Search

In marine benthic habitats, structural complexity is positively associated with diversity, and the area of hard surfaces modulates community structure. These principles are well developed on tropical coral reefs, where stony corals create complex surfaces that are favored for settlement. This study explores the relationship between substratum orientation and the distribution of arborescent octocorals on shallow reefs (

Ocean acidification is expected to affect coral reefs in multiple ways, in part, by depressing the calcification of scleractinian corals. To evaluate how coral communities will respond to ocean acidification, research into the effects on ecological processes determining community structure is now needed. The present study focused on corals utilizing soft tissues (i.e., mesenterial filaments) as agonistic mechanism, and evaluated their ability to compete for space under ocean acidification. Using aquarium-reared specimens in Monaco, single polyps of Galaxea fascicularis were paired with branch tips of Acropora hyacinthus to stimulate competitive interactions, which were evaluated through the production and use of mesenterial filaments in causing tissue damage under ambient (~ 600 μatm) and elevated pCO (~ 1200 μatm). At 1200 μatm pCO, and when paired with A. hyacinthus, the extrusion of mesenterial filaments from G. fascicularis occurred 2 days earlier than under ambient pCO, although ultimately the mesenterial filaments caused the same amount of tissue necrosis on A. hyacinthus under both pCO regimes after 7 days. This outcome supports the hypothesis that some kinds of competitive mechanisms utilized by scleractinian corals (i.e., mesenterial filaments) will be unaffected by short exposure to pCO as high as 1200 μatm.

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 ( 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.

Ocean acidification (OA), attributed to the sequestration of atmospheric carbon dioxide (CO 2 ) into the surface ocean, and coastal eutrophication, attributed in part to land-use change and terrestrial runoff of fertilizers, have received recent attention in an experimental framework examining the effects of each on coral reef net ecosystem calcification (G net ). However, OA and eutrophication in conjunction have yet to receive attention from the perspective of coral reef sediment dissolution. To address this omission, CO 2 and nitrate (NO 3 − ) addition experiments were performed in Mo'orea, French Polynesia. Incubation chambers were used to measure sediment G net during the day and night under three different [NO 3 − ] (0, 9.8, and 19.7 μM) that were nested within four separate constructed coral reef communities maintained at different P CO2 levels (417, 721, 1030, and 1333 μatm, respectively). P CO2 negatively affected sediment G net during the day and night, resulting in a shift to diel net dissolution at a P CO2 of 1030 μatm. Elevated NO 3 − alone, and the combination of NO 3 − and P CO2 , both negatively affected sediment G net at night. However, the response of G net to NO 3 − was less clear during the day, where diurnal sediment G net was enhanced under the combined treatment of elevated NO 3 − and P CO2 , resulting in no net effect of NO 3 − on sediment G net on diel timescales. Overall, these results show that ocean acidification represents a greater threat to the balance of calcification and dissolution in Mo'orea's back reef sediment communities than the potential impact of NO 3 − enrichment on relatively short timescales.

This study tested the hypothesis that intraspecific morphological plasticity within a scleractinian coral elicits differential responses to elevated PCO2 and temperature. In Mo'orea, French Polynesia, two short-term laboratory experiments (21 and 14 days) were conducted to test the effects of PCO2 (400 vs. 700 μatm), and PCO2 (400 vs 1000 μatm) combined with temperature (27.0 vs. 29.8 °C), on branches and plates of Porites rus. Experiments employed two irradiances (~ 1000 vs 200 μmol photons m− 2 s− 1), which characterized the microenvironments on the shallow fringing reefs where branching and plating morphologies are common, respectively. Calcification of both morphologies was insensitive to PCO2, as well as the combined effects of elevated PCO2 and temperature. Mean calcification rates were faster in high light than in low light for both morphologies, and biomass was greater in plates than branches in all treatments. Together, our results suggest P. rus is robust to increased PCO2 and high temperature within the constraints of the treatments applied. Morphological plasticity in this species does not mediate physiological resistance to low pH and high temperature.

