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Growth rates (linear extension) reported for all zooxanthellate corals at depths > 40 m and shallow-water agariciids (Vaughan 1915; Mayor 1924; Ma 1937; Bak 1976; Wellington 1982; Glynn and Wellington 1983; Wellington and Glynn 1983; Hubbard and Scaturo 1985; Hughes and Jackson 1985; Huston 1985; Guzmán and Cortes 1989; de Villiers et al. 1994; Crabbe 2009; Manzello 2010). The recently reported growth rates for Leptoseris spp. in Hawaii (this study and Kahng 2013) and for Agaricia grahamae in the Caribbean (Bongaerts et al. 2015b) have substantially altered the knowledge and expectations for potential coral growth rates at depth. For studies with multiple conspecies growth rates reported at the same depth, only maximum extension rate is shown. References for mesophotic growth rates given in Table 1
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8 Abstract The ecology of phototrophic corals in the lower 9 photic zone remains poorly understood. Studies to date 10 indicate that growth rates generally decrease as available 11 light attenuates with depth and are very slow at depths [ 12 40 m. Here, we provide detailed evidence for moderate 13 growth for obligate zooxanthellate corals at extrem...
Citations
... R. Soc. B 291: 20231534 [3,38,57]. Hawaiian Leptoseris colonies probably invest heavily in skeletal extension to increase lateral growth relative to thickness due to the stable environment of their deep habitat that is minimally impacted by wave action [38,45]. Low metabolic demand for carbon resources [16,28], coupled with increased nitrogen exchange, may enable Leptoseris to survive and grow in light-limited conditions. ...
In mesophotic coral ecosystems, reef-building corals and their photosynthetic symbionts can survive with less than 1% of surface irradiance. How depth-specialist corals rely upon autotrophically and heterotrophically derived energy sources across the mesophotic zone remains unclear. We analysed the stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values of a Leptoseris community from the ‘Au‘au Channel, Maui, Hawai‘i (65–125 m) including four coral host species living symbiotically with three algal haplotypes. We characterized the isotope values of hosts and symbionts across species and depth to compare trophic strategies. Symbiont δ¹³C was consistently 0.5‰ higher than host δ¹³C at all depths. Mean colony host and symbiont δ¹⁵N differed by up to 3.7‰ at shallow depths and converged at deeper depths. These results suggest that both heterotrophy and autotrophy remained integral to colony survival across depth. The increasing similarity between host and symbiont δ¹⁵N at deeper depths suggests that nitrogen is more efficiently shared between mesophotic coral hosts and their algal symbionts to sustain autotrophy. Isotopic trends across depth did not generally vary by host species or algal haplotype, suggesting that photosynthesis remains essential to Leptoseris survival and growth despite low light availability in the mesophotic zone.
... However, almost nothing is known about metabolic demand including rates of calcification and biomass production, and whether they are proportionally reduced at these depths. Below 60 m, only a few studies have measured growth rates, and no measurements of calcification exist (Fricke et al. 1987;Kahng 2013;Bongaerts et al. 2015;Kahng et al. 2020). In Hawaii, the growth rates (radial extension) of deep-water (70-110 m) obligate photosynthetic corals from the genus Leptoseris were recently demonstrated to be surprisingly robust (0.8-2.5 cm yr −1 ) despite the low light in situ environment (Kahng 2013;Kahng et al. 2020). ...
... Below 60 m, only a few studies have measured growth rates, and no measurements of calcification exist (Fricke et al. 1987;Kahng 2013;Bongaerts et al. 2015;Kahng et al. 2020). In Hawaii, the growth rates (radial extension) of deep-water (70-110 m) obligate photosynthetic corals from the genus Leptoseris were recently demonstrated to be surprisingly robust (0.8-2.5 cm yr −1 ) despite the low light in situ environment (Kahng 2013;Kahng et al. 2020). Leptoseris spp. ...
