The stakes of this work are high. Acropora palmata, listed as critically endangered by the International Union for Conservation of Nature since 2008, once built vast, wave-resistant thickets on Caribbean reef crests. Its branching morphology and rapid growth generate the complex three-dimensional structure that shelters juvenile fish and invertebrates and protects coastlines. Over the past 75 years, disease, pollution, warming waters and hurricanes have reduced populations to near local extinction in some regions. In Cuba, nationwide assessments before the 2023 mass bleaching event already described most A. palmata populations as being in poor or only moderate condition, with low colony densities and high partial mortality. That bleaching event then eliminated most remaining populations across the island, leaving only a handful of refugia, including the Laguna de Maya Wildlife Refuge in Matanzas Province and reefs at Santa Lucía in Camagüey Province.

Restoration programs throughout the Caribbean have responded by outplanting fragments of opportunity, cut from healthy mother colonies, or by raising corals in land-based and in-water nurseries. A. palmata fragments have historically shown high survival, between 63 and 95 percent on metal frames, and growth of around 21 centimeters per year in suspended mid-water line nurseries. Yet most of these projects have lasted only one to two years, and fragment mortality has been attributed to storms, heat, predation and poor water quality. What has remained poorly understood is how ecological processes, particularly competition with neighboring benthic organisms, shape the critical early establishment phase of outplanted fragments. The Cuban team set out to answer exactly that question.

The experiment took place on three shallow reef crests, one to three meters deep, chosen for their contrasting environmental and management contexts. Playa Baracoa and Rincón de Guanabo lie in northwestern Cuba, near Havana, where untreated wastewater discharges, coastal development and nearby industrial activity degrade water quality. Playa Baracoa sits just two kilometers east of the Santa Ana River, which receives untreated wastewater from an institution with roughly ten thousand students. The third site, La Puntica, lies within Jardines de la Reina National Park, a relatively pristine, oligotrophic marine protected area about fifty miles from the mainland, with high fish biomass and one of the largest and healthiest A. palmata populations on Cuban crests at the time of the study.

At each crest, the researchers randomly collected healthy, unbranched apical fragments from local colonies and attached them to cleared substrate with epoxy, following a standardized protocol. Control fragments were placed on substrate cleared of all sessile organisms, while treatment fragments were positioned less than one centimeter from locally abundant competitors: the coral Porites astreoides, the fire coral Millepora complanata, the green alga Cladophora sp., the brown algae Sargassum polyceratium, Dictyota sp. and Stypopodium zonale, and the zoanthid Palythoa caribaeorum. Each treatment and control at each crest comprised seven replicates, yielding 35, 42 and 28 fragments at Playa Baracoa, Rincón de Guanabo and La Puntica respectively. Fragments were monitored over periods ranging from roughly 320 to 423 days, with survival assessed as the retention of any living tissue and growth measured as changes in width, height and live tissue area.

The statistical analysis, based on Kaplan–Meier survival estimates, log-rank tests and Cox proportional hazards regression, produced strikingly uneven results. In Playa Baracoa, fragments paired with Millepora complanata suffered significantly higher mortality than controls, with a hazard ratio of 5.7, and survival fell from 0.5 at 135 days to zero by 275 days. The same pattern emerged at Rincón de Guanabo, where Millepora-paired fragments dropped from 0.6 survival at 125 days to none at 245 days. Field observations showed the fire coral progressively overgrowing the coral fragments between 90 and 180 days, and fragments paired with Millepora consistently exhibited the lowest live tissue area across all three crests. Although the study could not determine the precise mechanism of mortality, Millepora species are known strong spatial competitors that store toxins in their nematocysts, and secondary infections following overgrowth cannot be excluded.

In sharp contrast, Porites astreoides proved to be a remarkably tolerant neighbor. Fragments paired with this weedy coral showed survival similar to controls at Playa Baracoa and significantly higher survival at La Puntica, where 0.9 of the fragments were still alive at 358 days compared with 0.5 for controls. Most remarkably, the A. palmata fragments actually overgrew the living tissue of the Porites colonies, a classic case of asymmetric interference competition. This aligns with earlier work identifying Porites as one of the least aggressive coral species on the reef. The result carries practical weight: in restoration settings where P. astreoides has expanded its cover since the 1960s and 1980s, or in multi-species outplanting designs, this interaction appears unlikely to undermine the transplanted elkhorn coral.