Scleractinian corals on tropical reefs are exposed to many natural and anthropogenic disturbances, and while much is known about their responses to such conditions, it is unclear whether the responses will remain the same in a future affected by climate change and ocean acidification. To evaluate how one aspect of these effects — wound repair — might be influenced by ocean acidification, small colonies of massive Porites spp. from the back reef of Moorea, French Polynesia, were damaged to simulate the effects of single bites by corallivorous fishes, and healing was measured under contrasting P CO2 regimes. Using experiments lasting 19–20 d and employing superficial (2013) or deep (2014) lesions, the effects of damage were evaluated at ~ 400 μatm (ambient) and ~ 1000 μatm P CO2 (both at ~ 28.5 °C) using calcification and healing as dependent variables. Damage reduced calcification of massive Porites spp. in 2013, but not in 2014, and P CO2 had no effect on area-normalized calcification or healing in both years, although biomass-normalized calcification was reduced by high P CO2 in 2014. Overall these results reveal the physiological resilience of this functional group of corals to the extent of ocean acidification expected by the end of this century, and suggest that over this period small colonies will remain capable of recovery from minor damage arising from fish corallivory.

To evaluate the possibility that density-associated effects modulate octocoral abundance on a Caribbean coral reef, we tested the hypothesis that the density of octocoral recruits (colonies ≤ 4 cm tall) and adult colonies are positively associated on shallow reefs (≤ 14 m depth) in St. John, US Virgin Islands. Both life stages were censused for density at 8–10 sites along 7 km of shore in 2013 and 2014, and a correlative approach was used to evaluate the extent to which the densities were associated using sites as replicates. For 8 sites censused in both years, mean densities of adults (pooled among taxa) varied from 2.95 ± 1.16 colonies m− 2 to 20.60 ± 2.62 colonies m− 2 in 2013, and from 3.20 ± 0.75 colonies m− 2 to 13.00 ± 1.04 colonies m− 2 in 2014; for recruits, mean densities varied from 1.05 ± 0.34 colonies m− 2 to 4.25 ± 0.81 colonies m− 2 in 2013, and from 0.60 ± 0.31 colonies m− 2 to 1.44 ± 0.40 colonies m− 2 in 2014 (all ± SE). The most common taxa in both years among all sites were Antillogorgia spp., Gorgonia spp., and plexaurids. Density of recruits was significantly and positively correlated with population density of adult octocorals (pooled among taxa) and plexaurids in both years, and for Gorgonia spp. in 2013 (with a similar trend in 2014). Densities of recruits and adults of Antillogorgia spp. were not associated in either year. Together, these data suggest that densities of adult octocorals positively influence the density of co-occurring octocoral recruits, thereby potentially promoting population growth.

Coral reefs experience biologically-driven pCO2 oscillations that are predicted to become more extreme in magnitude and duration under ocean acidification (OA) regimes. Understanding the plasticity of responses in common reef-building corals to oscillations in pCO2 will allow for better predictions of their function in future seawater conditions. This study explored the effects of variation in seawater pCO2 on coral calcification using experiments conducted over one month between 9 April 2018 and 18 May 2018. Branches (~4-cm long) of Acropora retusa were sampled from colonies at 10-m depth on the fore reef of Mo'orea, French Polynesia (17° 28′ 53.9004" S, 149° 49′ 50.5992" W). We tested the hypothesis that depressed calcification caused by elevated pCO2 (~1000 μatm) is relaxed (i.e., calcification increases) upon return to ambient pCO2 (~400 μatm). Corals first were incubated in ambient or elevated pCO2 for 19 days, with the result that calcification integrated over this period was reduced by 31% under elevated pCO2. The same corals were then incubated at ambient pCO2 for 11 days, during which calcification was independent of the experimental pCO2 exposure history. Our results suggest that a quick relaxation of pCO2-depressed calcification in A. retusa following cessation of high pCO2 indicates that corals are capable of a reversible plastic response of calcification when confronted by pCO2 oscillations.

The recruitment of scleractinian corals to settlement tiles is widely used to infer relative rates of recruitment to natural reef surfaces. On tiles fixed approximately horizontally, the majority of corals settle to lower surfaces, and it is assumed that this distribution reflects the benefits of avoiding algal competition, grazers, and sedimentation on upper surfaces. Using settlement tiles with and without refuges (shallow depressions, 2–10 mm in width and depth), we tested the hypothesis that coral recruits are not found on the upper surfaces of smooth settlement tiles because these surfaces lack refuges suitable for small colonies. Tiles were deployed for 4 to 14 months over 2 years in near-horizontal orientations on shallow reefs (~ 5-m depth) of one Caribbean and six Pacific islands, and following collection, tiles were inspected for coral recruits. In eight of nine deployments and in both regions, densities of recruits in upward-facing refuges were indistinguishable from densities on lower surfaces, or were several-fold higher in upward-facing refuges. In the ninth deployment (lasting 14 months in the Caribbean) refuges were occluded by sediment and calcareous algae, and on these tiles densities of coral recruits were higher on the lower surface. Together, these results expand from a local-scale (Nozawa, 2008) to a global-scale the conclusion that the highest area-normalized density of coral recruits occurs in upward-facing refuges. An important implication of this finding is that coral recruitment on upward-facing natural reef surfaces will be increased by refuges that can be occupied by small corals.

