Introduction

Tropical coral reefs are renowned for being among the most biologically diverse and productive marine ecosystems, despite inhabiting just 0.1% of the ocean floor (Gattuso et al. 2014; Hughes 1994; Spalding et al. 2017). Scleractinian corals, however, are extremely vulnerable to environmental change and the continued deterioration of coral reefs threatens the wider ecosystem as well as the livelihoods of millions of people living in tropical coastal regions who depend on their persistence (Lesser and Farrell 2004; Mumby 2006; Mumby et al. 2007; Woodhead et al. 2019). Among the greatest threats to corals reefs is warming induced coral bleaching, which is responsible for significant mortality on the world’s reefs and is exacerbated by high solar irradiance (Coles and Jokiel 1977; Hoegh-Guldberg 1999; Hoegh-Guldberg et al. 2007). Shallow coral reefs, located between the depths of 0–30 m, are particularly vulnerable due to their proximity to natural and anthropogenic stressors. Yet, corals persist beyond this arbitrary depth limitation to deeper mesophotic reefs that exist between roughly 30 and 150 m, with records of photosynthetic corals found as deep as 172 m (Rouzé et al. 2021).

In recent years, mesophotic coral ecosystems (MCEs) have gained considerable interest due to their hypothesized potential to provide refuge for shallow water corals against disturbance (Bongaerts et al. 2010). However, long-term acclimation to environmental conditions on MCEs may also make these communities less resilient to future disturbance events, such as unprecedented thermal anomalies and disease outbreaks (Bongaerts et al. 2013; Smith et al. 2016). The physical environment of MCEs often includes sharp thermoclines, strong gradients in nutrients, and limited penetration of solar irradiance, all of which can impact the physiology of corals and determine their vertical distribution (Anthony and Larcombe 2000; Englebert et al. 2017; Lesser et al. 2009; Slattery and Lesser 2012). Thus, the maximum depth distribution of corals is location-specific (Laverick et al. 2020) and coral distribution is primarily driven by light availability (Carpenter et al. 2022; Tamir et al. 2019), as zooxanthellate corals rely on light-driven photosynthesis by symbiotic dinoflagellate algae in the genus Symbiodiniaceae to meet up to 90% of their metabolic demands (Muscatine and Porter 1977). To persist in MCEs, therefore, corals must adapt or acclimatize to the physical conditions present across depths through variability in morphology and physiology.

Several species that exist along a broad depth distribution have been shown to differ in skeletal morphology at different depths (Bruno and Edmunds 1997; Einbinder et al. 2009; Einbinder et al. 2009; Goodbody-Gringley et al. 2015; Goodbody-Gringley and Waletich 2018; Malik et al. 2020). For example, in Bermuda and the Red Sea, corals at increased depth had more laminar colony morphology, with smaller calyxes and greater distance between polyps compared to those at shallower depths (Goodbody-Gringley and Waletich 2018; Kramer et al. 2022; Malik et al. 2020). Likewise, changes in morphology and physiology such as nutrient acquisition, calcification and photosynthesis are reported to differ over increasing depth in the Gulf of Aqaba (Einbinder et al. 2009; Malik et al. 2020; Mass et al. 2007). These variations are often suggested to result from morphological variability in response to differing environmental conditions at depth, such as low light intensity (e.g., Goodbody-Gringley and Waletich 2018). For instance, Montastraea cavernosa exhibits divergent morphotypes associated with its feeding strategy: one morphotype features smaller polyps that are continuously open, optimized for light capture and daytime feeding, while the other has larger polyps that expand primarily at night, optimizing for zooplankton feeding (Lasker 1979, 1980, 1981). Despite the lack of conclusive evidence for cryptic speciation (Budd et al. 2012; Eckert et al. 2019; Goodbody-Gringley et al. 2012; Sturm et al. 2020), the distinct vertical distributions of these divergent morphotypes suggest an adaptation strategy to varying light conditions and depth-related environmental factors (Ruiz-Torres 2004).