... The bleached skeletons of three deep water Leptoseris samples (Leptoseris sp. 1 at 70 m, Leptoseris cf. foliosa at 95 m, and Leptoseris hawaiiensis at 111 m) were previously collected from Maui, Hawaii and measured for linear extension rates using Uranium-Thorium radiometric dating (Kahng et al. 2020). To measure for planar area, mass, and density, two fragments (on average ~ 28 cm 2 each) of each coral sample were cut to remove any portion that was non-living at the time of sampling, subject to secondary calcification from epiphytic and epifaunal organisms, or subject to bioerosion. ...
Photosymbiotic corals inhabit depths 0–170 + m and form the foundation of coral reef ecosystems by creating complex habitats with their calcium carbonate skeletons. While well studied in shallow waters, almost nothing is known about their basic biology & ecology at depths > 60 m. Here, we report on the first measurements of skeletal density and coral calcification rates from Leptoseris spp. growing at depths of 70–111 m in Hawaii. These corals have very thin, nonporous, skeletons that are considerably denser (2.7 g cm⁻³) than most shallow water corals. Their calcification rates (0.042–0.085 g cm⁻² yr⁻¹) are the lowest ever reported for a photosymbiotic scleractinian coral and ~ 20–40 times lower than the dominant shallow water corals in Hawaii. Given their colony geometry, calcification rate and tissue biomass productivity (per unit area) are tightly coupled, and a constant calcification rate leads to an increasing radial extension rate with colony size. These growth parameter relationships contrast sharply with hemispheroidal colonies in shallow water. Despite their extremely low calcification rates, these corals are very productive at increasing planar area over time, which is consistent with their phototrophic strategy at depth.
... To calculate surface area productivity or SAP ∆ = , the calcification equations was solved for rt+1 and substituted into the equation where surface area S = πr . The effect of skeletal thickness (h) on SAP was calculated within the range of values empirical values reported(Kahng et al. 2020). ...
Clonal organisms like reef building corals exhibit a wide variety of colony morphologies and geometric shapes which can have many physiological and ecological implications. Colony geometry can dictate the relationship between dimensions of volume, surface area, and length, and their associated growth parameters. For calcifying organisms, there is the added dimension of two distinct material components of growth, biomass production and calcification. For reef building coral, basic geometric shapes can be used to model the inherent mathematical relationships between various growth parameters and how colony geometry determines which relationships are size-dependent or size-independent. Coral linear extension rates have traditionally been assumed to be size-independent. However, even with a constant calcification rate, extension rates can vary as a function of colony size by virtue of its geometry. Whether the ratio between mass and surface area remains constant or changes with colony size is the determining factor. For some geometric shapes, the coupling of biomass production (proportional to surface area productivity) and calcification (proportional to volume) can cause one aspect of growth to geometrically constrain the other. The nature of this relationship contributes to a species’ life history strategy and has important ecological implications. At one extreme, thin diameter branching corals can maximize growth in surface area and resource acquisition potential, but this geometry requires high biomass production to cover the fast growth in surface area. At the other extreme, growth in large, hemispheroidal corals can be constrained by calcification. These corals grow surface area relatively slowly, thereby retaining a surplus capacity for biomass production which can be allocated towards other anabolic processes. For hemispheroidal corals, the rate of surface area growth rapidly decreases as colony size increases. This ontogenetic relationship underlies the success of microfragmentation used to accelerate restoration of coral cover. However, ontogenetic changes in surface area productivity only applies to certain coral geometries where surface area to volume ratios decrease with overall colony size.
... However, the relationship between coral growth rate and depth is not straightforward in all species. Recently, Kahng et al. (2020) reported linear extension rates for Leptoseris at depths of 70 to 110 m, similar to those in shallow, well-lit environments (i.e. 8 to 25 mm yr -1 ). Although this reflects horizontal rather than vertical growth, sustained coral growth at mesophotic depths does suggest that some species adapted to low PAR can compensate the effect of bioerosion and maintain their reefbuilding potential. ...