We tested experimentally if irradiance can modulate the response of coral reef calcifiers to seawater warming and ocean acidification. Nubbins of the coral Acropora pulchra and individuals of the calcifying alga Hydrolithon reinboldii were incubated for 20 d under 2 irradiances (150 and 650 μmol quanta m− 2 s− 1) in a matrix of duplicate treatments crossing 2 temperatures (27.2 °C and 29.8 °C) with 3 pCO2 levels (400, 750 and 1100 μatm). To determine the effects of the treatments, net calcification was measured in A. pulchra and H. reinboldii, and biomass in A. pulchra. High temperature and low irradiance caused a significant decrease in coral net calcification, whereas only low irradiance resulted in a significant decrease in algal net calcification. The biomass of A. pulchra was affected significantly by pCO2 and light (separately and in synergy), with maximum biomass measured at 750 μatm pCO2 in 3 out of 4 combinations of light and temperature. Light intensity adds complexity to the response of reef calcifiers to ocean acidification through indirect effects on coral biomass, which will need to be considered in future studies.

We tested the effects of pCO2 on Seriatopora caliendrum recruits over the first 5.3 d of post-settlement existence. In March 2011, 11–20 larvae were settled in glass vials (3.2 mL) and incubated at 24.0 °C and ~ 250 μmol quanta m− 2 s− 1 while supplied with seawater (at 1.4 mL s− 1) equilibrated with 51.6 Pa pCO2 (ambient) or 86.4 Pa pCO2. At 51.6 Pa pCO2, mean respiration 7 h post-settlement was 0.056 ± 0.007 nmol O2 recruit− 1 min− 1, but rose quickly to 0.095 ± 0.007 nmol O2 recruit− 1 min− 1 at 3.3 d post-settlement, and thereafter declined to 0.075 ± 0.002 nmol O2 recruit− 1 min− 1 at 5.3 d post-settlement (all ± SE). Elevated pCO2 depressed respiration of recruits by 19% after 3.3 d and 12% overall (i.e., integrated over 5.3 d), and while it had no effect on corallite area, elevated pCO2 was associated with weaker adhesion to the glass settlement surface and lower protein biomass. The unique costs of settlement and metamorphosis for S. caliendrum over 5.3 d are estimated to be 257 mJ recruit− 1 at 51.6 Pa pCO2, which is less than the energy content of the larvae and recruits.

article i nfo Caribbean Hydrozoa Long-term Millepora In this study, the abundance of Millepora spp. on the shallow (b9-m depth) fringing reefs of St. John, US Virgin Islands, was measured from 1992 to 2008 using photoquadrats recorded annually. The objectives were to describe how a rare, yet ecologically important invertebrate has changed in abundance over 16 years, and evaluate the extent to which the changes were associated with seawater temperature and storm intensity. Millepora spp. covered ≤1.5% of the benthos throughout the study, but it increased in cover as much as 306% between some consecutive years, while decreasing 67% between other years. Overall, mean Millepora spp. cover declined 49% from 0.99±0.33% in 1992 to 0.51±0.15% in 2008, in part because colonies became 47% smaller through shrinkage and fission. The percentage cover, number of branches, and population den- sity (i.e., colonies m −2 )o fMillepora spp. decreased in years characterized by warm seawater and intense storm effects, but the size of Millepora spp. colonies increased in years characterized by large numbers of cold days. Changing abundances of scleractinian corals are well documented on coral reefs, but the present study is unusual in demonstrating strong temporal variation in abundance of a rare taxon, probably as a result of some of the same physical conditions affecting scleractinians. As scleractinian corals decline in abun- dance, the dynamics of rare taxa are likely to play increasingly important roles in the community structure and function of tropical reefs.

article i nfo The benthic community structure of the shallow reefs of St. John, US Virgin Islands, was studied from 1992 to 2007 to test the hypotheses that the abundances of non-scleractinian invertebrates have changed, and further, that the changes are associated with variation in the percentage cover of scleractinian coral and macroalgae. The study utilized photoquadrats (0.25 m 2 ) from fringing reefs (7-9 m depth) characterized by igneous

