Coral is often mistaken for a rock or a plant, but it is actually a marine animal, a colonial organism composed of thousands of genetically identical polyps. These tiny polyps build the massive calcium carbonate structures that form reefs over generations. Most reef-building corals sustain themselves through a mutually beneficial partnership with microscopic algae called zooxanthellae. These algae live within the coral’s tissues, performing photosynthesis and transferring up to 90% of the organic material they produce to the host animal. Growing coral, whether for a home aquarium or large-scale conservation, relies on understanding and manipulating this unique animal-algae relationship.
Essential Environmental Conditions for Coral Life
Maintaining the correct environment is necessary for any coral growth, as these organisms are highly sensitive to physical and chemical parameters. The zooxanthellae require intense light penetration to perform photosynthesis, meaning the water must be clear with low turbidity. Corals thrive in shallow, tropical waters where the light spectrum and intensity are sufficient; for many species, this means depths less than 60 meters. The light requirements are often measured in Photosynthetically Active Radiation (PAR).
Temperature stability is paramount for coral survival, with optimal growth occurring within a narrow range of 73°F to 84°F (23°C to 29°C). Temperatures outside this range, especially prolonged high temperatures, cause thermal stress, leading to the expulsion of zooxanthellae in a process called bleaching. Corals require a stable salinity, between 32 and 42 parts per thousand, because sudden fluctuations disrupt the osmotic balance within the coral cells.
Water chemistry is also a major factor, particularly the concentration of elements necessary for skeletal construction. Corals secrete calcium carbonate to build their hard skeletons, a process called calcification. This requires maintaining optimal levels of calcium and alkalinity. Adequate water flow is necessary to deliver nutrients, remove waste products, and keep the coral surface free from sediment.
The Practical Science of Asexual Propagation (Fragging)
The most common method for cultivating coral is asexual propagation, or “fragging,” which is the fragmentation of a parent colony to create a new, genetically identical piece. This technique works by exploiting the coral’s natural ability to regenerate and recover from physical damage. This method is the primary driver of both the aquarium trade and large-scale conservation efforts.
The tools and techniques for fragging vary based on the coral’s skeletal structure. Small Polyp Stony (SPS) corals, like Acropora, are branched and can be cut cleanly with bone cutters or specialized clippers, aiming for a small branch or tip. Large Polyp Stony (LPS) corals, such as brain or torch corals, have larger polyps and require more careful cutting, using a wet saw or rotary tool to slice through the dense skeleton between polyps.
Soft corals are the simplest to propagate and may only require a sharp scalpel or scissors to cut a portion of tissue. Once the fragment, or “frag,” is separated, it must be mounted quickly onto a stable substrate, such as a ceramic plug. A reef-safe cyanoacrylate gel adhesive is commonly used to bond the coral to the plug.
After the frag is mounted, it requires a period of healing in a stable environment. The exposed skeletal tissue is susceptible to algae growth or infection, so monitoring the frag for signs of tissue regrowth is important. When the coral has fully healed and shows signs of new growth, it is ready to be moved to a permanent location.
Coral Growth Through Sexual Reproduction
While fragging creates clones, sexual reproduction is the natural process that introduces new genetic diversity into coral populations. Corals reproduce sexually through two main strategies: broadcast spawning and brooding. Broadcast spawners release massive quantities of egg and sperm bundles into the water column simultaneously.
Fertilization occurs externally in the water. The resulting embryos develop into a mobile, planktonic larval stage called a planula. These larvae can drift before seeking a suitable place to settle.
Brooding corals utilize internal fertilization, releasing planula larvae that are already fully developed. The planulae of brooding species settle much faster and closer to the parent colony, leading to less dispersal than broadcast-spawned larvae. Once a planula finds a hard bottom substrate, it attaches and undergoes metamorphosis to become a single polyp, beginning a new colony.
Scaling Up: Growing Coral for Reef Restoration
The techniques of asexual and sexual propagation are applied on a massive scale for reef restoration projects, utilizing specialized facilities called coral nurseries. These nurseries can be in-situ (underwater structures) or ex-situ (land-based aquaculture facilities). This controlled environment protects young corals during their most vulnerable growth stages.
The practice of “coral gardening” involves growing fragments in these nurseries until they are large enough to be resilient. Branching corals, such as Acropora, are commonly grown on PVC trees or line nurseries underwater, a method that accelerates their growth rate. Once the nursery-grown corals reach a sufficient size, they are permanently attached, or “outplanted,” back onto degraded reef areas using epoxy.
Larval seeding applies the knowledge of sexual reproduction to restoration, focusing on genetic diversity. Scientists collect gametes during mass spawning events, fertilize them in a lab, and then settle the resulting planulae onto artificial substrates. This use of sexually produced corals introduces new genetic combinations that may be better suited to a changing ocean environment.