At Little Cayman Island, benthic surveys linked coral community distribution and the boundary of shallow-mesophotic reefs with light availability and differences in coral photophysiology, with a distinct shift in coral community composition at 25–35 m and significant differences in quantum yield and maximum photosynthetic rate for two species (Porites astreoides and Montastraea cavernosa) across depth (Carpenter et al. 2022). Here, the differences in skeletal morphology, photophysiology, and symbiont physiology of the same two coral species were examined across depth to further examine phenotypic variability across a bathymetric gradient and explore the relationship between morphology and photophysiology in response to low light. A greater capacity to modify skeletal features along with symbiotic associations is expected to result in broader environmental tolerance, which may translate to higher long-term resilience to changing conditions.

Methods

Site and sample selection

The Cayman Islands are located 200 miles northwest of Jamaica and 150 miles south of Cuba in the Caribbean Sea. Little Cayman lies 80 miles northeast of Grand Cayman and is the smallest of the Cayman Islands with roughly 10 square miles of landmass and an estimated resident population of 160 people. For this study, two sites were selected that lie outside of the marine protected areas (MPAs), Martha’s Finyard (N19.66487, E80.11118) and Coral City (N19.68075, E80.02330) (Fig. 1A). Martha’s Finyard has a midday average photosynthetic active radiation (PAR; μmol/m−2 s−1) measurement of 566 μmol/m−2 s−1 at 12 m and 206 μmol/m−2 s−1 at 36 m, as measured between October 2021 and September 2022 (Fig. 1B). Nearly, 57% of the coastal waters in Little Cayman are classified as no-take MPAs. Both sites consist of sloping inner reef spur and groove formations leading to a near vertical fore reef wall which extends beyond 2000 m of depth.

Fig. 1
figure 1

A Map of survey sites in Little Cayman. B Weekly median photosynthetically active radiation (PAR; µmol/m2/s−1) from 17th of October 2021 to 22nd of September 2022

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Montastraea cavernosa is a gonochoric broadcast spawning coral that forms mounding boulders or encrusting plates and is recognized for having large polyps (Sebens et al. 1998). In the wider Caribbean, M. cavernosa has a vast depth distribution, can be found between 3 and 113 m and is also one of the most abundant corals at mesophotic depths (Reed 1985; Lesser et al. 2010). Porites astreoides, a fast-growing hermaphroditic brooding coral, rapidly colonizes reefs following disturbance (Green et al. 2008; Knowlton 2001). The species grows in mounding, encrusting or plate-like morphologies (Chornesky and Peters 1987), and can be found at depths around 5–40 m in the Cayman Islands (Carpenter et al. 2022).

Using a hammer and chisel, a small fragment (~ 5 × 5 cm) was removed from each of 8 colonies at 10, 20, 30, 40, and 50 m depth using closed-circuit rebreathers (Hollis Prism2, Hollis Rebreathers, Salt Lake City, UT, USA), between the 25th of February and the 2nd of March 2022. Samples were selected haphazardly with a minimum of 10 m between sampled colonies. Samples were then transported to the wet lab facility at CCMI in coolers where they were immediately frozen. Upon defrosting, the tissue of each frozen coral fragment was water picked using filtered seawater. A 50-ml aliquot of the total tissue slurry was transferred to centrifuge tubes and electrically homogenized for 20 s before centrifugation at 4,400 rpm for 8 min to separate the symbiotic algae from the host tissue. The pellet containing symbiotic algae cells was resuspended in 1 mL distilled water (DW); 50 μL was taken for algal cell count and 500 μL to measure chlorophyll a concentration. Algal cells were imaged on a hemocytometer grid using a Nikon Eclipse (Nikon Eclipse Ti–S Inverted Microscope System) microscope to excite the chlorophyll autofluorescence with blue light (n = 4 counts per sample). The color threshold of images was adjusted to isolate algal cells using ImageJ software, and the number of cells in four replicate corner squares were counted using the ‘Analyze Particles’ function. The chlorophyll aliquot was first centrifuged to remove remaining DW before extraction in 1 mL 100% acetone at 4 °C for 24 h. Sample absorbance was determined in a 96-well plate using the equations of Ritchie (2008). The planar surface area of the coral was measured using Fiji software (Schindelin et al. 2012). Acknowledging the inherent topographical complexity of coral structures, the most planar orientation of each fragment was selected prior to imaging. Symbiodiniaceae concentration per milliliter (cells/mL) was calculated by dividing the total Symbiodiniaceae cell count by the adjusted volume. To assess the distribution of Symbiodiniaceae, the density per square centimeter of surface area was determined by normalizing cells/mL to the surface area (cm2). Furthermore, chlorophyll content per zooxanthellae cell (µg/cell) was calculated by dividing the µg/mL by cells/mL. An additional metric, chlorophyll content per square centimeter (µg/cm2), was derived by dividing the µg/cell by the cell density per cm2 (Fig. S1).

