paths in the absence of top-down control.

The outcomes of species interactions can vary greatly in time and space with the outcomes of some interactions determined by priority effects. On coral reefs, benthic algae rapidly colonize disturbed substrate. In the absence of top-down control from herbivorous fishes, these algae can inhibit the recruitment of reef-building corals, leading to a persistent phase shift to a macroalgae-dominated state. Yet, corals may also inhibit colonization by macroalgae, and therefore the effects of herbivores on algal communities may be strongest following disturbances that reduce coral cover. Here, we report the results from experiments conducted on the fore reef of Moorea, French Polynesia, where we: (1) tested the ability of macroalgae to invade coral-dominated and coral-depauperate communities under different levels of herbivory, (2) explored the ability of juvenile corals (Pocillopora spp.) to suppress macroalgae, and (3) quantified the direct and indirect effects of fish herbivores and corallivores on juvenile corals. We found that macroalgae proliferated when herbivory was low but only in recently disturbed communities where coral cover was also low. When coral cover was <10%, macroalgae increased 20-fold within 1 year under reduced herbivory conditions relative to high herbivory controls. Yet, when coral cover was high (50%), macroalgae were suppressed irrespective of the level of herbivory despite ample space for algal colonization. Once established in communities with low herbivory and low coral cover, macroalgae suppressed recruitment of coral larvae, reducing the capacity for coral replenishment. However, when we experimentally established small juvenile corals (2 cm diameter) following a disturbance, juvenile corals inhibited macroalgae from invading local neighborhoods, even in the absence of herbivores, indicating a strong priority effect in macroalgae-coral interactions. Surprisingly, fishes that initially facilitated coral recruitment by controlling algae had a net negative effect on juvenile corals via predation. Corallivores reduced the growth rates of corals exposed to fishes by ~30% relative to fish exclosures, despite increased competition with macroalgae within the exclosures. These results highlight that different processes are important for structuring coral reef ecosystems at different successional stages and underscore the need to consider multiple ecological processes and historical contingencies to predict coral community dynamics.

Coral epigenetic responses to nutrient stress: Histone H2A.X phosphorylation dynamics and DNA methylation in the staghorn coral Acropora cervicornis.

Nutrient pollution and thermal stress constitute two of the main drivers of global change in the coastal oceans. While different studies have addressed the physiological effects and ecological consequences of these stressors in corals, the role of acquired modifications in the coral epigenome during acclimatory and adaptive responses remains unknown. The present work aims to address that gap by monitoring two types of epigenetic mechanisms, namely histone modifications and DNA methylation, during a 7-week-long experiment in which staghorn coral fragments (Acropora cervicornis) were exposed to nutrient stress (nitrogen, nitrogen + phosphorus) in the presence of thermal stress. The major conclusion of this experiment can be summarized by two main results: First, coral holobiont responses to the combined effects of nutrient enrichment and thermal stress involve the post-translational phosphorylation of the histone variant H2A.X (involved in responses to DNA damage), as well as nonsignificant modifications in DNA methylation trends. Second, the reduction in H2A.X phosphorylation (and the subsequent potential impairment of DNA repair mechanisms) observed after prolonged coral exposure to nitrogen enrichment and thermal stress is consistent with the symbiont-driven phosphorus limitation previously observed in corals subject to nitrogen enrichment. The alteration of this epigenetic mechanism could help to explain the synergistic effects of nutrient imbalance and thermal stress on coral fitness (i.e., increased bleaching and mortality) while supporting the positive effect of phosphorus addition to improving coral resilience to thermal stress. Overall, this work provides new insights into the role of epigenetic mechanisms during coral responses to global change, discussing future research directions and the potential benefits for improving restoration, management and conservation of coral reef ecosystems worldwide.

Is a community still a community? Reviewing definitions of key terms in community ecology.

Community ecology is an inherently complicated field, confounded by the conflicting use of fundamental terms. Nearly two decades ago, Fauth et al. (1996) demonstrated that imprecise language led to the virtual synonymy of important terms and so attempted to clearly define four keywords in community ecology; "community," "assemblage," "guild," and "ensemble". We revisit Fauth et al.'s conclusion and discuss how the use of these terms has changed over time since their review. An updated analysis of term definition from a selection of popular ecological textbooks suggests that definitions have drifted away from those encountered pre-1996, and slightly disagreed with results from a survey of 100 ecology professionals (comprising of academic professors, nonacademic PhDs, graduate and undergraduate biology students). Results suggest that confusion about these terms is still widespread in ecology. We conclude with clear suggestions for definitions of each term to be adopted hereafter to provide greater cohesion among research groups.

Fish-derived nutrient hotspots shape coral reef benthic communities.

Animal-derived nutrients play an important role in structuring nutrient regimes within and between ecosystems. When animals undergo repetitive, aggregating behavior through time, they can create nutrient hotspots where rates of biogeochemical activity are higher than those found in the surrounding environment. In turn, these hotspots can influence ecosystem processes and community structure. We examined the potential for reef fishes from the family Haemulidae (grunts) to create nutrient hotspots and the potential impact of these hotspots on reef communities. To do so, we tracked the schooling locations of diurnally migrating grunts, which shelter at reef sites during the day but forage off reef each night, and measured the impact of these fish schools on benthic communities. We found that grunt schools showed a high degree of site fidelity, repeatedly returning to the same coral heads. These aggregations created nutrient hotspots around coral heads where nitrogen and phosphorus delivery was roughly 10 and 7 times the respective rates of delivery to structurally similar sites that lacked schools of these fishes. In turn, grazing rates of herbivorous fishes at grunt-derived hotspots were approximately 3 times those of sites where grunts were rare. These differences in nutrient delivery and grazing led to distinct benthic communities with higher cover of crustose coralline algae and less total algal abundance at grunt aggregation sites. Importantly, coral growth was roughly 1.5 times greater at grunt hotspots, likely due to the important nutrient subsidy. Our results suggest that schooling reef fish and their nutrient subsidies play an important role in mediating community structure on coral reefs and that overfishing may have important negative consequences on ecosystem functions. As such, management strategies must consider mesopredatory fishes in addition to current protection often offered to herbivores and top-tier predators. Furthermore, our results suggest that restoration strategies may benefit from focusing on providing structure for aggregating fishes on reefs with low topographic complexity or focusing the restoration of nursery raised corals around existing nutrient hotspots.