The Physical Basis of Coral Reefs

Maureen Awad, Evan Chow, Luke Gailloux, Rylee McKay, Adrian Yehia

Abstract

The physical complexity of the coral reef cannot be overstated. The term coral itself is loosely spread across several phylogenies and is still rooted in scientific debate. This paper aims to characterize and analyse two specific variants of coral from a physical context: soft and stony. Even with such a simple anatomy, the dynamics that define each polyp’s development, tissue thickness, and shape in response to environmental stimuli, specifically pH, ambient temperature, and tidal forces, is relentlessly complicated. The development and organization of the reef also strongly relates to environmental stimuli, leading to four recognized archetypes. As heterotrophic invertebrates, coral depend on a mutualistic relationship with the Dinoflagellate genus Symbiodinium. This relationship defines the health of a coral reef, and disputes can lead to the bleaching and consequent death of the entire biome. Soft coral, though they do not form reefs, can be an important part of the reef community and exhibit unique physical properties with their pulsation pattern. The structural basis of the reef itself, the aragonite crystal, is an extraordinary result of carefully monitored biomineralization. Mechanical integrity properties, such as fracture and compressive strength, of the biomaterial depends on polyp orientation and organic inclusions. This leads to emergent properties, such as anisotropy, at the microscopic and macroscopic level.

Introduction

Coral reefs have been described as the “rainforests” of the sea, heralding over 93,000 endemic species and ranking second in biodiversity planetwide, following tropical forests. Both biomes are crucial carbon sink in Earth’s carbon cycle, which will be increasingly relevant with our current strife with climate change. Despite facing a multitude of anthropogenic threats – pollution, deforestation, overfishing, and global warming – both biomes continue to persist at a global scale. That is, however, where the similarities largely end. While the tropical rainforests of Brazil, Indonesia, and Sub-Saharan Africa are characterized by thousands of independent trees, the coral reefs of Australia and Mexico are defined by their super organismal colonial structure. Some examples of the colonial nature of the coral biome are displayed the Figure 1 shown below.

Figure 1 Examples of the coral reef biome in A) Hurgada, Red Sea, Egypt, B) Marsa Alam, Red Sea, Egypt; C & D) Weizhou Island, China (Mohammed, Chen, & Abd-Elgawad, 2021).

As the basis for such complex ecosystems, corals are some of the simplest multicellular invertebrates. Like other notable marine invertebrates such as jellyfish and anemone, they belong to the phylum Cnidaria. Their sessile, or stationary nature, often leads to coral being confused as a plant or sediment; however, Cnidaria represents one of the earliest diverging clades from the kingdom Animalia (Moore, 2006). Cnidarians, and consequently coral, are among the most underexplored groups in biosystematics, primarily due to the field’s longstanding emphasis on Bilateria, the clade human’s reside in. The slow evolutionary change of the mitochondrial DNA of these biosystematics groupings also slows phylogenic research in Cnidaria. For example, only within the past couple of years was the genomic evidence needed to split the phylum Coelenterata found. Now, Ctenophores (comb jellies) form a separate clade from Cnidarians (Stampar et. Al, 2014).

A key feature of Cnidaria is their polymorphic life cycle, with most species dividing their time between sessile polyp and swimming medusae forms. The classes of Cnidaria are defined by the relationship between these forms: Scyphozoans (jellyfish) and Cubozoans (box jellies) emphasize their medusae form, while Anthozoans (coral and anemone) reside entirely as polyps. Coral, as a classification, is paraphyletic and is split between three monophyletic subclasses of Anthozoa: Octocorallians, Hexacorallians, and Ceriantharians. The defining feature of these clades lies in their polyps’ symmetry. Octocorallians display 8-fold symmetry while Hexacorallians display 6-fold symmetry. Ceriantharians, however, do not display any symmetry in their polyps, as both daughter orders consist of tube-dwelling anemone (Moore, 2006). This paper will focus on two key groupings of coral: the Hexacorallia order Scleractiniatia, which form coral reefs, and the Octocorallia order Alcyonacea, commonly known as soft coral. The evolutionary relationships discussed above are shown below in Figure 2.

Figure 2 A phylogenic tree of the phylum Cnidaria. The two key subclasses that coral reside in, Hexacorallia and Octocorallia, are shown as monophyletic and form a clade as Anthozoa (García-Linares et al., 2013).

Corals, like many other Cnidarians, form colonies of genetically identical polyps. Each polyp is incredibly small, only a couple millimeters in diameter, but together they hold the record for the largest structures created by a living organism, being even visible from space. Sexual reproduction can either emphasize gonochoristic (unisexual) or hermaphroditic reproductive methods. Most reef corals “broadcast spawn” and freely release their gametes into surrounding seawater. To maximize the chance that gametes meet, corals practice reproductive synchrony and tend to release their spawn at similar times. The timing of this release event has been linked to moonlight, triggering light-dependent reactions in key flavin-containing proteins. Once a zygote is formed, a planulae larva develops at the surface. This ciliated larva is the only non-sessile segment of the coral’s life cycle. Once the coral larva finds a suitable substrate to reside on, it eventually descends to the shallow sea floor. If it manages to avoid a sleuth of physical and allelopathic barriers, a new polyp will form and eventually form a new reef (Moore, 2006).

Polyp and Reef Structure

Each coral reef starts from a single polyp. Each polyp contains three layers of tissue: the outer epidermis, the intermediate mesoglea, which is an extracellular layer of connective tissue, and an inner layer of cells that line the gastrovascular cavity (National Oceanic and Atmospheric Administration, 2024). The gastrovascular cavity, also known as the coelenteron, is the hollow interior of the polyp that opens at one end, allowing the polyp to consume its prey and excrete metabolic waste. Surrounding the cavity is a ring of tentacles which assist in the process of capturing food, clearing debris, and possibly defense; however, many species of coral do not display these tentacles. (National Oceanic and Atmospheric Administration, 2024). Tentacles can be single or branched and have a tube-like structure.

Polyp Development

As mentioned above, polyps develop from the ciliated larval stage. Specifically, coral colonies grow through budding in various patterns depending on the species. Budding can occur at the base of the polyp, the lateral polyp body wall, or within the ring of the tentacles. Each new polyp that buds form a cylindrical wall around itself, and vertical plates called septa that radiate from the center under this new polyp. These walls are called corallites and are joined together by skeletal elements called coenosteum, shown below in Figure 3.

Figure 3. Skeletal features of hard coral (Digital Atlas of Ancient Life).

