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As the major form of coral reef regime shift, stony coral to macroalgal transitions have received considerable attention. In the Caribbean, however, regime shifts in which scleractinian corals are replaced by octocoral assemblages hold potential for maintaining reef associated communities. Accordingly, forecasting the resilience of octocoral assemblages to future disturbance regimes is necessary to understand these assemblages' capacity to maintain reef biodiversity. We parameterised integral projection models quantifying the survival, growth, and recruitment of the octocorals, Antillogorgia americana, Gorgonia ventalina, and Eunicea flexuosa, in St John, US Virgin Islands, before, during, and after severe hurricane disturbance. Using these models, we forecast the density of populations of each species under varying future hurricane regimes. We demonstrate that although hurricanes reduce population growth, A. americana, G. ventalina, and E. flexuosa each display a capacity for quick recovery following storm disturbance. Despite this recovery potential, we illustrate how the population dynamics of each species correspond with a longer-term decline in their population densities. Despite their resilience to periodic physical disturbance events, ongoing global change jeopardises the future viability of octocoral assemblages. [ABSTRACT FROM AUTHOR]

Within the past decade, a functional group of encrusting red macroalgae defined as peyssonnelid algal crusts (PAC) has been rapidly spreading on shallow Caribbean reefs, frequently encrusting corals and sponges. This study focused on PAC growing on the reefs of St. John, US Virgin Islands, to (1) expand the monitoring of PAC abundance in this location, and (2) test two mechanisms, rapid linear growth and competitive dominance, by which PAC is hypothesized to enhance its capacity to spread. In August 2019, we found that the percentage coverage of PAC reached 46.6% at Tektite (3 m depth), and 63.8% at Cabritte Horn (3 m depth). Overall, the cover of PAC at these sites in 2019 was identical to, or up to 2.4-fold higher than in August 2017, and 2.1–3.7-fold higher than in November 2017, just after two category 5 storms impacted St. John. PAC was 1.8–29.1-fold more abundant at 3 m versus 9 m depth, and 1.4–22.0-fold more abundant at sites close to the tips of headlands than sites sheltered within bays. From August 2019 to January 2020, the rate of linear extension of PAC at each site and depth ranged from 26 to 43 µm d − 1, and of 80 PAC-coral interactions that were tagged in August 2019, none involved corals that had become completely overgrown by PAC by January 2020. The slow linear growth and absence of cases in which PAC completely overgrew coral colonies in 6 months are inconsistent with the high rate at which PAC has occupied space on Caribbean reefs over the past decade. One means to reconcile these observations is the possibility that recruitment of peyssonnelid spores to benthic surfaces facilitates the rapid spread of PAC. [ABSTRACT FROM AUTHOR]

Genetic variation underlying differences in organism performance is subject to natural selection, and organisms with high values of genetically determined phenotypic measures of fitness should perform better than those that do not. Using small scleractinian corals (i.e., ≤40-mm diameter), this principle was tested with 20 yr of census data from St. John, US Virgin Islands. Using growth rate (change in diameter) as a measure of fitness, growth in one year was tested for association with growth and survivorship in the following two years, and this process was repeated over 20 yr using a 3-yr sliding window. Virtually all variation in growth was independent of colony size, and growth among pairs of years was highly variable, with corals that grew fast in one year rarely growing fast in the next 2 yr. While growth in some pairs of years was positively correlated, ≤4% of the growth variance was explained by growth in the preceding 2 yr. Survivorship was related positively to growth in the preceding year, but the association was weak, it did not extend over 3 yr, and was inconsistent over the study. These results demonstrate the importance of the environment in translating phenotypic measures of fitness into future performance, and for small Caribbean corals, they suggest that environmental conditions may preempt genotype in determining short-term success. [ABSTRACT FROM AUTHOR]

The article studies the impact of anthropogenic disturbances on two Caribbean corals, specifically Porites astreoides and Diploria strigosa, on a shallow Caribbean reef. The population density of juvenile Porites spp. and Diploria spp. is estimated. The demography of Porites astreoides and Diploria strigosa is analyzed. The population projections for both coral species were determined by increases in the number of colonies, elevated coral cover, and retention of the larger colonies regardless of the disturbance.

Tropical reef corals are well known for their sensitivity to rising temperature, yet surprisingly little is known of the mechanisms through which temperature acts on intact coral colonies. One such mechanism recently has been suggested by the association between the growth of juvenile corals and seawater temperature in the Caribbean, which suggests that temperature causes a transition between isometric and allometric growth scaling in warmer versus cooler years, respectively (Edmunds in Proc R Soc B 273:2275–2281, ). Here, this correlative association is tested experimentally for a cause-and-effect relationship. During April and May 2006, juvenile colonies (8–35 mm diameter) of massive Porites spp. from Moorea, French Polynesia, were incubated at warm (27.8°C) and cool (25.7°C) temperatures for 15 days, and their response assessed through the scaling of growth (change in weight) with colony size. The results reveal that the scaling of colony-specific growth (mg colony−1 day−1) was unaffected by temperature, although growth absolutely was greater at the cool compared to the warm temperature, regardless of colony size. This outcome was caused by contrasting scaling relationships for area-specific growth (mg cm−2 day−1) that were negatively allometric under warm conditions, but independent of size under cool conditions. In April 2007, a 22 days field experiment confirmed that the scaling of area-specific growth in juvenile Porites spp. is negatively allometric at a warm temperature of 29.5°C. Based on strong allometry for tissue thickness, biomass, and Symbiodinium density in freshly collected Porites spp ., it is hypothesized that the temperature-dependency of growth scaling in these small corals is mediated by the interaction of temperature with biomass. [ABSTRACT FROM AUTHOR]