... However, direct comparison with the growth potential of modern coralgal frameworks may not be appropriate since the mesophotic reef section of Hole 40A is a microbialite boundstone that has no known equivalent today. In coral-dominated reef frameworks, although the growth rate of non-branching corals tends to decrease with depth (Pratchett et al., 2015), those of species well adapted to mesophotic conditions, such as Leptoseris, can remain high and contribute to increase the vertical accretion potential of mesophotic reefs (Kahng et al., 2020). Mesophotic reef growth continued until at least 24 ka. ...
Reef communities at intermediate (10 to 30 m) and mesophotic (~ 30 to 150 m) depths occupy large areas of sea floor but little is known about their potential to accrete vertically, their response to sea-level change and other environmental perturbations. In this study, the authors have examined cores from two holes, M0040A and M0041A, drilled by the International Ocean Drilling Program Expedition 325 along the shelf edge of the modern GBR at 131 m water depth. The objective was to investigate reef growth history at palaeo-water depths > 20 m over a time period spanning 30,000 years, from the end of the Last Glacial period through the last deglaciation. Based on changes in lithologies and biotic components, and a robust chronostratigraphic framework supported by 47 radiometric ages, the authors have identified two episodes of reef growth, one during Marine Isotope Stage (MIS) 2 and the other at the onset of the deglaciation, both characterised by abundant microbialite crusts and distinct coral assemblages. Palaeo-water depths range from 30 to 60 m and from 20 to 30 m for the MIS 2 and early deglacial reef sections, respectively. The first episode of reef growth documented in the cores initiated at 27 to 25 ka, possibly in response to increased light availability and change in sedimentation resulting from falling sea-level between 32 and 29 ka from MIS 3 and also to low atmospheric pCO2 at the end of the Last Glacial period. Reef accretion was reduced or ceased sometime between 24 and 19 ka, coinciding with the rapid 20 m sea-level fall of the peak Last Glacial Maximum and minima of SSTs. Reef growth resumed at 19.5 to 18.5 ka, influenced by a period of moderate sea-level rise and increasing sea surface temperatures at the onset of the deglaciation. Reef growth termination at ca. 17 ka correlates with a major episode of reef demise previously identified in adjacent mid and outer terrace cores and linked to reduced water quality combined with rapid deglacial sea-level rise. Vertical accretion (VA) rates were calculated based on two methods: linear visual fitting and Bayesian modelling. The findings show that the highest VA rates are associated with microbialite boundstone. Reef ecosystems dominated by microbialite and corals developed at intermediate and mesophotic depths, and grew vertically at maximum rates of 2 to 5 mm yr-1 depending on the method used, over a period of rapid environmental change during the transition from MIS 3 to MIS 1. Further study needs to explore the potential of modern-type deep coralgal communities to cope with higher rates of sea-level rise predicted this century.
... In addition, coral morphology can change in response to irradiance levels. In low irradiances, for example, corals increase their area-to-volume ratio by shifting to a flattened or plate-like morphology, which is considered energetically more efficient for the capture of incident light when its availability is low 34,35 . Thus, modular photosynthetic corals can regulate their internal light regime by varying the extent of self-shading surface on the colony scale towards a photosynthetic optimum 8,20,34,36 . ...
The morphological architecture of photosynthetic corals modulates the light capture and functioning of the coral-algal symbiosis on shallow-water corals. Since corals can thrive on mesophotic reefs under extreme light-limited conditions, we hypothesized that microskeletal coral features enhance light capture under low-light environments. Utilizing micro-computed tomography scanning, we conducted a novel comprehensive three-dimensional (3D) assessment of the small-scale skeleton morphology of the depth-generalist coral Stylophora pistillata collected from shallow (4–5 m) and mesophotic (45–50 m) depths. We detected a high phenotypic diversity between depths, resulting in two distinct morphotypes, with calyx diameter, theca height, and corallite marginal spacing contributing to most of the variation between depths. To determine whether such depth-specific morphotypes affect coral light capture and photosynthesis on the corallite scale, we developed 3D simulations of light propagation and photosynthesis. We found that microstructural features of corallites from mesophotic corals provide a greater ability to use solar energy under light-limited conditions; while corals associated with shallow morphotypes avoided excess light through self-shading skeletal architectures. The results from our study suggest that skeleton morphology plays a key role in coral photoadaptation to light-limited environments.