Together, the entire skeletal structure of the coral formed by corallites is referred to as the corallum. Some of the basic corallite shapes of corals are: plocoid/phaceloid (where all corallites have their own walls), meandroid/ceroid (polyps share common walls), and flabello-meandroid. Colony growth varies depending on the type of coral colony. For example, hexacorals, which are characterized by their polyps having tentacles in multiples of six, have multiple growth forms. One of these forms is massive, which grow in similar shapes in all directions, as can be seen in Figure 4. There are also encrusting, laminar, arborescent, and branched growth forms. The varying polyp sizes, branching patterns, and branch morphology/texture allow scientists to differentiate between species.

Figure 4. Massive coral. (James Cook University, 1995-2024).

Polyp Structure

Polyps receive structural support from internal tissues called mesenteries which extend from the body wall to the center of the polyp. Mesenteries also encourage gas exchange and nutrient absorption by increasing the surface area of the internal epithelia. Mesenteries have mesenterial filaments, shown in Figure 5, which form from the thickening and modification of the free-rounded edge of a mesentery into an elongated ruffle structure. These filaments can be stretched to extend loops of the filament out of the mouth or through pores in the surface body wall called cinclodes to help capture and digest food inside and outside the polyp. Polyps also have a passageway called the actinopharynx, which connects the gastrovascular cavity to the mouth and is lined with cilia that move particles and waste into the gastrovascular cavity. Some corals also have siphonoglyphs, which are grooves along the actinopharynx that help move water in and out of the polyp. Apart from these grooves, corals utilize its hydrostatic skeleton to control water flow and facilitate feeding, protection, and excretion. When the polyp relaxes its muscle fibers, its column and tentacles extend, and water moves into the gastrovascular cavity. Water is expelled when the polyp contracts its muscle fibers, causing the column and tentacles to retract back into the colony. When tentacles and the body column retract, they fold down onto the oral disc, but are not pulled into the actinopharynx, allowing the upper polyp column wall to stretch over contracted tentacles to protect them (Peters, 2015).

Figure 5. Coral polyp anatomy (K. Sugihara, H. Yamano, K. Choi, K. Hyeong, 2014).

Tissue Thickness and Moderating Light Stress

Light is important for photosynthesis since corals host algal symbionts and other photosynthetic organisms (which will be further discussed below); however, excess light can cause thermal stress and bleaching. A study looked at tissue thickness in relation to photoprotective capacity and bleaching resistance for anemone, providing relevant findings for cnidarian symbioses in general. It mesogleal and epidermal tissue layers have significant impact on light attenuation, meaning they greatly influence the organism’s ability to gradually decrease the intensity of light that it is exposed to mitigate thermal stress. The aximum quantum yield of photosystem II (𝐹𝑣/Fm ) (𝐹𝑣 is the variable fluorescence after draw-adaptation; 𝐹𝑚 is the maximum fluorescence after dark-adaptation) of anemone species with different tissue thicknesses was observed and a strong correlation was found between 𝐹𝑣/𝐹𝑚, tissue light attenuation, and tissue thickness, indicated by similar differences in the magnitude of these properties between species. Lower 𝐹𝑣/𝐹𝑚 was found in species with lower tissue thickness and was the result of a decrease in photochemical efficiency (Dimond, 2012), which is the balance between efficiency of light absorption and its use for energy in an organism (da Rocha Nina Junior, 2020). These results indicate that corals with greater tissue thickness are able mitigate thermal stress due to greater light attenuation.

Reef Formation/Growth Rates

Coral reefs form and grow as polyps secrete calcium carbonate (CaCO₃) skeleton and form connected corallites. Some polyps will lift off their base and secrete a new floor, forming a new layer in the basal plate above the old one and elevating the coral. The shape a reef takes as it forms depends on environmental factors such as light exposure, temperature, wave action, and the density of surrounding corals. As mentioned, there are various general shapes into which corals are categorized such as massive and branched. Massive corals tend to grow slowly, with a size increase of 0.5 cm to 2 cm per year. Depending on the conditions, some can grow up to 4.5 cm per year in favorable conditions. On the other hand, branching colonies grow much faster, and can grow vertically to as much as 10 cm per year (National Oceanic and Atmospheric Administration, 2024). Vertical growth rates are greatly influenced by space, which depends on sea levels. Rapidly rising sea levels can hinder reef growth, and if the sea level falls, the reef must step down to accommodate the change. Sea level also impacts light exposure, so increased water depth usually results in decreased growth rates as the corals receive less light (Dullo, 2005).

To analyze these growth rates, scientists drill out the cores of corals to study the band (Figure 6) layers through X-ray or UV light; these can be seen in Figure 6. Dark bands generally show slow, high-density (HD) growth, while light bands show faster, low-density (LD) growth (National Marine Sanctuaries, 2024). One study looked at three different species of massive coral: Favia pallida, Goniastrea retiformis, and Porites lutea. X-rays of samples of the corals showed alternating high- and low-density bands, as a band pair usually represents a year’s growth. The average growth rates were measured by counting the band pairs and measuring the distance between them, with results showing that growth rates decreased overall with depth because of decreased light available. 

Figure 6. A slice of Great Barrier Reef coral under ultraviolet light (Joelle Gergis).

Light intensity declines exponentially with depth, given by the equation (equation 1)

(1) $\frac{I(x)}{I_0}=e^{-\mu x}$

This equation models the exponential decay of light intensity with depth, where 𝑥 represents depth and 𝜇 is the linear attenuation coefficient that characterizes the absorption and scattering of light by a material (Priamo F, 2014). The study predicted that low density bands would be smaller in deeper water, as they usually only occur when light intensity is greater. The widths of high- and low-density portions in annual growth bands were measured, and true to this prediction, the LD:HD ratio was lower in samples collected at 22-30 m depths, as compared to samples collected at 6-17m, where the HD growth was higher. These results indicated a strong correlation between low growth rates and low LD:HD ratios, indicating that corals in deeper water experience slower growth. However, there was still some high variability in overall growth rates, indicating that environmental conditions also play a significant role and can sometimes override the effects of light intensity on growth rate (Highsmith, 1979). 

Figure 7. Growth rates of massive corals: Favia pallida, Goniastrea retiformis Porites lutea (R. Highsmith, 1979).