On Caribbean reefs, a striking trend of the last 25 years has been the decline in cover of the framework-building coral Motastraea annularis, a species that has dominated reefs throughout the region for millennia. Clearly, such losses are important ecologically, but to evaluate their significance fully, they need to be placed in the context of the proximal causes and balanced against the potential for gains in cover through growth and recruitment. In this study, a population of M. annularis in St. John, U.S. Virgin Islands, was censused annually from 1988 to 2003 to quantify coral cover and construct a size-based demographic model. The model was developed to explore the mechanisms of change in coral cover and to ascertain likely trajectories for future population growth. Over the study period, the cover of M. annularis declined from 41% in 1988, to ~12% by 1999 (a 72% decline) but remained unchanged statistically for the last five years of the study. Between 1988 and 2003, colony abundances declined by 57% (from 47 colonies/m² to 20 colonies/m²), and the losses were driven mostly by the death and fission of medium to large colonies (i.e., ≥ = 151 cm²). By 2003, the population had proportionally more small colonies (70% were ≤50 cm²) and fewer large colonies (3% were >250 cm²) than in 1988 (60% and 6%, respectively), and the changes in population structure had accelerated ≥=14% in terms of the rate of change in population size and the time necessary to attain equilibrium of colony size structure. Importantly, this analysis revealed an ongoing and imminent population decline coincident with the recent period of apparently stable coral cover. Fifty-year projections indicate the strong likelihood of extirpation of M. annularis at this particular site in St. John (in contrast to a continuation of constant low cover) and suggest that the 1988 population structure cannot be restored by recruitment. It is unlikely that the population decline will reverse until there is an amelioration of the conditions that kill individual colonies. [ABSTRACT FROM AUTHOR]

Comments on the role of the sea urchin, Diadema antillarum in boosting the number of juvenile corals in Caribbean Region reefs by reducing macroalgal cover. Comparison of the density of the sea urchin and the density of juvenile corals; Significance of the findings for reef conservation efforts.

Peyssonnelid algal crusts (PAC) have rapidly spread on coral reefs throughout the Caribbean since 2010 and have become dominant space holders in multiple locations. In 2019, PAC covered 31–86% of the shallow reefs (< 6-m depth) at two sites in St. John, US Virgin Islands, but within halos around aggregates of the echinoid, Diadema antillarum , PAC was absent and small corals were common. To test the hypothesis that the density of small corals is greater in Diadema halos versus on adjacent substrata, and that this effect is accentuated by high coverage of PAC, the shallow reefs of St. John were surveyed in July/August 2019 and January 2020. Densities of small corals (pooled among taxa) in Diadema halos were equal to, or greater than, densities on adjacent substrata, and were highest in Diadema halos adjacent to areas with high coverage of PAC. Where the cover of PAC was ≥ ~ 10%, the mean (± SE) density of small corals was 23.3 ± 2.1 colonies m−2 in Diadema halos, but was 67% lower at 7.5 ± 1.2 colonies m−2 on adjacent substrata; where the cover of PAC was < ~10%, densities of small corals were nearly identical in Diadema halos (9.1 ± 0.9 colonies m−2) and on adjacent substrata (9.0 ± 1.1 colonies m−2). Similar results were obtained when the analyses were repeated for the common corals, Porites spp., Siderastrea radians , and S. siderea. As Diadema halos remained in the same location for at least 6 months, they may function as refuges for coral recruitment in locations where PAC is becoming spatially dominant. • Peyssonnelid algal crusts (PAC) are rapidly occupying Caribbean reefs. • PAC overgrows corals and other organisms and deters coral settlement. • Grazing halos around sea urchins provide coral recruits with refuges from PAC. [ABSTRACT FROM AUTHOR]

On most tropical coral reefs, decades of disturbances have ratcheted down coral cover to create low abundance communities. In such a state, the reefs of St. John, US Virgin Islands, were hit by two Category 5 hurricanes in September 2017, yet the effects on two sites dominated by Orbicella annularis were minor in terms of coral cover. To explore the implications of this outcome, the fates of O. annularis colonies were determined from photoquadrats and used to prepare size‐based matrix models for the year preceding the storms and the four months bracketing the storms. The populations displayed contrasting dynamics from 1988 to July 2017, with coral cover declining from 43% to 5% at Yawzi Point but remaining at ~30% at Tektite. Over this period, colony sizes declined, with ≥82% having planar areas ≤50 cm2 (i.e., the smallest size class) by July 2017, and while densities declined from 47 to 8 colonies/m2 at Yawzi Point, they increased from 36 to 51 colonies/m2 at Tektite. Hurricanes Irma and Maria depressed coral cover by 1–4%, transitioned colonies into the smallest size class (>87% by November), killed 27% and 5% of the colonies in the smallest size class at Yawzi Point and Tektite, respectively, and depressed the 5‐yr intrinsic rate of population growth (λ) to 0.53–0.87. Twenty‐year projections suggested these demographic effects will not have ecologically meaningful impacts on population size, at least compared to projections initiated assuming Hurricanes Irma and Maria had not occurred. With low cover of O. annularis distributed among many small colonies, future disturbances may play more important roles in winnowing the few remaining host genotypes rather than further depressing coral cover. [ABSTRACT FROM AUTHOR]