... Some of the most common scleractinian species at steep slopes or near-vertical walls were laminar Leptoseris solida, L. hawaiiensis, Echinophyllia aspera, Oxypora echinata and Cycloseris wellsi, whereas P. "speciosa" and Porites rus were associated with more moderate slopes at 40 and 60 m. The dominance of large laminar/plating corals at mesophotic depths has been previously reported in the Caribbean and the Indo-Pacific (Bouchon, 1983;Faure and Laboute, 1984;Hoeksema et al., 2017;Hopley et al., 2007;Kahng et al., 2014Kahng et al., , 2010Kahng and Kelley, 2007;Kühlmann and Chevalier, 1986;Pyle et al., 2016;Rooney et al., 2010;Zlatarski and Estalella, 1982), and hypothesised to be related to potential physiological adaptations inherent to the host (Kahng et al., 2020), or acquired through symbiosis (Gonzalez-Zapata et al., 2018). For example, the observed skeletal geometry in laminar Leptoseris and Montipora species confers a higher Figs. ...
... 6 and 7). efficiency in light harvesting (Kahng et al., 2012), promoting moderate growth rates of these genera (Kahng et al., 2020), which could result in advantage while recruiting at mesophotic depths. Further, potential symbiosis with the endolithic green algae Ostreobium, observed as part of the benthic community in some of the positive outliers (e.g., outliers III, IV, VII, VIII, IX; Sup. ...
The rapid decline of shallow coral reefs has increased the interest in the long-understudied mesophotic coral ecosystems (MCEs). However, MCEs are usually characterised by rather low to moderate scleractinian coral cover, with only a few descriptions of high coral cover at depth. Here, we explored eight islands across French Polynesia over a wide depth range (6 to 120 m) to identify coral cover hotspots at mesophotic depths and the co-occurrent biotic groups and abiotic factors that influence such high scleractinian cover. Using Bayesian modelling, we found that 20 out of 64 of studied deep sites exhibited a coral cover higher than expected in the mesophotic range (e.g. as high as 81.8 % at 40 m, 74.5 % at 60 m, 53 % at 90 m and 42 % at 120 m vs the average expected values based on the model of 31.2 % at 40 m, 22.8 % at 60 m, 14.6 % at 90 m and 9.8 % at 120 m). Omitting the collinear factors light-irradiance and depth, these 'hotspots' of coral cover corresponded to mesophotic sites and depths characterised by hard substrate, a steep to moderate slope, and the dominance of laminar corals. Our work unveils the presence of unexpectedly and unique high coral cover communities at mesophotic depths in French Polynesia, highlighting the importance of expanding the research on deeper depths for the potential relevance in the conservation management of tropical coral reefs.
... Furthermore, a decrease in growth rates with increases in water depth has also been detected on gorgonian corals [57] and algae [58], among other photosynthetic organisms. Therefore, an increase in depth (and consequently a decrease in light availability) will cause a decrease in photosynthesis and growth rates in most zooxanthellate corals due to a reduction in the resources allocated to growth [56,59]. However, see contrasting results for example from Torres et al. [60] where A. cervicornis colonies were transplanted to deeper zones grew more apparently due to a reduction in UV radiation causing a significant reduction in the production of UV-absorbing compounds and consequently a higher availability of resources for photosynthesis and growth. ...