Variants of Coral Reefs

Coral reefs are classified into four types: fringing, barrier, atoll (annular), and patch reefs each with distinct characteristics. Fringing reefs are the most common type, growing straight out from the shore of the landmass which it borders. These reefs are generally found in shallow, clear waters, at up to 30 meters of depth. As sea levels rise and the distance between the reef and shoreline increases, fringing reefs may eventually evolve into other types of reefs. Barrier reefs are fringing reefs that have gained a further separation from shore, often due either to the erosion of the landmass to which the reef was attached or to a rise in sea level. These reefs are separated from shore by a deep, wide lagoon and are often formed by a combination of multiple different fringing reefs. Atolls are ring-shaped reefs that form around a lagoon, often following the complete submersion of a volcanic island. As the island sinks, the reef continues to grow upwards, leaving behind a ring that can sometimes be observed near the surface of the water. The outer part of the reef generally contains hardier strains of coral which can survive wave breakage, creating a calm habitat for marine life within the inner ring. Finally, patch reefs are small formations of coral, often found in shallow waters within lagoons or between other types of reefs. (How do coral reefs form?, 2024). Figure 8 (below) provides a visual representation of fringing, barrier, and atoll reefs.

Figure 8 Formation patterns of fringing, barrier, and atoll reefs (Earle, 2019).

Reef Complexity

Reef complexity plays a significant role in larval recruitment, affecting wave behavior and subsurface water motion. A study by Carlson et al. (Carlson et al., 2024) examined the relationship between reef rugosity and coral larvae recruitment. Reef rugosity, defined as the ratio of the surface length of reef to its straight-line length, appeared to have a positive correlation with larval recruitment. Small-scale complexity, including small cracks and crevices and coarseness in the coral surface, showed to have less of an effect on larval recruitment in most species of coral. In the study, small-scale complexity was measured up to a 32-centimeter scale and showed to have a weak positive correlation with larval recruitment. The increased settlement area theoretically would provide more area for larvae to attach to the reef structure, but, in most species, this has only a slight positive correlation with larval settlement. However, in the Pocillopora species, whose larvae is subjected to increased predation, higher small-scale reef complexity provided larvae with refuge from predators, leading to increased larval recruitment. Large-scale complexity, measured on a scale of meters, had a significant positive effect on larval settlement. Boulders and large coral structures generate drag, which leads to water turbulence. More complex structures have a higher coefficient of friction, which leads the velocity of water near the reef to be slower than the rest of the current. This turbulence helps retain particles, such as coral larvae and nutrients, near the reef, facilitating coral settlement. Without drag forces, coral larvae would struggle to attach to the reef, likely being swept away by ocean currents, due to them being poor swimmers. A study by Hata et al. (Hata et al., 2017) compared larval swimming speeds to the velocity of ocean currents, finding that coral larvae lack the swimming strength to overcome ocean currents, instead relying on the turbulence generated by both large-scale and fine-scale rugosity. Turbulence near reef structures can generate chemical cues to help in both the settlement and development of coral larvae (Quinlan et al., 2023).

Coral Shapes and Dispersal Due to Wave Energy

Wave energy significantly influences coral morphology and dispersal, as certain species and shapes thrive under different conditions. Massive corals form large spherical or hemispherical colonies that can survive continuous disturbance from wave action. Their compact structure distributes wave energy across the surface of the coral, as shown in Figure 9.

Figure 9 Pressure levels, measured in pascals, on a spherical model of massive coral assuming an incoming current from the left side. Positive values, towards red on the color bar, indicate compressive forces while negative values(blue) represent tensile forces. (Baldock et al., 2014).

This ability of massive corals to distribute wave energy across their surface makes them exist more commonly in high-energy zones of reefs, such as reef fronts, crests, and outer slopes.

Branching corals, on the other hand, form thin, branchlike structures. These thinner branches allow for more surface area, maximizing their light intake for photosynthesis, but they are more delicate and susceptible to breakage, leading them to be found in low-energy zones like lagoons or the back reef. In calmer waters, the risk of breakage is significantly lower, allowing the corals to grow more rapidly and to a greater height. Encrusting corals, similarly, to massive corals, are also found in high-energy zones. However, instead of withstanding the forces of the waves, encrusting corals form flattened colonies, minimizing the surface area that is exposed to wave forces. These corals are mostly found in reef crests due to their high resistance to physical stress (Dao-ru et al., 2013).

Wave energy not only affects coral shape and dispersal, but also larval dispersal. Because they are weak swimmers, larvae are heavily reliant on ocean currents for their settlement. In high-energy zones like the reef crest, strong wave action can prevent larvae from settling on the reef substrate, often carrying them far from their origin—sometimes even to entirely different reefs. Conversely, in low-energy zones, coral larvae are less likely to be swept away, leading to localized recruitment and growth. Coral populations become more concentrated and less diverse as larvae find settlements near their parent colonies.

Dinoflagellate Mutualism and Soft Coral

Coral reefs are comprised of more than just their polyps. To function, coral forms a variety of symbiotic relationships with key protists, viruses, and bacteria. Irregularities in environmental conditions often disrupt these interactions, leading the death of the entire colony. As climate change continues to shift the temperature, pH, and physical conditions in the ocean, the characteristics that define each of these relationships becomes more relevant as the reefs continue to suffer from our extended influence.

Coral-Algal Symbiosis and Dynamics

As heterotrophic invertebrates, coral require external sources of fixed carbon and chemical energy to grow. The rate that sessile polyps can capture and consume prey, however, is much too slow for the rate of reef formation we see today. Hermatypic, or reef-building, coral depends on a mutualistic relationship with a variety of endosymbiotic dinoflagellates in the genus Symbiodinium for this need. When nutrient availability is low, or oligotrophic, these protists perform photosynthesis and provide close to all the fixed carbon, oxygen gas, and chemical energy necessary for the coral’s metabolism. In return, coral provides its endosymbiont essential nutrients it garners from its prey and well-lit shelter from the open ocean. Note that there are variants of coral that do not depend on zooxanthellae and, thus, reside primarily in deeper oceans. For the purposes of this paper, however, we will focus on benthic, or shallow, coral reefs (NOAA National Ocean Service Education: Corals, n.d.).

The Local Light Environment and Aragonite

The rate of photosynthesis displayed by these Symbiodinium depends significantly on both the quantity and quality of light that interacts with their pigments. Most coral can only survive in depths under 30 meters due to the attenuating effects of ocean water that were previously mentioned. Specifically, solar irradiation diminishes exponentially from the surface and most coral cannot survive without significant changes to their skeletal structure, respiratory rates, or growth rates under around 2000 μmol photons per square meters per second. Coral that can survive at levels below this are known as mesophotic coral and depend more heavily on stronger, sideways diffuse light. Additionally, different wavelengths of sunlight are attenuated at different rates, with photosynthetically active blue light (400-500nm) diminishing the least and UVR (200-400nm) and red (620-740mn) attenuating the fastest. While the open ocean tends to be oligotrophic for pioneer corals, mature coral reefs have high concentrations of dissolved organic matter (DOM) from terrigenous processes and upwelling. Therefore, shallow corals will experience with higher UVR and blue light levels while mesophotic corals will struggle with low levels of blue light. The characteristics of incoming light also change rapidly with the size and shape of ocean waves, interactions with marine organisms above, and the tidal cycle (Roth, 2014).