The remaining skeletons were then bleached and dried overnight. A fragment from each colony (n = 8) was subsequently photographed using the Aven Mighty Scope 5 M Digital Microscope and scanning electron microscope (SEM) ZEISS SigmaTM SEM (Germany), by using an In-lens detector (5 kV, WD = 6–8 mm). For each fragment of M. cavernosa (Fig. 2A), and P. astreoides (Fig. 3A), corallite diameter, columella diameter, septa length, and distance between polyps were recorded at four random polyps, methods per Goodbody-Gringley and Waletich, (2018). Additionally, septa width was measured for both species, and the spike-like structures on the septa were counted exclusively for P. astreoides. Digital microscope photographs were scaled to mm using a transparent plastic ruler placed directly on the fragment and calibrated in the same area for each image using ImageJ software (Rasband 2018). Fiji (Schindelin et al. 2012) was then used to acquire polyp measurements.

Fig. 2
figure 2

A Colony and skeletal morphology of M. cavernosa with focus on the microstructures within the columella and how these change across depths. B Skeletal morphology measurements of M. cavernosa polyps at varying depths. The parameters measured include corallite diameter (n = 313), columella diameter (n = 312), septa length (n = 314), distance between polyps (n = 299), and septa width (n = 67). The significance of variation in these measurements across different depths is denoted by letters, with non-overlapping letters indicating significant variations (p < 0.05)

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Fig. 3
figure 3

A Colony and skeletal morphology of P. astreoides with focus on the spike-like microstructures visible on the septa. B Skeletal morphology measurements of P. astreoides (PAST) polyps taken across various depths. The evaluated parameters encompass corallite diameter (n = 493), columella diameter (n = 491), septa length (n = 480), distance between polyps (n = 495), septa width (n = 481), and septa spikes (n = 64). The significance of variation in these measurements across different depths is denoted by letters, with non-overlapping letters indicating significant variations (p < 0.05)

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Light and photophysiology

The peak daily values of light (PAR, 400–700 nm) data were collected using a PME miniPAR logger from Martha’s Finyard throughout the year at 12 and 36 m (Fig. 1B). A median of these peak values was then determined to represent typical exposure. To extend understanding to unmeasured depths, the following equations were utilized:

The light attenuation coefficient (Kd) was calculated using the logarithmic ratio of Photosynthetic Active Radiation (PAR) at two different depths (12 m and 36 m), according to the equation described by Kirk 1994:

$$Kd=\frac{1}{z}. \text{ln}\left(\frac{{E}_{36}}{{E}_{12}}\right)$$

where E12 and E36 are the known PAR values at 12 m and 36 m, respectively, and z is the difference in depths (36 m–12 m).

The vertical profile of light intensity within the water column was modeled using the exponential attenuation of light, derived from the Beer–Lambert law (Domingues and Barbosa, 2023):

$$I\left(z\right)= {I}_{0}\cdot {e}^{-Kd\cdot z}$$

where I(z) is the intensity of light at depth, I0 is the initial light intensity (PAR at 12 m), and Kd is the light attenuation coefficient calculated as above.