Perhaps the most counterintuitive finding concerned the macroalgae. Despite their reputation as major coral competitors, capable of abrasion, shading, sediment trapping and allelopathy, none of the algal treatments significantly impaired fragment survival or growth. Fragments paired with Cladophora sp. and Sargassum polyceratium at Playa Baracoa increased their live tissue area 2.4-fold and 2.1-fold respectively, and at Rincón de Guanabo the Cladophora treatment produced a 7.8-fold increase. Even more intriguingly, fragments paired with the brown alga Stypopodium zonale at Rincón de Guanabo had significantly higher survival than controls, with a hazard ratio of 0.2. The authors caution that algal contact in the experiment occurred mainly along one side of each fragment, which may have limited competitive intensity, and that coral–algal interactions are known to be density- and context-dependent. Corals themselves can suppress some macroalgae through competitive or allelopathic mechanisms, so the relationship may be reciprocal rather than one-sided.

The zoanthid Palythoa caribaeorum, widely regarded as one of the most successful competitors on reefs and a producer of the potent biotoxin palytoxin, also failed to harm the fragments at La Puntica, where paired fragments retained 0.8 survival at 231 days and grew 3.6-fold in live tissue area. Because the organisms were placed adjacent rather than overlapping, direct overgrowth never developed, and the authors note that negative effects might emerge over longer contact periods. Acropora species are known to possess defensive strategies, including sweeper tentacles, mesenteric filaments and nematocyst discharge, which may help explain their resilience.

Underlying all of these species-level patterns was a powerful site effect. Fragments at Rincón de Guanabo had 6.2-fold lower survival than those at Playa Baracoa, and live tissue area was significantly lower at both Rincón de Guanabo and La Puntica compared with Playa Baracoa. Significant interactions between the La Puntica crest and the Porites and Millepora treatments indicate that local ecological conditions modulate the outcome of biotic interactions. The authors suggest that differences in reef community structure, herbivory, nutrient availability, temperature and anthropogenic stress, including untreated wastewater discharges and overfishing in the northwest, likely shaped these contrasting outcomes. Growth rates in the study were slower than those recorded for wild A. palmata, resembling storm-generated fragments, possibly reflecting transplantation shock, and negative growth in some periods may reflect fish bites observed on fragment tips.

For restoration practitioners, the message is clear and actionable. Simply clearing substrate before outplanting, a commonly recommended intervention, may not be enough to maximize fragment performance, because the ecological condition of the crest and its local stressors play a decisive role. The team recommends conducting small-scale pilot outplanting trials and incorporating assessments of benthic competitor abundance into site selection before scaling up, with periodic monitoring and adaptive management where fire coral or other aggressive competitors are abundant. As Caribbean restoration programs confront the functional extinction of Acropora corals across much of the region, this Cuban experiment demonstrates that knowing not just where to plant, but what to plant next to, could determine whether restored elkhorn thickets survive their first year.

Subject of Research: Effects of benthic competition on the survival and growth of outplanted Acropora palmata coral fragments on Cuban reef crests

Article Title: Effect of benthic organisms on Acropora palmata (Lamarck 1816) fragment survival and growth

Article References: Romero, A. R., González-Díaz, P., Pérez, G. A., & Banaszak, A. T. (2026). Effect of benthic organisms on Acropora palmata (Lamarck 1816) fragment survival and growth. Coral Reefs. https://doi.org/10.1007/s00338-026-02957-1

Image Credits: AI Generated

DOI: 10.1007/s00338-026-02957-1

Keywords: Acropora palmata, coral restoration, benthic competition, Millepora complanata, macroalgae, Porites astreoides, Palythoa caribaeorum, reef crests, Cuba, fragment survival, coral growth, marine ecology