Populations of Acropora cervicornis, one of the most important reef-building corals in the Caribbean, have been declining due to human activities and global climate change. This has prompted the development of strategies such as coral farms, aimed at improving the long-term viability of this coral across its geographical range. This study focuses on comprehending how seawater temperature (ST), and light levels (LL) affect the survival and growth of A. cervicornis fragments collected from three reefs in Culebra, Puerto Rico. These individuals were fragmented into three pieces of the similar sizes and placed in farms at 5, 8, and 12 m depth. The fragments, ST and LL were monitored for 11 months. Results show that fragments from shallow farms exhibit significantly higher mortalities when compared to the other two depths. Yet, growth at shallow farms was nearly 24% higher than at the other two depths. Corals grew fastest during winter, when temperature and LL were lowest, regardless of the water depth. Fragment mortality and growth origin were also influenced by reef origin. We conclude that under the current conditions, shallow farms may offer a slight advantage over deep ones provided the higher growth rate at shallow farms and the high fragment survival at all depths.
... Houlbrèque & Ferrier-Pagès, 2009). However, since heterotrophic feeding is thought to be species specific, the strategy of increased reliance on heterotrophy versus autotrophy with depth does not appear to be a primary trophic strategy for some depth generalists, and particularly not in deep-water specialists (Kahng et al., 2020). The photosynthetic and growth efficiencies of the strictly mesophotic Leptoseris species, for example, were shown to be facilitated mainly by their skeletal optical geometry (Kahng et al., 2020). ...
... However, since heterotrophic feeding is thought to be species specific, the strategy of increased reliance on heterotrophy versus autotrophy with depth does not appear to be a primary trophic strategy for some depth generalists, and particularly not in deep-water specialists (Kahng et al., 2020). The photosynthetic and growth efficiencies of the strictly mesophotic Leptoseris species, for example, were shown to be facilitated mainly by their skeletal optical geometry (Kahng et al., 2020). ...
Sustained light‐dependent coral reef communities can be found at a wide range of light environments, extending from the sea level to as deep as 150 m (i.e. esophotic). How mesophotic corals thrive despite extremely limited light conditions still requires further investigation.
Here, we undertook a comprehensive ecophysiological and bio‐optical study on four depth‐generalist coral species aiming to delineate the functional role that optical trait properties have in light‐harvesting, at contrasting light regimes.
We show that the optical traits of coral skeletons are adjusted to their ambient light conditions and complement the microalgal demands for sufficient light, thus exhibiting a spatially efficient photosymbiotic system.
In contrast to shallow corals, mesophotic corals absorbed up to three fold more light, resulting in excellent photosynthetic response under light conditions of only ~3% of the incident surface irradiance. The enhanced light‐harvesting capacity of mesophotic corals was achieved by redistributing light in the coral skeleton through optical scattering, thereby facilitating light transport and absorption by densely pigmented host tissue.
Our findings provide fundamental insight into the light‐harvesting mechanisms underlying the productivity of mesophotic coral reef ecosystems, yet also raise concerns regarding their ability to withstand prolonged environmental disturbances.
A free Plain Language Summary can be found within the Supporting Information of this article.
... Degrees of freedom (Df), not available (NA, the test statistic was negative). . Leptoseris can span the whole photic zone as seen throughout many Indo-Pacific reefs (Kahng et al., 2019), thriving with little light as a result of the optical geometry of their thin skeletons allowing for efficient light absorption while minimizing calcification (Kahng et al., 2020). While we acknowledge light is a primary factor controlling the species zonation along the depth gradient , we suggest other factors play a role such as substrate type (e.g. ...
... This suggests limited modern carbonate/relief production. Horizontal linear extension rates in the GoA of Leptoseris (0.2-0.8 mm/yr; Fricke et al., 1987), the coral genera most commonly associated with MR features, and generally thin skeletal plate calcification (Kahng et al., 2020) would prevent accretion rates needed to create the recorded MR relief during the past~6000 years that sea level has remained relatively stable (Shaked et al., 2002). Even if GoA Leptoseris grew as rapidly as on Hawaiian mesophotic reefs (Kahng et al., 2020), current abundances atop MR features could not have produced notable topographic relief in 6000 years. ...