Photophysiology data were measured at Martha’s Finyard on the 10th of July 2023 and at Coral City on the 11th of July 2023. Due to the period between morphometric evaluations and photophysiological assessments, it was not possible to correlate data with the individual colonies. Active fluorescence was measured with a fluorescence induction and relaxation (FIRe) instrument, using the same techniques described in Carpenter et al. (2022), Goodbody-Gringley et al. (2024), and Gorbunov and Falkowski (2005). Phase 1 of the FIRe technique employs a robust short pulse with a 100-ms duration, known as a single turnover flash. This phase is used to cumulatively saturate Photosystem II (PSII) and assess various properties of the system. These include the quantum yield of photochemistry in PSII [Pmax (electron S−1 PSII−1)], the functional absorption cross section of PSII (σPSII(A2)) and the connectivity parameter (p), which indicates the likelihood of excitation energy transfer between individual photosynthetic units. Phase 2 of the process uses weak modulated light to observe the relaxation kinetics of fluorescence yield over a 500-ms duration. This measurement helps in determining the maximum photosynthetic rate (Pmax), expressed as electrons per second per PSII. The process considers the acclimation of different areas of a coral. For instance, areas that are pre-acclimated to low light (such as shaded parts) will yield more precise readings for physiology in dark or low-light states (e.g., Fv/Fm). Conversely, high-light acclimated areas (e.g., those exposed to sunlight) will more accurately reflect parameters adapted to high light, such as maximum photosynthetic turnover rates. To ensure a comprehensive analysis that accounts for intra-colony differences in light exposure, three readings were taken from each colony and averaged to represent the photophysiology of the entire colony. The study targeted between 5 and 7 colonies of each species at varying depths of 10, 20, 30, 40 and 50 m.

Statistical analysis

All data analyses and management were performed using the RStudio data analysis software (R Core Team, team 2022). Firstly, outliers were removed using the interquartile range. Data were then tested for normality using a Shapiro–Wilk test on residuals. Where non-normal distribution was identified by indication of a significant p value (p < 0.05), a common logarithm was used to transform data and re-tested. For normally distributed data, a one-way ANOVA was used to test between depths, and for non-normally distributed data, a Kruskal–Wallis test was used. Where significance was identified using the ANOVA or Kruskal–Wallis test (p < 0.05), the appropriate post hoc test in form of a Tukey’s Honestly Significant Difference test, or Dunn’s test as the nonparametric equivalent was performed. Photophysiology parameters were tested using Kruskal–Wallis and Dunn’s, per Carpenter et al. (2022). Due to equipment failure in the initial round of field work, photophysiology measurements do not correspond to the exact same individuals sampled for morphology and physiology; thus, medians of all measured parameters were taken across the depths and used in Spearman’s rank correlative analyses.

Results

Skeletal morphology

All skeletal features of M. cavernosa (Fig. 2A) were found to vary significantly across depths, including corallite diameter, columella diameter, septa length, septa width, and distance between polyps. Generally, these features increased in size, peaked around the median depth, and decreased there after (Fig. 2B; Table 1). Skeletal features of P. astreoides (Fig. 3A) also showed significant variations across depths, including corallite diameter, columella diameter, septa length, septa width, distance between polyps, and spikes per septa. All features showed a general increase in size with depth (Fig. 3B; Table 2).

Table 1 Significant variations in morphometrics and photophysiology of M. cavernosa
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Table 2 Significant variations in morphometrics and photophysiology of P. astreoides
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Photophysiology

All photophysiological parameters had significant variations across depths for M. cavernosa, including photosynthetic efficiency (Fv/Fm), functional absorption (σPSII), connectivity parameter (p), and maximum rate of photosynthesis [Pmax (electrons−1 PSII−1)] (Fig. 4; Table 1). For P. astreoides, Fv/Fm, σPSII, p, and Pmax all showed significant variation across depths (Fig. 4; Table 2).

Fig. 4
figure 4

Photophysiological measurements of M. cavernosa (n = 226 across all measurements) and P. astreoides (n = 161 across all measurements). The significance of variation in these measurements across different depths is denoted by letters, with non-overlapping letters indicating significant variations (p < 0.05)

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Zooxanthellae and chlorophyll

No significant variation across depths was identified in associated Symbiodiniaceae cells per cm2 or chlorophyll per symbiont cell for either M. cavernosa or P. astreoides (Fig. S1).

Correlation matrix

For M. cavernosa, PAR was found to have a significant negative correlation with depth and Chlorophyll a per Symbiodiniaceae, and a positive correlation with Pmax, while σPSII, and p had a significant positive correlation (Fig. 5A; Table 3). In P. astreoides, PAR had a signficant negative correlation with depth, Fv/Fm, σPSII, and p; and a positive correlation with Pmax. Depth positively correlated with Fv/Fm, σPSII, and p, but negatively correlated with Pmax; and Fv/Fm. A significant correlation was also found between σPSII and p, which both had significant negative correlations with Pmax. Symbiodiniaceae/cm2 negatively correlated with Pmax (Fig. 5B; Table 3).

