Coral reefs, vital ecosystems supporting diverse marine life, are primarily shaped by the clonal expansion of coral colonies. Although the principles of coral clonal growth, involving polyp division for spatial extension, are well-understood, numerical modelling efforts are notably scarce in the literature. In this article, we present a parsimonious numerical model based on the cloning of polyps, using five key parameters to simulate a range of coral shapes. The model is agent-based, where each polyp represents an individual. The colony’s surface expansion is dictated by the growth mode parameter ( s ), guiding the preferred growth direction. Varying s facilitates the emulation of diverse coral shapes, including massive, branching, cauliflower, columnar and tabular colonies. Additionally, we introduce a novel approach for self-regulatory branching, inspired by the intricate mesh-like canal system and internode regularity observed in Acropora species. Through a comprehensive sensitivity analysis, we demonstrate the robustness of our model, paving the way for future applications that incorporate environmental factors, such as light and water flow. Coral colonies are known for their high plasticity, and understanding how individual polyps interact with each other and their surroundings to create the reef structure has been a longstanding question in the field. This model offers a powerful framework for studying these interactions, enabling a future implementation of environmental factors and the possibility of identifying the key mechanisms influencing coral colonies’ morphogenesis.

Corals are modular organisms that use asexual reproduction (i.e., budding) to increase the size of the colony by adding new polyps. Clonal modularity offers several advantages, such as the ability to sustain partial mortality, redistribute resources internally, replace or repair single modules, and delay senescence, potentially supporting millenary growth. Despite the global importance of coral reefs, little is known about clonal growth rules in anthozoan corals. Coral clonal growth was reviewed to synthetise current understandings and identify gaps driving future research efforts. Despite corals present high plasticity, their growth is dictated by strong intrinsic regularities at different life stages and modularity levels. For example, a six-polyps crown with fixed distances among polyps is typically formed in the early development stages of Stylophora pistillata larvae. Similarly, specific developmental regularities are observed in the budding of azooxanthellate Dendrophylliidae polyps, which are consistently maintained across generations and species. In Octocorallia, colony shape is preserved by maintaining a constant ratio between the total number of branches and mother branches. Concurrently, environmental factors (i.e., light and hydrodynamics) play a fundamental role in shaping the final morphology of the colony, driving the architectural design at different levels of modularity. Some species revealed higher plasticity at the branching level in contrast with the predetermined shape assumed by the colony. Several models have been proposed to describe the environmental modulation of coral growth, mostly in branching forms. However, a holistic, universal model applicable to a broader range of coral taxa is still lacking. Understanding the fundamental rules underpinning coral clonal growth is essential to improving predictions of coral reef recovery, inferring stress on coral colonies and guiding restoration efforts. <!----> <!----> <!----> <!---->

Coloniality and clonality in marine sessile organisms offer several advantages, such as better space occupation and directional growth. In coral colonies, species-specific functional connections are maintained among polyps, allowing for resource translocation and colony architecture coordination. A potential whole-colony integration mechanism is apical dominance, a phenomenon controlling branching patterns through hormonal signaling in plants and seagrass, yet unconfirmed in scleractinian corals. This study aims at investigating the occurrence of apical dominance in corals, hypothesizing that highly integrated species exhibit this mechanism. We experimentally tested this hypothesis in situ by removing the apical tip in three different species (Stylophora sp., Acropora hemprichii, A. pharaonis), presenting two contrasting levels of integration and monitoring their branching morphogenesis over time. After 74 days, the null hypothesis that apical dominance does not occur could not be rejected for A. hemprichii and Stylophora sp., likely due to experimental limitations. However, A. pharaonis exhibited accelerated apical regrowth and increased lateral branching after tip removal, suggesting that apical dominance-like mechanisms may operate in this species. These findings highlight the importance of addressing potential Type 1 and Type 2 errors in experimental design to improve reliability while addressing the emergence of apical dominance in highly integrated coral colonies. Further long-term experiments are needed to capture morphometric changes in slow-growing species, such as A. hemprichii. These findings suggest novel endogenous mechanisms coordinating complex three-dimensional morphogenesis in clonal organisms and offer valuable application in the growing field of coral farming and restoration.

Coral bleaching and mortality are commonly associated with heat stress during marine heatwaves (MHWs), yet attributing these responses solely to heat stress may overlook the complexity of contemporary MHWs. Here, we document a severe MHW affecting a central Red Sea reef in mid‐2024 that coincided with repeated nighttime low‐oxygen events. By integrating satellite and in situ records of temperature, dissolved oxygen and wind, alongside repeated surveys of in situ nursery‐reared coral fragments, we document the temporal coincidence of these conditions with rapid coral bleaching and mortality. As temperatures approached and exceeded 33°C during the MHW peak, nighttime oxygen minima were extreme (down to 15.2–57.8 μmol O ₂ kg ⁻¹ ) and diel temperature variability diminished (down to 0.31°C–0.67°C), indicating a loss of nighttime thermal reprieves. Coincident responses of nursery corals were atypical, with transitions from healthy or bleached states to mortality occurring within 7–8 days. Bleaching and mortality also occurred across diverse coral species in nearby habitats after the MHW peak, with < 1% live coral cover documented eight months later. Sustained high temperatures, reduced vertical mixing and increased nighttime respiratory demand likely constrained oxygen availability, resulting in low‐oxygen extremes. We posit that similar compound MHW extreme events may have gone undetected in the Red Sea and elsewhere, potentially obscuring the contribution of co‐occurring low‐oxygen extremes to coral responses attributed to heat stress.

Colonial corals display diverse growth forms shaped by both environmental conditions and intrinsic developmental mechanisms. Understanding these internal morphogenetic rules is critical for accurately modeling coral colony formation. In this study, we investigated the intrinsic budding patterns of Dendrophyllia cf. horsti , an azooxanthellate deep-sea coral from the Red Sea. Using detailed morphological analyses of colony architecture, we identified consistent intrinsic constraints that govern colony development. Lateral corallites bud at a consistent angle (mean 15.9°) relative to the growth axis, and the orientation of the directive septa in daughter corallites remains stable across generations (mean 19.8°). Budding was consistently restricted to two specific sites per internode located near the lateral primary septa. In most colonies, budding occurred on the same side at successive internodes, although occasional observations suggested variation in the relative position of budding sites. These findings suggest that even in deep-sea, non-photosynthetic species, colony morphology is strongly regulated by intrinsic developmental constraints, as previously demonstrated in congeneric species. The results contribute valuable data for simulating coral growth and highlight the importance of intrinsic constraints in shaping colonial forms across Scleractinia.

Body size is a fundamental driver of metabolism, yet it remains unclear whether colonial organisms such as corals conform to the universal ¾-power scaling law. As climate change accelerates metabolic demands, characterizing these scaling relationships is essential to identifying which species are most physiologically vulnerable to environmental shifts. Here, we test whether coral polyp morphological traits can predict aerobic metabolism across a diverse range of reef-building species. We examine relationships between respiration and polyp biovolume, surface area, and corallite width, finding isometric scaling with biovolume and slight positive allometry with surface area, with both exponents close to one. Using median corallite width, we further extrapolate our model to theoretically predict per-polyp respiration for 727 coral species from a publicly available trait database.