All these factors have been found to induce various adaptations in the superorganism’s metabolism and structure. For instance, the quantity of light inside the coral superstructure is much higher than would be expected. The light quality is also heavily weighted towards the photosynthetic region of the algae’s pigments. The basis of the preservation of these ideal conditions lies in the structural and physical properties of the coral’s skeleton. As relatively fragile organisms, it would be reasonable to assume that coral would utilize the tougher, more resistant material calcite, but they instead produce the polymorph aragonite. Aragonite, as a light-emitting semiconductor, was experimentally determined to exhibit a much lower energy gap 2.46eV than calcite’s 3.93eV, as shown in figure 10.

Figure 10 A diagram representing the difference in energy gaps between aragonite and calcite in comparison to an energy spectrum of the energetic distributions of photons on the surface and in deep-water. Photons with a higher energy level than the energy gap of the material will be able to excite the electron and interact with the material sub-atomically (Neumann-Micheau & Tributsch, 2018)

The energy gap of a substance corresponds to the minimum energy needed to excite an electron from the valence to the conducting energy level of the semiconductor (Neumann-Micheau & Tributsch, 2018). This relationship is emphasized in Figure 9. Thus, this enables coral aragonite to absorb and re-emit light from lower wavelengths than calcite, specifically in the blue region of the visible spectrum. Note that wavelengths with lower energy levels than the energy gap for aragonite do not experience the same boost in light quantity.

Additionally, aragonite benefits from its ability to accommodate the fractal structure of the skeleton. Unlike calcite, aragonite can crystallize in a variety of forms including spindles, feathers, and needles. Fractal structures prevent the coral from self-shading and encourage a variety of optical processes that prevent light from radiating and decaying outwards. The fractal structure of aragonite is especially effective at creating localized electromagnetic fields that increase key interactions between dipoles responsible for electron transitions. The higher the density of these dipoles, the higher capacity for light absorbance and luminescence. This relationship can be modelled with the equation (eq. 2) shown below. Here, the time-dependent decay function of the excited states, or electrons that have absorbed photons, is modelled on the constant A, the Euclidean dimension d, and the number of multipolar interaction S. With fractal geometries, the Euclidean dimension trends towards 2 instead of 3. Hence, coral that display increased fractal natures can be characterized by a higher S and a lower d, reducing the rate of light decay and energy dispersion (Neumann-Micheau & Tributsch, 2018).

(2) $f(t)=e^{-At^{\frac{d}{s}}}$

Coral Bleaching

Coral bleaching occurs when corals expel their Symbiodinium due to environmental stressors such as elevated temperatures and increased light intensity. Specifically, if the environment halts or forbid the fair exchange of resources and energy between the two symbiotic partners, the relationship is broken off in bleaching. By doing so, each species chooses to focus entirely on their own survival in trying times. Each coral species has a specific heat tolerance. When water temperatures exceed this range, corals experience thermal stress, leading to coral bleaching. When water temperatures rise beyond the optimal range for coral health, the coral absorbs higher amounts of thermal energy. This excess heat can lead to stress in the Symbiodinium. Elevated temperatures can affect the efficiency of photosynthesis. (Porter, 1976).

As the Symbiodinium become stressed, they may produce reactive oxygen species, which are highly reactive molecules, due to excessive light absorption and inefficient photosynthesis. This oxidative stress can damage both the dinoflagellates and the coral, leading to a breakdown of the symbiotic relationship. Without the dinoflagellates, which provide essential nutrients through photosynthesis, the corals become increasingly vulnerable and may expel the Symbiodinium, resulting in bleaching (Porter, 1976).

Coral reefs can respond negatively to excess nutrients. High nutrient levels (particularly Nitrogen and Phosphorus) can lead to increased growth of algae, which competes with corals for space and resources. This can diminish light availability for the dinoflagellates within the coral tissues, disrupting the symbiotic relationship and potentially leading to coral stress and bleaching.

In warmer waters, nutrient pollution can lead to an increase in algal blooms. While some nutrients are beneficial for coral growth, excessive amounts can create imbalances that favour algae over corals. This phenomenon is increased by climate change, as warmer temperatures can enhance the growth rates of algae. Algal overgrowth can smother corals, block sunlight, and reduce the availability of space for new coral recruits. This competition negatively affects the already stressed coral-dinoflagellate relationship, leading to further declines in coral populations (Shantz & Burkepile, 2014).

Global warming negatively impacts corals and their symbiotic relationship with Symbiodinium. Additionally, it contributes to ocean acidification, a process in which increased CO₂ from the atmosphere dissolves in seawater, lowering its pH. This change reduces the availability of carbonate ions, which are necessary for corals to build their calcium carbonate skeletons. Weakened skeletal structures hinder the ability of corals to grow and maintain their reefs, making them more vulnerable to erosion and other stressors. Additionally, ocean acidification can disrupt the symbiotic relationship with dinoflagellates. Altered chemistry may affect the photosynthetic efficiency of these organisms, which diminishes the energy supply to corals (Fisher et al., 2011).

Stony and Soft Coral

Corals primarily fall under the orders of Scleractinia (stony corals) and Alcyonacea (soft corals). These groups differ in structure and composition, affecting their interactions with the environment from the conditions under which each thrives to the role each plays in the ecosystem. Stony coral polyps secrete calcium carbonate (limestone) to form the skeletons in which they live. Each polyp, about 1-3 millimeters in diameter, periodically lifts the existing layers of limestone, depositing a new layer underneath, expanding the colony by about 0.3-2 centimeters per year. When the polyp dies, the limestone skeleton is left behind, providing an anchor point where new coral larvae, both hard and soft, can attach themselves (Coral Facts, n.d.). The mineralized skeletons of stony coral polyps’ form reefs, whose hard structure can resist forces from ocean currents and waves, providing habitat and protection for other marine life. Figure 11 below depicts an example of stony coral.

Figure 11 A formation of brain coral from the Great Barrier Reef (Great Barrier Reef Tours, n.d.)