Fig. 5
figure 5

Spearman's rank correlation coefficients for phenotypic variability in A Montastraea cavernosa and B Porites astreoides. The strength of positive correlations is indicated by a gradient of red shades, with darker tones representing stronger correlations, and the strength of negative correlations is depicted with a gradient of blue shades, where darker tones indicate stronger negative correlations

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Table 3 Significant correlations between environmental and physiological parameters for M. cavernosa and P. astreoides
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Discussion

Phenotypic variability refers to the capacity of an organism to exhibit differences in its morphological and/or physiological characteristics (Wagner and Altenberg 1996). This capacity to alter phenotype in response to change is an important attribute and a significant mechanism for determining distribution and abundance on a reef (Foster 1979). Skeletal features of corals are directly affected by environmental condition, with light being one factor influencing morphology (Todd 2008). Light attenuation decreases with depth, with 10% of surface light available in the upper mesophotic (30–60 m) and roughly 1% in the lower mesophotic (below 60 m), where it is hypothesized that organismal respiration is equal to symbiont photosynthesis (Lesser et al. 2009). Here, both M. cavernosa and P. astreoides showed differences in skeletal features and photophysiology across depth, suggesting high degrees of variability in response to limited light availability corresponding with previous work on these species (e.g., Goodbody-Gringley et al. 2015; Goodbody-Gringley and Waletich 2018). Likewise, several other studies have found similar results documenting correlative changes in skeletal morphology with decreased light availability for a variety of coral species (Kramer et al. 2022; Pisapia et al. 2012; Studivan et al. 2019). For example, four corals species in Indonesia showed variable polyp morphology along both a depth gradient and a turbidity gradient, suggesting that light was the main driver of morphological plasticity (Pisapia et al. 2012). Likewise, transplantation of the coral Goniastrea pectinata across depth in Singapore resulted in changes to polyp morphology that resulted in increased self-shading for shallow corals and increased light capture for deeper corals (Ow and Todd 2010). Finding similar patterns in light-driven changes to skeletal morphology, Kramer et al. (2022) used 3D simulations of light propagation and photosynthesis for the coral Stylophora pistillata to show that diverging morphotypes either enhance light-use efficiency or reduce UV damage, providing further evidence that light availability for photosynthesis is a key driver of morphological variability. A significant decline in maximum photosynthetic yield (Pmax) for both species beyond 30 m found here, accompanied by a shift in skeletal features for M. cavernosa beyond 40 m, where deeper colonies more closely resemble the shallowest colonies, may suggest that deeper colonies are less reliant on photosynthesis, potentially alternating to greater reliance on heterotrophy. Likewise, convergent morphologies also suggest that photoinhibition may be occurring in the shallowest colonies, indicating that the moderate light levels found at the 20–30 m depth ranges are optimal for photosynthesis in M. cavernosa, which also corresponds to the depth range where the species is most abundant (Carpenter et al. 2022). Porites astreoides, on the other hand, was previously not documented to occur deeper than 35 m in Little Cayman (Carpenter et al. 2022), suggesting that changes in skeletal morphology alone are not sufficient for the species to obtain adequate nutrients from photosynthesis in low light environments, which likely results in the limited depth distribution. Contrary to these findings, however, one colony of P. astreoides was documented to reside at 50 m during photophysiological measurements, which is evidence of the potential for this species to exist deeper, likely dependent on bottom topography affecting colony orientation toward the light.

The reduction in the maximum rate of photosynthesis (Pmax) at 30 m and beyond indicates that M. cavernosa may require alternative sources of energy other than photosynthesis to survive at depth. In Panama, an increase in heterotrophic feeding rates was noted across increasing depth, the degree of which was found to be dependent on polyp size, with corals featuring smaller polyps exhibiting significantly enhanced feeding rates (Palardy et al. 2005). Lesser et al. (2010) used stable isotope analyses to demonstrate that primary productivity in M. cavernosa sharply declines at upper mesophotic depths, indicating a shift to heterotrophy. The data presented here indicate a general decrease in polyp size beyond 30 m, which could be an adaptation to enhance food capture at greater depths, lending further support to the trophic shift hypothesis; yet, relying solely on polyp size might not accurately reflect trophic strategies (Lasker 1981; Lesser et al. 2000; Studivan et al. 2019). Previous analyses of M. cavernosa in Little Cayman found a decrease in δ13C with increasing depth, suggesting reduced reliance on autotrophy, but no corresponding increase in δ15N, which would indicate a transition to heterotrophy (Crandall et al. 2016). Alternatively, it is possible that beyond 40 m M. cavernosa has reduced energy available to allocate toward skeletal growth due to reduced photosynthesis, and the smaller polyp sizes seen at these increasing depths could therefore be due to reductions in energy rather than a switch to heterotrophy. Thus, while M. cavernosa is likely primarily phototrophic down to at least 60 m, the role of heterotrophy remains unclear. Regardless, the capacity of M. cavernosa to maintain higher photosynthetic efficiency and total photosynthetic output into the lower mesophotic zone compared to P. astreoides may explain its persistence and prevalence at greater depths.