In this study, coral recruitment was measured on a kilometer-wide scale over two years on shallow (5–6 m depth) fringing reefs in St. John, US Virgin Islands, with the objective of determining the extent to which variation in recruitment was affected by biophysical coupling involving temperature and flow. Coral recruitment was measured using settlement tiles deployed at 10 sites along 10 km of shore. The tiles were first deployed in August 2006, and thereafter replaced every ≈ 6 months to sample from either August to January, or January to August over 2 years. Seawater temperature was recorded at the 10 sites using logging thermistors, and flow was quantified using drogues. Overall, corals recruited at a rate equivalent to 76 corals m− 2 6 months− 1, and were represented mostly by poritids (43% of recruits), agaricids (29%), faviids (17%) and siderastreids (7%). Although the density of recruits differed among sites in a pattern that varied among periods and years, there was a consistent trend for mean density to decline from ≈ 4 corals tile− 1 at eastern sites, to ≤ 1 coral tile− 1 at western sites. One aspect of seawater temperature – the daily range – differed among sites and was greater at western compared to eastern sites, and while it was related inversely to recruitment over one of the sampling periods, it was equivocal as a physical process affecting recruitment. Instead, our results are consistent with biophysical coupling involving patch depletion and downstream filtering, whereby patches of coral larvae are delivered to the south shore of St. John and depleted of larvae through settlement as the water progresses westward.

An opportunity to explore the effects of fluctuating temperatures on tropical scleractinian corals arose when diurnal warming (as large as 4.7 °C) was detected over the rich coral communities found within the back reef of Moorea, French Polynesia. In April and May 2007, experiments were completed to determine the effects of fluctuating temperature on Pocillopora meandrina and Porites rus , and consecutive trials were used to expose them for 13 days to 26 °C, 28 °C (ambient conditions), 30 °C, or a fluctuating treatment ranging from 26 to 30 °C over 24 h. The multivariate response was assessed using maximum dark-adapted quantum yield of PSII (F V /F M ), Symbiodinium density, chlorophyll-a content, and calcification. In trial 1, multivariate physiology of both species was significantly affected by treatments, with the fluctuating temperature resulting in a 17–45% decline in Symbiodinium density (relative to the ambient) matching that occurring at a constant 30 °C; F V /F M , chlorophyll-a content, and calcification, did not differ between the fluctuating and the steady treatments. In contrast, in trial 2 that utilized corals collected two weeks after those used in trial 1, the corals were unaffected by the treatments, likely due to an environment × trial interaction caused by seasonal declines in Symbiodinium density. Together, these results demonstrate that short transgressions to ecologically relevant high and low temperatures can elicit a potentially detrimental response equivalent to that occurring upon exposure to a constant high temperature. The dissimilar responses among dependent variables and consecutive trials underscore the importance of temporal replication and multivariate approaches in coral ecophysiology. It is likely that recent history has a stronger effect on the response of corals to treatments than is currently recognized.

article i nfo Coral larvae are selective with regards to the surfaces upon which they settle, but little is known about the outcome of these choices. In this study, we explored the implications for juvenile scleractinians (b40-mm diameter) of growing on igneous versus carbonate rock on the shallow reefs (5-m depth) of St. John, US Virgin Islands. Surveys revealed that juvenile corals occurred at densities of 16 colonies m −2 and were distributed on igneous and carbonate rocks in proportion to the abundance of these surfaces, suggesting that larvae do not discriminate between rock types at settlement. Repeated surveys demonstrated that all juvenile corals (i.e., pooled among taxa) grew 41% slower on igneous versus carbonate rock between January and August, but not between August and January when the growth was statistically indistinguishable between rock types. Although the growth of the most common juvenile coral, Porites astreoides, was similar on both substrata, the photophysiology of this species was affected by the type of rock. The maximum relative electron transfer rate (rETR, a proxy for photosynthesis) of P. astreoides was down-regulated 30% on igneous compared to carbonate rock. Phylogenetic analyses of the Symbiodinium community sequence profiles within P. astreoides revealed significant differences between substrata, with a greater diversity of co- occurring ITS-2 sequences in corals growing on carbonate compared to igneous rock. While substratum- dependent patterns in the characteristics of juvenile corals suggested there is selective value to the settlement choices made by larvae, these trends did not translate into differences in survival, at least over the time scale investigated. It remains uncertain what features of the rocks affected coral performance, but differences in the temperature of the rock may be an important feature during the warmest period of the year.