Soft corals, on the other hand, do not produce rigid skeletons. Instead, specialized cells called scleroblasts produce sclerites, protein matrices around which calcite crystals are deposited. Colonies of coral polyps branch together via these sclerites to form what appears to be a single large organism. As new coral larvae attach to the structure and mature into polyps, new sclerites are formed, growing the colony at a rate of 2-4 centimeters per year. (France, 2019). While soft corals’ flexible structure does not offer the same wave-resistant properties as stony corals, soft corals are important habitats in deeper waters, where stony corals struggle to thrive. Soft corals can come in a variety of different shapes including mushroom corals, tree corals, tube corals, and bubble corals, some of which are shown in figure 12 below.

Figure 12 A collection of soft coals in the Great Barrier Reef (Bruckner, 2015).

Pulsation in Soft Corals

Certain soft corals demonstrate perpetual and non-synchronous polyp pulsation, caused by the extension and contraction of their tentacles. A study performed on Heteroxenia fuscescens used in situ underwater particle imaging to understand the benefit of this movement pattern. Pulsation was found to move water upward, increasing mixing across the coral-water boundary layer and preventing refiltration of water by neighboring polyps, meaning that post-filtered water from other polyps is moved away. Results found that the respiration rate during pulsation is two times higher than during rest periods, and overall photosynthesis is sevenfold higher. During pulsation, stems of active polyps are extended, and tentacles change posture from being fully extended to being tightly packed, which increases the surface area exposed to downwelling light. The flow pattern above polyps is also altered due to pulsation, as water is propelled away from the oxygen-rich water-coral interface. Due to these effects, photosynthesis is enhanced, since there is a greater release of oxygen from coral tissues, lowering internal oxygen concentration and mitigating the effects of RuBisCo having a decreased affinity for CO₂ in high oxygen environments. This allows for more efficiency in the Calvin cycle, and therefore, in photosynthesis. When no pulsation occurs, oxygen produced by photosynthesis accumulates in tissues and reduces the ability for RuBisCo to bind to CO₂. Furthermore, when ambient oxygen concentrations were artificially raised, there was no increase in photosynthesis. This indicates that the benefit of pulsation is the ability of the tentacles to push oxygen rich water away from the polyps, decreasing the amount of oxygen immediately surrounding the polyp as opposed to the high concentrations of oxygen ranging from 550-600µM under no-flow conditions (Kremien, 2013).

Figure 13. A. Photosynthesis rates during rest vs. pulsation under normal $O_2$ conditions and artificially raised $O_2$ conditions. B. Comparison of areal gross photosynthesis rates at rest and during pulsation (M. Kremien, U. Shavit, T. Mass, and A. Genin, 2013).

Environmental Conditions for Reefs

Favorable water conditions are critical for the development and survival of coral reefs. Factors such as water salinity, water temperature, light levels, and nutrient content must fall within specific ranges for coral to be able to grow. Corals thrive in the tropics, where the mean annual water temperature is between 23-25℃, although a few species can survive in environments as low as 18℃. Coral health is also highly sensitive to changes in temperature. A study found that temperatures above 29.5℃ are correlated with coral bleaching events (Winter et al., 1998). Bleaching is a phenomenon where corals expel the zooxanthellae living in their tissues. Without the energy supply provided by zooxanthellae, corals experience increased stress and are more susceptible to disease and death (What is coral bleaching? 2024). The importance of zooxanthellae’s presence to coral health also dictates that corals must grow where there are adequate light levels. Zooxanthellae produce energy through photosynthesis, a process that requires ample sunlight, by converting the carbon dioxide produced by coral to sugars that the corals use for energy. For this reason, corals struggle to survive in deeper waters where light is scarce, often being limited to depths of about 50-70 meters. In fact, they struggle in nutrient-rich environments due to the presence of algae, which compete with coral for space and light. Since algae grows significantly faster than coral, they can outcompete corals for these resources. Because of this, corals are confined to nutrient-poor environments, where algae do not dominate. Water salinity is also an important constraint for coral growth. Coral grows best under normal oceanic salinity, approximately 30-40 parts per thousand. Under normal ocean salinity, the water is highly saturated with ionized calcium, which is critical in the formation of calcium carbonate that stony corals use to form their skeletons. Drastic changes to water salinity can affect corals’ calcification process, leading to the death of large parts of the reef (Britannica, 2024).

Mechanical Properties, Hydrodynamic Stress Response

Polyp Orientation and Skeletal Porosity

A keystone property of corals are their hard exoskeletons. Their exoskeletons, made up of aragonite, protect the polyps from the harsh environment that is the ocean. Extreme weather events such as cyclones and tsunamis cause damage to colony branches by either breaking off segments of the coral or uprooting the coral substrate (Madin, 2005). Research has illustrated that coral skeleton is weaker than previously thought. Its strength is superior to modern-day, carbonate engineered materials such as concrete or limestone. However, coral skeleton lacks strength in comparison to other ceramic skeletal material (Chamberlain, 1978).

Coral skeletal fracture strength is determined by polyp orientation and skeletal porosity. According to Chamberlain, vectorial polyp growth creates a “grain” in coral skeleton like that of wood (Chamberlain, 1978). Vectorial poly growth along the grain essentially means that polyp’s growth is oriented parallel to stress. Figure 14 illustrates the inverse relationship between fracture strength and polyp orientation (measured in the degrees between polyps’ growth direction and stress direction). As depicted in the graph, fracture strength significantly decreases between 60 and 90 degrees, demonstrating the point that a polyp’s “along-the-grain growth” increases skeletal strength (Chamberlain, 1978).

Figure 14 Fracture strength as a function of polyp orientation. Polyp orientation is the angle between the growth direction of the polys and the stress direction. Each symbol represents one test core. Curve fit by eye. A – massive corals, dots – M. annularis, circles – S. radians, B – branched corals: dots – A. palmata (Chamberlain, 1978)

Coral skeleton’s compressive strength was experimentally measured to range from 12 to 81 meganewtons per square meter (MNm^-2) in a limited sample size of corals (Chamberlain, 1978). To put that in context, compact bone is typically in the range of 200 MNm^-2while mollusc shells are in the range of 88 to 170 MNm^-2 (Chamberlain, 1978). Coral’s compressive strength is partially attributed to its porosity. Porosity, defined to be pore volume divided by total skeletal volume, helps indicate the amount of stress a material is under. As expected, porosity and strength have an inverse relationship (like in most other materials), calculated from equation 3.

$P=-m_c(\pi r^2l\rho_a)^{-1}$

Equation 3 Porosity equation where P is porosity, m_c is the mass of the core, ρ_a is the density of aragonite, r is the radius (of the core), l is the length (of the core) (Chamberlain, 1978).