Interestingly, differing responses in P. astreoides polyp morphology across depth were observed in Little Cayman, compared to Bermuda, where Scucchia et al. (2023) observed a decrease in corallite diameter but no variation in skeletal thickness or aragonite crystal morphology from shallow to mesophotic depths. These differences are likely due to variations in the environmental conditions at each location. The open ocean surrounding Bermuda is characterized by exceptional clarity leading to increased depth of the euphotic zone with the 10% PAR depth ranging from 30 to 60 m and the 1% depth reaching 120 m (Coates et al. 2013; Goodbody-Gringley et al. 2019; Siegel et al. 1995), while on the reef platform, 1% light availability reaches MCEs at 90–100 m. Although coral skeletal accretion is expected to cease below 20% of surface light, M. cavernosa and A. fragilis are documented below 70 m at the 5% light level, while P. astreoides is typically limited to < 60 m (Fricke and Meischner 1985). Furthermore, the topography of MCEs in Bermuda is characterized by a gradual slope from 30 to 60 m that results in orientation of benthic organisms toward the ocean surface (Goodbody-Gringley et al. 2019). In contrast, Little Cayman is surrounded by the deep Cayman Trench and is characterized by a spur and groove forereef zone that drops quickly to steep nearly vertical wall features. Vertical walls not only facilitate upwelling of nutrient-rich turbid waters that reduce water clarity, but also orient benthic organisms perpendicular to the ocean surface affecting the angle of incident light reaching the benthos that can result in vertical surfaces receiving just 25% of the irradiance that horizontal surfaces at the same depth are exposed to (Lesser et al. 2000). Thus, it can be expected that light availability on MCEs in Little Cayman is less than that in Bermuda. In fact, the maximum light availability is roughly 450 and 280 µmol/m2/s−1 at 30 m and 45 m depth in Bermuda, respectively (Goodbody-Gringley, unpublished data), compared to 206 µmol/m2/s−1 at 36 m in Little Cayman. These differential gradients in light availability across depth likely influence physiological and morphological responses leading to the different patterns found between the present study and those of Scucchia et al. (2023).

Light availability (PAR), known to impact light-driven photosynthesis dynamics (Roth 2014), was found to positively associate with maximum photosynthetic yield in both P. astreoides and M. cavernosa in this study, which is consistent with previous finding in Little Cayman (Carpenter et al. 2022). However, light availability negatively correlates with photosynthetic efficiency (Fv/Fm), functional absorption (σPSII), and the connectivity parameter (p), as well as Symbiodiniaceae cell density and chlorophyll concentration per symbiont in P. astreoides, suggesting that despite a lower total photosynthetic yield, deeper colonies are more efficient at transforming the limited light that is available into photosynthate. Although the majority of photophysiology metrics were not found to correlate with light availability for M. cavernosa, a negative correlation was found with chlorophyll concentration per symbiont cell, which may be indicative of an increase in photochemical conversion efficiency under high light intensities, possibly to mitigate photodamage on shallow reefs (Mass et al. 2007). These disparities between species-specific responses suggest unique cellular strategies to optimize energy production from available light, hinting at evolutionary adaptations to differing light environments (Neder et al. 2022).