... Horizontal linear extension rates in the GoA of Leptoseris (0.2-0.8 mm/yr; Fricke et al., 1987), the coral genera most commonly associated with MR features, and generally thin skeletal plate calcification (Kahng et al., 2020) would prevent accretion rates needed to create the recorded MR relief during the past~6000 years that sea level has remained relatively stable (Shaked et al., 2002). Even if GoA Leptoseris grew as rapidly as on Hawaiian mesophotic reefs (Kahng et al., 2020), current abundances atop MR features could not have produced notable topographic relief in 6000 years. As such, it is unknown if some or all of these outcrops were originally built during the last~6000 years at mesophotic depths by benthic organisms with different abundances than what is presently available. ...
Antecedent topography such as relic reef terraces as well as biogenic carbonate relief-forming deposits ~30–150 m deep, referred to as mesophotic reefs, provide structural support for diverse mesophotic coral ecosystems (MCEs) that may serve as coral refuges for select light-dependent species. Although terraces at mesophotic depths are found globally, an understanding of their spatial distribution, formation, and relationship with living community composition and lithology is generally lacking. Herein, 2 × 2 m resolution bathymetry from the Gulf of Aqaba (GoA) was examined to define geomorphology features spanning mesophotic depths and compare geomorphology relationships to overlying benthic and lithologic cover. Analysis led to the production of a new map categorizing 12 geomorphology features, including upper mesophotic terraces harboring thriving MCEs. Additionally, a large collection of still imagery (1726 pictures) was obtained at 94 sites and used to define eight unique habitats at mesophotic depths and lithological and biological distribution patterns over vertical and horizontal scales. Study area benthic and lithologic cover was found to be significantly different between geomorphology features and related to GoA geomorphology as well as to seafloor depth and slope, and light attenuation. While these relationships indicated modern cover could not provide a model for producing most underlying geomorphology in the study area, results provided data needed to enhance understanding of geomorphology feature formation history and reef accretion at mesophotic depths. Study results also detailed benthic cover and geomorphology features critical for better identifying and mapping unknown MCE habitats, and for recognizing mesophotic reef spatial relationships and biodiversity patterns in the GoA. These results are especially important considering most northern GoA reefs act as potential refuges, but local anthropogenic development continually stresses shallow GoA reefs and most other shallow coral reefs around the globe continue to degrade.
Clonal organisms like reef building corals exhibit a wide variety of colony morphologies and geometric shapes which can have many physiological and ecological implications. Colony geometry can dictate the relationship between dimensions of volume, surface area, and length, and their associated growth parameters. For calcifying organisms, there is the added dimension of two distinct components of growth, biomass production and calcification. For reef building coral, basic geometric shapes can be used to model the inherent mathematical relationships between various growth parameters and how colony geometry determines which relationships are size-dependent or size-independent. Coral linear extension rates have traditionally been assumed to be size-independent. However, even with a constant calcification rate, extension rates can vary as a function of colony size by virtue of its geometry. Whether the ratio between mass and surface area remains constant or changes with colony size is the determining factor. For some geometric shapes, the coupling of biomass production (proportional to surface area productivity) and calcification (proportional to volume) can cause one aspect of growth to geometrically constrain the other. The nature of this relationship contributes to a species’ life history strategy and has important ecological implications. At one extreme, thin diameter branching corals can maximize growth in surface area and resource acquisition potential, but this geometry requires high biomass production to cover the fast growth in surface area. At the other extreme, growth in large, hemispheroidal corals can be constrained by calcification. These corals grow surface area relatively slowly, thereby retaining a surplus capacity for biomass production which can be allocated towards other anabolic processes. For hemispheroidal corals, the rate of surface area growth rapidly decreases as colony size increases. This ontogenetic relationship underlies the success of microfragmentation used to accelerate restoration of coral cover. However, ontogenetic changes in surface area productivity only applies to certain coral geometries where surface area to volume ratios decrease with colony size.