Organic Material

Mechanical integrity – “the ability to withstand breakage caused by hydrodynamic force” (Madin, 2005) – are affected by a variety of factors including, but not limited to, porosity/micro-flaws, organic content, or weak organic/inorganic and inorganic/inorganic interfaces. Interestingly, organic macromolecules are not only responsible for regulating crystal disposition (on the micro-level), but also helps to redistribute internal stresses (Moynihan et al., 2021). Moynihan and her colleagues demonstrated an inverse relationship between annually averaged organic content and micro-cracking stress. This relationship provides evidence that by increasing organic content, skeletal flexibility also increases (figure 15 graphically illustrates this concept) (Moynihan et al., 2021). However, greater skeletal flexibility comes at the expense of hardness, leaving the coral skeleton more prone to frequent micro-cracking (Moynihan et al., 2021).

Figure 15 Correlations between Young’s modulus (GPa) and micro-cracking stress (MPa) with density (gcm^-3) and sum organic content (%). Correlations with organic content data excluded the tissue layer, whereas bulk density correlations included the tissue layer (Moynihan et al., 2021).

Mechanical Integrity at the Microscopic Level

At the microscale, corals produce calcium carbonate in the form of aragonite, forming the exoskeleton. The exoskeleton, itself, is composed of two components – centers of calcifications (COCs) and fibers – which is differentiated by their nano-structural organizations and growth rate of aragonite (Moynihan et al., 2022). Structurally, the fibers, composed of aragonite crystals, are arranged in fan-like bundles around COCs. Each coral species has a unique ratio of COCs and fibers, giving them different mechanical properties (Moynihan et al., 2022). For example, Porites, a massive coral species, have lower compressive and tensile strength compared to branching coral species (Moynihan et al., 2022). To clarify, massive coral species are boulder-shaped and grow at a slower rate compared to branching coral species, which grow branches, as the name suggests (Massive Corals).

The microscale material properties of coral can also explain coral exoskeleton’s macro-anisotropic properties. Anisotropy refer to “the quality of exhibiting properties with different values when measured along axes in different directions” (Britannica, 2021). In this case, coral exoskeletons exhibit a greater Young’s Modulus, indicating the ability to easily stretch and deform, (Physics A level revision resource: Introduction to Young’s Modulus) and hardness value, indicating greater resistance to deformation (Hardness) when measured parallel to the primary macroscale direction of coral growth. These anisotropic properties are attributed to the packing and microarchitectural organization of the crystals and crystal bundles at the microscale rather than intrinsic crystal anisotropy. More specifically, Moynihan and her colleagues noted that Young’s modulus and crystal orientation (where individual crystal bases faced upward relative to COCs) held a correlative relationship (Moynihan et al., 2022).

Mechanical Integrity Related to Coral Colonies

In engineering design, the mechanical integrity is dictated by its “weakest link” or its weakest component. That logic similarly applies to corals. Coral’s overall mechanical strength is not limited by the strength of its carbonate exoskeleton, but by the reef substrate, which is much weaker. In an experiment, researchers defined the mechanical integrity of a coral colony to be based on size, morphology, water velocity, gravity and strength of material (Madin, 2005). Size (of a whole colony) was defined to be A = hd, where A is the projected area of the colony perpendicular to the horizontal water motion on the reef, h is height, and d is diameter (Madin, 2005) (Equation 4). Morphology was defined to be expressed by a shape-index, where shape-indexes greater than 1 indicated a taller, thinner colony and less than 1 indicated a short, stubby colony (Equation 5). Water velocity was calculated using the hydrodynamic force equation (Equation 6). Force of gravity was calculated to account for buoyancy (Equation 7). Finally, strength of material was defined by the maximum internal sheer stress equation, maximum tensile strength and maximum bending stress (Madin, 2005) (Equations 8, 9, 10, respectively) .

(4) $A = hd$

Equation 4 Calculating size of a whole colony or branch where A is projected area, h is height, d is diameter (Madin, 2005).

(5) $S = \frac{h}{d}$

Equation 5 Shape-index equation (S) where h is height, d is diameter. Higher shape-index means a structure that is more tall than wide. (Madin, 2005).

(6) $F_h=\frac{1}{2}\rho_whdu^2C_d$

Equation 6 Maximum hydrodynamic force a coral or branch can handle. ρ_w is density of seawater, u is horizontal water velocity, C_d is drag coefficient (Madin, 2005).

(7) $F_g=\frac{(\rho_a-\rho_w)g\pi hd^2}{4}$

Equation 7 Gravitational force (minus buoyancy). πhd²/4 is colony’s volume, g is gravitational constant (Madin, 2005).

(8) $\tau_{max}=\frac{8\rho_wu_{max}^2S}{3\pi}$

Equation 8 Maximum internal shear stress equation. Notice how it is independent of colony size (Madin, 2005).

(9) $\sigma_{max}=\frac{8\rho_wu_{max}^2S^2}{\pi}$

Equation 9 Maximum tensile stresses equation (Madin, 2005).

(10) $\sigma_{\max,g}=(\rho_a-\rho_w)hg$

Equation 10 Maximum bending stress equation (Madin, 2005).

Madin ultimately found that the tensile strength of the basal materials (in this case the colony/substrate interface) is the limiting factor of coral’s mechanical integrity with coral skeleton being three to ten times stronger than the coral substrate on average. Since bending stresses act parallel to a colony’s central axis, bending stress is a nonfactor (Madin, 2005). This case study also illustrated that corals that have higher shape-index (Fig. 16) were more likely to have a larger disparity between the skeletal and substrate strengths due to their proneness of water-induced breakage (Madin, 2005). Higher shape index (i.e. branching corals) were found to correlate with higher bending stress.

Figure 16 The average compressive (open bars) and tensile (shaded bars) strengths of reef substrate and coral skeleton (Madin, 2005).

Response to Hydrodynamic Stress

In response to hydrodynamic stress, coral growth has adapted to survive in its environment. Researchers have theorized that corals minimize and reduce mechanical stress by changing its branching phenotype to become more compact/massive or reorienting the branches toward the direction of the current or wave (Baldock et al., 2014). Branches grown in the lateral direction (with respect to the coral base) are subject to “pruning,” or breaking off, due to the greater hydraulic stress, thus leading to the evolutionary reorientation of branch growth (Graus et al., 1977). Wave-induced orbital velocities as low as 0.5 m/s could cause coral breakage. Conversely, massive corals are unlikely to experience failure stresses and thus not susceptible to wave-induced structural damage. Overall, wave loads are significantly higher in branching corals than massive corals (Figure 17), contributing to evolutionary adaptations (Baldock et al., 2014; Graus et al., 1977).