In examining depth-related variations, this study discovered that M. cavernosa exhibits enhanced efficiency in photochemical conversion at 50 m, suggesting its capability to thrive at deeper depths where light intensity diminishes. Conversely, P. astreoides displays no significant alterations in Symbiodiniaceae density or chlorophyll concentration with depth. Consistent with Carpenter et al. (2022) and Warner et al. (2006), photosynthetic efficiency (Fv/Fm) and maximum photosynthetic yield (Pmax) remain notably higher for P. astreoides compared to M. cavernosa. However, contrary to findings by Carpenter et al. (2022), which reported no significant depth-related differences in Pmax for P. astreoides, this study shows significantly lower Pmax values at upper mesophotic depths. While Carpenter et al. (2022) reported an increase in Fv/Fm beyond 15 m for M. cavernosa, these findings revealed no significant differences between 10 and 40 m, albeit with significantly higher rates observed at 50 m. The consortia of Symbiodiniaceae species may differ across coral depth distributions, potentially increasing photosynthate transfer rates to the host (Bongaerts et al. 2013; Goulet et al. 2019), unfortunately, such assessments were not conducted in this study. Both M. cavernosa and P. astreoides generally exhibit genetic variability across depths at several locations within the Caribbean, which could impact genomic responses to environmental gradients (Serrano et al. 2016; Studivan et al. 2019; Sturm et al. 2020), warranting further investigation into coral host genomics and symbiont consortia.

Despite increased photosynthetic efficiency at 50 m for M. cavernosa, reliance on autotrophy as the primary source of nutrients under decreasing light availability suggests that a metabolic trade-off would be expected, such as reduced calcification or reproduction in deeper corals. Reproduction by M. cavernosa is documented to change along a depth gradient in the US Virgin Islands, where fecundity and oocyte size decreased with depth and the sex ratio became male dominated on mesophotic reefs (Bloomberg and Holstein 2021). While reproductive output by the confamilial species Orbicella faveolata was found to increase by over an order of magnitude on mesophotic reefs in the USVI compared to shallow reefs (Holstein et al. 2019), calcification was found to be significantly reduced (Groves et al. 2018). Fecundity of P. astreoides was also found to increase with depth in the USVI and Bermuda (Holstein et al. 2016; Goodbody-Gringley et al. 2018), and ex situ experiments showed increased fitness for mesophotic P. astreoides larvae compared to shallow larvae (Goodbody-Gringley et al. 2018, 2021). Porites astreoides exhibited a notable increase in calcification within the septa at increasing depths, as well as enhancement in the complexity of its internal microstructures, suggesting a depth-related adaptation in the calcification process. Similarly, there is a clear difference in the structure of the internal columella for M. cavernosa along the depth gradient in this study, where internal septa changes from being vertical to swirling in a spiral formation. There is also a reduction in the complexity and presence of centers of calcification. These findings suggest that the mechanisms of calcification are changing for both species with increased depth. Therefore, while the species may be able to persist at greater depths, the overall function of lower mesophotic M. cavernosa colonies may be altered due to limited energetic reserves.

Overall, these results highlight the range of phenotypic variability that exists within corals, including skeletal formation, photophysiology, and symbiotic associations. Both M. cavernosa and P. astreoides have highly variable skeletal morphologies. Differences in variability and overall capacity to efficiently use light in a light-limited environment may help explain differences in the vertical distribution of these two coral species along a depth gradient. Montastraea cavernosa and P. astreoides are considered more resilient to disease and temperature stress compared to other Caribbean species such as Acropora spp. and Orbicella spp., which have experienced significant declines in recent years (Aronson and Precht 2001; Edmunds and Elahi 2007). Porites astreoides thrives in a variety of habitats and temperatures, indicating that acclimatization may be occurring (Gallery et al. 2021). In recent years, P. astreoides has become a dominant species on Caribbean reefs, with increasing relative abundance as other coral species have declined (Gallery et al. 2021; Goodbody-Gringley and Manfrino 2020; Green et al. 2008). This suggests that P. astreoides may be able to survive and even thrive in shallow water ecosystems despite human-induced disturbance; however, the species may lack the ability to reside at depths beyond 35 m (Carpenter et al. 2022; Holstein et al. 2016). In contrast, M. cavernosa is noted to have an established population on mesophotic reefs, potentially due to the lack of competition from shallow-water specialist coral species that cannot adapt. As the impacts of climate change continue to intensify, understanding the physical and physiological factors that lead to species distributions will aid in predictions of future persistence and help uncover the potential for MCEs to serve as refuges.