For many marine invertebrates, the maximum size of an individual is influenced heavily by environmental factors and may be limited by energetic constraints. In this study, an energetic model developed originally for anemones was applied to the free-living scleractinian Fungia concinna (Verrill) from Moorea (French Polynesia) to test the hypothesis that energetic constraints limit the size of this solitary coral. The modified model assumed that photosynthesis was the primary source of metabolic energy, and that metabolic costs were represented by aerobic respiration; these sources and sinks of energy were compared using daily energy budgets that were analyzed using double logarithmic regressions of energy against coral size. With this approach, energy limitation is characterized by a scaling exponent for energetic cost ( b cost ) that is larger than the scaling exponent for energy intake ( b intake ). For the size range of F. concinna studied, b intake = 0.73 ± 0.09 and b cost = 0.46 ± 0.10, thereby demonstrating that large individuals accumulated an energetic surplus, even when the expenditure associated with host tissue and symbiont growth was included in the model. The surplus of energy that this coral acquires as it grows appears to be driven by the scaling of traits associated functionally with the scaling of respiration and photosynthesis. Specifically, tissue biomass displayed a strong positive allometry with respect to surface area ( i.e. , b > 1), and this constraint on surface area may be the mechanistic basis of the low scaling exponent for metabolic cost. In contrast, the capacity for autotrophy – defined indirectly as Symbiodinium population density and chlorophyll content – increased isometrically with surface area, and likely contributed to the higher scaling exponent for intake relative to cost. Our results suggest that growth in F. concinna is not limited strictly by energy, but instead maximum size must be determined by alternative physiological or ecological constraints.

Mass transfer characteristics of scleractinian corals are affected by their skeletal morphology and the concentration gradients that develop as a consequence of the interactions of their morphology and biomass with the overlying seawater. These interactions can have a profound effect on coral metabolism. In this study, boundary layer characteristics were compared between different size colonies of the corals Dichocoenia stokesii and Stephanocoenia michilini to determine the relative roles of colony size and corallite structures (i.e. surface roughness) in mass transfer. Colonies of both species were rounded in shape, but differed in small-scale roughness as measured by the elevation of corallites. Additionally, D. stokesii had a greater aspect ratio than S. michilini, and their colonies were slightly taller for a given diameter. Boundary layers were characterized by placing dead coral skeletons in a flow tank and estimating shear velocities (u(*)) at different flow speeds. The effects of flow speed, size, and roughness on shear velocities were estimated for two juvenile size classes (10-20 and 30-40 mm diameter) of each species that were exposed to unidirectional flow regimes (4 and 17 cm s(-1)). Shear velocities were significantly greater in high, compared to low flow, and there was a significant interaction between colony size and surface roughness; the interaction was caused by a difference in magnitude, rather than direction, of the effect of roughness and size on u(*). Thus, there was a greater degree of turbulence at high flow compared to low flow, regardless of roughness or size, and the greatest turbulence occurred over large colonies of D. stokesii at high flow. Together, these results suggest that boundary layers around small corals are heavily influenced by upstream roughness elements, and more strongly affected by flow regimes than skeletal features. The relationship between colony morphology (i.e. aspect ratio and, possibly, surface roughness) and boundary layer characteristics may be non-linear in small corals.

Phenotypic plasticity has been documented in a number of reef coral species for a variety of morphological traits, but its ecological importance is not well understood. In the branching coral Madracis mirabilis (Duchassaing and Michelotti) spacing among branches varies across environmental gradients and in general is inversely related to the rate of water movement. This polymorphism is due in part to variation in branch diameter which is phenotypically plastic in this species. Branch spacing can affect biomechanical processes such as the surface roughness of aggregated branches and flow dynamics within an aggregate, both of which could affect colony metabolism. We examined the metabolic consequences of variation in branch spacing and flow speed on small M . mirabilis aggregates to determine if plasticity for this trait could be beneficial. The dark respiration of aggregates with different amounts of branch spacing was measured under three flow speeds (3.1, 4.7 and 8.4 cm s −1 ) in respiration chambers. Aggregates with the greatest branch spacing had the highest respiration rates in all three flow conditions and the respiration of each morphology increased with flow speed. Increased branch spacing may decrease the thickness of the diffusive boundary layer, thereby maintaining mass transport and hence high respiration rates in low flow. This and previous studies suggest that phenotypically plastic branch spacing may represent an important adaptation in M . mirabilis , enabling aggregates to tolerate a variety of flow conditions.