Figure 17 Typical branching corals (Acropora spp.), left, and massive coral (Favia sp.), right (Baldock et al., 2014).

Conclusion

The survival of the coral reef, despite the apparent simplicity of each individual polyp, is heavily dependent on the interactions between various physiological, ecological, and structural factors. When these factors align, the coral reef displays unique design solutions and continues to support one of the most biodiverse ecosystems on the planet. Each coral species displays a different developmental pattern that allows coral reefs to colonize a variety of different benthic landscapes. The unique structure of the triple layered Cnidarian polyp allows it to bind to its substrate, capture and absorb its prey, and interchange metabolic products with their endosymbiotic partners. The growth and development of the reef are highly dependent on the sea level and the amount of light reaching the benthic zone. Even the shape of the reef is defined by a host of factors, with geological features like volcanoes and coastlines specifically defining their structure.

Ecologically, coral could not exist without their own community of symbionts in the ecosystem they tout. The defining relationship, with the photosynthetic protist genus Symbiodinium, is a unique and especially effective mutualistic relationship that improves the coral’s ability to grow and calcify. Without this relationship, coral would struggle to collect the energy and nutrients needed to sustain giant colonies. This bond is a crucial factor to the colony’s success. However, light diminishes exponentially with depth, and the quality of diffuse sunlight is often insufficient to support the photosynthetic needs of these protists. To address this, each polyp biomineralizes aragonite—a material that, while mechanically weaker, plays a crucial optical role by refracting and concentrating light onto the algal cells, thereby enhancing photosynthesis. If environmental factors, like temperature, nutrient concentrations, acidity, and salinity, breach the protist’s range of tolerance, the coral may eject the protist through coral bleaching. Another variant of coral, soft coral, also illustrate colony-forming abilities, but do not form coral reefs. Instead, they can inhabit deeper waters and display a specialized pulsation pattern to decrease photorespiration by adjusting oxygen and carbon dioxide concentrations.

Other factors like polyp orientation and the inclusion of organic matter can impact the physical basis of the reef. The mechanical integrity of the reef lies in the calcium carbonate crystal’s properties, including its anisotropy, fiber count, shape, width, substrate, and various other factors. Taken in isolation, the structure, interactions, and mechanical properties that characterize the coral reef lack robustness; it is their integration that gives rise to the reef’s strength and resilience. Each individual polyp is identical to every other polyp in the colony and would cease to exist on its own. Corals exists only through the interplay of numerous factors, making them a well-known example of emergent systems in nature. The reef is, from a physical standpoint, more than the sum of its parts.

References

Baldock, T. E., Karampour, H., Sleep, R., Vyltla, A., Albermani, F., Golshani, A., Callaghan, D. P., Roff, G., & Mumby, P. J. (2014). Resilience of branching and massive corals to wave loading under sea level rise – A coupled computational fluid dynamics-structural analysis. Marine Pollution Bulletin, 86(1), 91-101. https://doi.org/https://doi.org/10.1016
/j.marpolbul.2014.07.038

Britannica, T. (2021). anisotropy. Encyclodpaedia Britannica. https://www.britannica.com
/science/anisotropy

Britannica, T. Editors of Encyclopaedia (2024, September 24). coral reef. Encyclopedia Britannica. https://www.britannica.com/science/coral-reef

Bruckner, A. (2015, March 24). Swimming among soft corals of the Great Barrier Reef – KSLOF. Living Oceans Foundation. https://www.livingoceansfoundation.org/swimming-among-soft-corals-great-barrier-reef/

Chamberlain, J. A. (1978). Mechanical Properties of Coral Skeleton: Compressive Strength and its Adaptive Significance. Paleobiology, 4(4), 419-435. http://www.jstor.org.proxy3.
library.mcgill.ca/stable/2400190

Coral Cores: Ocean Timelines. (August 12, 2024). Flower Garden Banks National Marine Sanctuary. Retrieved September 26 from https://flowergarden.noaa.gov/science/coralcores.html

Coral facts. (n.d.)National Oceanic and Atmospheric Administration. Retrieved September 26 from https://www.coralreef.noaa.gov/education/#:~:text=Soft%20Coral%3A%20A%20soft%20coral,to%20four%20centimeters%20per%20year.

Coral Reef – Australia’s Great Barrier Reef. (n.d.) Great Barrier Reef Tours. Retrieved October 9 from https://greatbarrierreeftours.com/great-barrier-reef/coral-reef/

Dao-ru, W., Yuan-chao, L., & Jian-xin, L. (2013). Spatial differentiation of coral species related to wave energy along the Changqi coast, Hainan island, southern China. Continental Shelf Research, 57, 117-122. https://doi.org/https://doi.org/10.1016/j.csr.2012.12.004

da Rocha Nina Junior, A., Furtunato Maia, J. M., Vitor Martins, S. C., & Gonçalves, J. F. C. (2020). Photochemical Efficiency and Oxidative Metabolism of Tree Species during Acclimation to High and Low Irradiance. Plants (Basel), 9(8). https://doi.org/10.3390/plants9081047

Dimond, J. L., Holzman, B. J., & Bingham, B. L. (2012). Thicker host tissues moderate light stress in a cnidarian endosymbiont. J Exp Biol, 215(Pt 13), 2247-2254. https://doi.org/10.1242/jeb.067991

Dullo, W.-C. (2005). Coral growth and reef growth: a brief review. Facies, 51(1), 33-48. https://doi.org/10.1007/s10347-005-0060-y

Earle, S. (2019). Physical Geology – 2nd Edition. Victoria, B.C.: BCcampus. Retrieved from https://opentextbc.ca/physicalgeology2ed/

Fisher, P. L., Malme, M. K., & Dove, S, (2011). The effect of temperature stress on coral symbodinium associations containing distinct symbiont type. Coral Reefs, 21(2), 473-485. https://doi.org/10.1007200338-011-0853-0

France, S. C. (2019, September 4). What is a sclerite?: Deep connections 2019: Exploring Atlantic Canyons and seamounts of the United States and Canada: NOAA Office of Ocean Exploration and Research. What is a Sclerite?: Deep Connections 2019: Exploring Atlantic Canyons and Seamounts of the United States and Canada: NOAA Office of Ocean Exploration and Research. https://oceanexplorer.noaa.gov/okeanos/explorations/ex1905/logs/sept5/sept5.html#:~:text=Sclerites%20are%20small%20aggregates%20of,stone%2Dlike%20sclerites%20of%20Paracis.

