Award: OCE-1756381

Caribbean coral reefs have undergone dramatic changes over the past 40 years. Marine heat waves, diseases, hurricanes, and chronic degradation of reef habitats have led to the near continuous decline of reef-building corals. That decline has created ecological space for arborescent octocorals. Octocorals are now numerically and functionally dominant on many shallow Caribbean reefs, forming dense "animal forests." The 2023–2024 marine heat wave, the most intense on record in the region, illustrates this pattern: it measurably altered coral assemblages and deepened the ecological perils of rarity and potential local extinction for scleractinians. Understanding the processes controlling the success and fates of these communities is critical to understanding the future of Caribbean reefs.

Several interconnected biological traits explain why octocorals have flourished. A critical advantage lies in their symbiotic relationship with thermally tolerant algal symbionts in the genus Breviolum. For instance, during heat waves from 2014 to 2016, their symbiont densities declined but recovered rapidly with low colony. Dominant symbiont genotypes within host colonies were mostly stable, suggesting that the octocoral holobiont is resistant to thermal stress.

Central to octocoral success compared to hard corals is their capacity for high rates of recruitment. Recruitment is, by definition, the primary demographic driver of resilience, but development from larva to established colony is difficult. Larval supply is the dominant predictor of recruit densities, but post-settlement survival is extremely low. Demographic models show that only about 5% of recruits survive to reach 5 cm in height, a process that takes on average three years due to highly variable and frequently negative growth rates. Partial mortality further distorts the relationship between age and size, keeping many recruits in vulnerable size classes far longer than expected.

The physical and biological environment dramatically affects the success of recruits and small developing colonies. Algal turf, which has expanded alongside scleractinian decline, significantly impairs recruit survival — each 1% increase in turf cover accelerates polyp mortality by 1.3%. Paradoxically, declines in grazing fish and the sea urchin Diadema antillarum may have benefited octocorals: experimental exclusion studies demonstrate that grazing substantially reduces post-settlement survival of octocoral recruits. Thus reductions in grazing have the opposing effects of promoting algal turfs and reducing predation of settlers. Which of those effects is greater is likely to vary among reefs.

The success of octocorals has led to the dense, forest-like communities. “Octocoral forests.”  Like terrestrial forests, octocoral forests alter their physical. Studies around Puerto Rico and St. John found that as colony density increases, canopy cover, frontal area, and occupied volume all grow, with measurable effects on water flow. At densities above roughly 10 colonies per square meter, flow within the forest is significantly altered. Octocoral canopies increase turbulence, accumulate greater amounts of coarser, organically enriched sediment, and potentially influence larval settlement dynamics. Four of eight surveyed sites met criteria to be classified functionally as octocoral forests, underscoring that explicit ecological definitions of these communities are essential for tracking their trajectories under climate change.

At the community level, octocoral forests have show surprising stability through mechanisms analogous to financial portfolio effects. Multi-year monitoring at St. John sites from 2014 to 2019 revealed asynchronous fluctuations in species abundances, with different species compensating for one another's declines. Structural and functional redundancy among species means that the community's canopy and vertical structure remain relatively stable even as species composition shifts. Resilience to catastrophic disturbance was demonstrated vividly after Hurricanes Irma and Maria in 2017, which reduced colony densities by up to 47% but left species composition and spatial community structure largely intact, with recruit densities rebounding to pre-storm levels within two years.

The resilience of octocoral forests has limits. Models forecasting population dynamics under varying hurricane regimes suggest that despite recovery short term recovery from acute disturbances, all three modeled species — Antillogorgia americana, Gorgonia ventalina, and Eunicea flexuosa — show long-term declining population densities. The octocoral forest, it appears, is a resilient but not invulnerable. The community may be a precarious successor state with a fate that will be decided by the pace and severity of continuing environmental degradation, much caused by climate change.

Last Modified: 03/16/2026
Modified by: Howard R Lasker