García-Linares, S., Castrillo, I., Bruix, M., Menéndez, M., Alegre-Cebollada, J., Martínez-Del-Pozo, Á., & Gavilanes, J. G. (2013). Three-dimensional structure of the actinoporin sticholysin I. Influence of long-distance effects on protein function. Archives of Biochemistry and Biophysics, 532(1), 39–45. https://doi.org/10.1016/j.abb.2013.01.005

Graus, R. R., Jr, J., & Boker, A. M. (1977). Structural modification of corals in relation to waves and currents. Studies in Geology, 4, 135-153.

Hardness. University of Southampton. Retrieved September 26 from https://www.south
ampton.ac.uk/engineering/research/facilities/360/nCATS_facility/hardness.page

Hata, T., Madin, J. S., Cumbo, V. R., Denny, M., Figueiredo, J., Harii, S., Thomas, C. J., & Baird, A. H. (2017). Coral larvae are poor swimmers and require fine-scale reef structure to settle. Scientific Reports, 7(1), 2249. https://doi.org/10.1038/s41598-017-02402-y

Highsmith, R. C. (1979). Coral growth rates and environmental control of density banding. Journal of Experimental Marine Biology and Ecology, 37(2), 105-125. https://doi.org/https://doi.org/10.1016/0022-0981(79)90089-3

How do Coral Reefs Form?. (August 12, 2024). National Oceanic and Atmospheric Administration. Retrieved September 26 from https://oceanservice.noaa.gov/education/tutorial_corals/coral04_reefs.html

Howells, E. J., Beltran, V. H., Larsen, N. W., Bay, L. K., Willis, B. L., & van Oppen, M. J. H. (2011, December 18). Coral thermal tolerance shaped by local adaptation of photosynthesis of photosymbionts. Nature News. https://www.nature.com/articles/nclimate1330

Kremien, M., Shavit, U., Mass, T., & Genin, A. (2013). Benefit of pulsation in soft corals. Proceedings of the National Academy of Sciences of the United States of America, 110(22), 8978-8983. http://www.jstor.org/stable/42657199

Madin, J. S. (2005). Mechanical limitations of reef corals during hydrodynamic disturbances. Coral Reefs, 24(4), 630-635. https://doi.org/10.1007/s00338-005-0042-0

Massive Corals. National Oceanic and Atmspheric Administration. Retrieved September 26 from https://oceanservice.noaa.gov/education/tutorial_corals/media/supp_coral03h.html

Moore,J. (2006). An introduction to the invertebrates. https://doi.org/10.1017/cbo9780511754760

Moynihan, M. A., Amini, S., Goodkin, N. F., Tanzil, J. T. I., Chua, J. Q. I., Fabbro, G. N., Fan, T.-Y., Schmidt, D. N., & Miserez, A. (2021). Environmental impact on the mechanical properties of Porites spp. corals. Coral Reefs, 40(3), 701-717. https://doi.org/10.1007/s00338-021-02064-3

Moynihan, M. A., Amini, S., Oalmann, J., Chua, J. Q. I., Tanzil, J. T. I., Fan, T. Y., Miserez, A., & Goodkin, N. F. (2022). Crystal orientation mapping and microindentation reveal anisotropy in Porites skeletons. Acta Biomaterialia, 151, 446-456. https://doi.org
/https://doi.org/10.1016/j.actbio.2022.08.012

Peters, E. C. (2015). Anatomy. In Diseases of Coral (pp. 85-107). https://doi.org/https://doi.org/10.1002/9781118828502.ch6

Porter, J. W. (1976). Autotophy, heterotrophy, and resource partitioning in Caribbean Reef-building corals. The American Naturalist, 110(975), 735-742. https://doi.org/10.1086/283100

Physics A level revision resource: Introduction to Young’s Modulus. University of Birmingham. Retrieved September 26 from https://www.birmingham.ac.uk/study/undergraduate
/schools-and-colleges/post-16/a-level-stem-resources/youngs-modulus#:~:text=The%20Young%27s%20modulus%20(E)%20is,%CE%B5%20%3D%20dl%2Fl).

Priamo F, G. Y., Chieng R, et al. . (2014, 2024). Linear attenuation coefficient. Retrieved September 26 from https://radiopaedia.org/articles/linear-attenuation-coefficient

Quinlan, Z. A., Bennett, M. J., Arts, M. G. I., Levenstein, M., Flores, D., Tholen, H. M., Tichy, L., Juarez, G., Haas, A. F., Chamberland, V. F., Latijnhouwers, K. R. W., Vermeij, M. J. A., Johnson, A. W., Marhaver, K. L., & Kelly, L. W. (2023). Coral larval settlement induction using tissue-associated and exuded coralline algae metabolites and the identification of putative chemical cues. Proc Biol Sci, 290(2009), 20231476. https://doi.org/10.1098/rspb.2023.1476

Shantz, A.A., Burkepile,D.E. (2014). Context-dependant effecrs of nutrient loading on the coral-algal mutualism. Ecology, 95(7), 1995-2005. https://doi.org/10.1890/13-1407.1

Stampar, S. N., Maronna, M. M., Kitahara, M. V., Reimer, J. D., & Morandini, A. C. (2014). Fast-evolving mitochondrial DNA in Ceriantharia: a reflection of hexacorallia paraphyly?. PloS one, 9(1), e86612. https://doi.org/10.1371/journal.pone.0086612

Stat, M., Morris E., & Gates, R. D. (2008). Functional diversity in coral-dinoflagellate symbiosis. Proceedings of the National Academy of Sciences, 105(27), 9256-9261. https://doi.org/10.1073/pnas.0801328105

What are coral reefs. (August 28, 2024). National oceanic and atmospheric administration. https://www.coris.noaa.gov/about/what_are/#:~:text=The%20Structure%20of%20Coral%20Reefs,%E2%80%94fringing%2C%20barrier%20or%20atoll

What is coral bleaching?. (June 16, 2024). National Oceanic and Atmospheric Administration. Retrieved September 26 from https://oceanservice.noaa.gov/facts/coral_bleach.html#:~:text=Warmer%20water%20temperatures%20can%20result,This%20is%20called%20coral%20bleaching

Winter, A., Appeldoorn, R. S., Bruckner, A., Williams Jr, E. H., & Goenaga, C. (1998). Sea surface temperatures and coral reef bleaching off La Parguera, Puerto Rico (northeastern Caribbean Sea). Coral Reefs, 17(4), 377-382. https://doi.org/10.1007/s003380050143S