Coral Biology Glossary

Allelopathy

Allelopathy refers to the release of defensive chemicals by certain corals and other organisms to prevent the survival of nearby competitors. Allelopathy typically affects only organisms in close proximity to the source of the chemicals, but can be very detrimental to surrounding species over long periods of time. Allelopathy is specific to the chemical defenses released by corals to protect themselves, not to be confused with the stinging defenses provided by nematocyst cells within coral tentacles.

Aragonite

Aragonite is a crystalline form of calcium carbonate that is present in most active reefs. Aragonite is the primary mineral structure that contributes to reef formation, and is used by stony corals to create their skeletons. In addition to coral skeletons, aragonite can be found in rock substrate, sand, and animal shells.

Attachment

Attachment refers to the process by which corals secure themselves to a viable substrate. This process typically involves tissue adhesion using mucus, a stolon, or encrustation by depositing calcium carbonate. The security of attachment can affect a coral's rate of growth, as stability allows for optimal photosynthesis and metabolism.

Bleaching

Coral bleaching refers to the process by which corals respond to stress by expelling the zooxanthellae algae responsible for photosynthesis. This bleaching is almost always indicative of stress, and is noticeable through visible paling of the coral’s skeletal and tissue color. Bleaching is a temporary stress response, but prolonged bleaching can lead to coral death due to the loss of photosynthetic energy.

Branching

Branching refers to the process of growth employed by certain kinds of stony corals that produce elongated skeletal limbs. Branching corals typically grow more vertically than horizontally, building their skeletons upward and outward toward the light to increase surface area for light exposure and flow. These branching corals can be somewhat brittle, but their growth patterns are laced with many natural “break” lines that aid in the preservation of the colony as a whole. When branches break off or “frag”, the broken fragment is often capable of growing into a new colony if it’s able to find a suitable substrate.

Budding

Budding is a type of asexual reproduction marked by the development of a new coral polyp from an existing polyp. Budding is common in many kinds of coral, and allows for the growth of a full colony from the new polyp. Unlike most forms of fragging, budding does not require human intervention.

Calcification

Calcification describes the process by which corals process calcium and carbonate ions from their surrounding water and convert them into aragonite to create their skeletons. Calcification is dependent on specific aquarium parameters being in healthy ranges and remaining stable, including calcium, magnesium, alkalinity, flow, and lighting. Corals and other animals perform calcification at different rates and in different patterns.

Colony

A colony is a group of polyps with fundamentally identical genetics that are connected by living tissue. Individual polyps can be detached from the colony without permanently injuring it, and those polyps can grow to produce new colonies through the coral’s natural reproductive process. Although colonies tend to grow collectively, individual polyps are subject to receive varying nutrients from the water and light they’re exposed to. This means that colony growth can be shaped by the environment, with polyps exposed to favorable conditions growing faster than polyps surrounded by less favorable conditions. Although most are immobile, some corals can slowly reposition themselves, detach under stress, or intentionally displace themselves to find more suitable conditions or substrates. 

Coenosarc

The coenosarc is the connective tissue that allows individual polyps within a colony to share resources with one another. The coenosarc is an essential organ in coral because it allows all of the polyps to collectively benefit from the nutrients absorbed by any one polyp. This tissue helps the colony heal from damage and grow efficiently. 

Encrustation

Encrustation is the method through which corals attach to the surface of a substrate and spread outward across the substrate by depositing skeletal mass. Encrustation requires calcification to produce skeletal mass, which consumes calcium carbonate as the skeleton is formed. This term is typically used in reference to stony corals.

Encrusting

Not to be confused with encrustation, encrusting describes a pattern of growth in which the coral grows outward rather than upward like branching corals. Encrusting corals often grow laterally to form a thin layer over the substrate. The process of encrusting requires calcification, meaning this term is used to describe stony corals.

Fragging

Fragging, short for “fragmenting”, is a term used to describe the process of coral propagation where a polyp or portion of a coral colony is intentionally separated. Corals can be fragged by either cutting them or splitting them along natural break lines in their skeletons. This process does not injure the coral if done correctly, but requires basic knowledge of coral anatomy. Corals are colonial animals, and their asexual reproduction allows them to reproduce lost polyps over time. Individual polyps that are separated from the colony are also capable of growing into a completely independent colony, which makes the process a sustainable means of reproduction. Improper fragging, however, can result in prolonged tissue damage or the death of the colony, so it’s important that fragging is preceded by research.

Heterotrophy

Heterotrophy is the process through which corals gain energy by consuming food particles or small organisms. All corals are capable of some level of heterotrophy, though reliance on feeding varies widely by species. Heterotrophy is considered a secondary yet beneficial process for energy absorption, with photosynthesis being the primary driver of growth for coral. Some corals can expel their stomachs during heterotrophy to assist in the capture and digestion of food.

Large Polyp Stony Coral (LPS)

Large polyp stony corals, also known as LPS, are a variation of corals distinguished by their large, fleshy polyps that are supported by calcium carbonate skeletons. LPS corals are typically sensitive to instability and prefer moderate light and flow. However, flow that is too strong or direct can sometimes cause tissue damage in the fleshy polyps of LPS corals. Because these corals deposit calcium carbonate skeletons to grow, maintaining proper calcium and alkalinity levels is essential for optimal coral growth.

Matting

Matting refers to the process of growth through which certain corals spread a connective layer of tissue across the surface of a substrate. This process is distinguished by more lateral growth than vertical, and matting corals often prefer lower light intensity. Matting is typically used in reference to soft corals that rely on tissue adhesion for their primary means of attachment, while stony corals that grow in this pattern are typically described as encrusting or plating. 

Mesenterial Filaments

Mesenterial filaments are digestive tissues present in some corals that can be extended outward to digest food or attack predators. Mesenterial filaments can be used both offensively and defensively to capture food and attack nearby organisms. These filaments contain stinging cells called nematocysts and enzymes that protect the coral, which helps it remain competitive in its environment, despite its lack of mobility.

Mouth

The mouth is the primary orifice of the coral polyp that allows for both waste removal and heterotrophy, or the digestion of food particles. This mouth is typically located in the center of the oral disc and often surrounded by tentacles that help capture and direct food toward the mouth. Certain kinds of coral are most effectively fragged by cutting the polyp at the mouth to split it into two separate halves of the original polyp. In this process, both halves of the polyp will heal to form a new second half of the polyp if conditions are favorable.

Nematocysts

Nematocysts are the cells within a coral’s tentacles that deliver stinging toxins to other organisms they come into contact with. Nematocysts are used defensively and to incapacitate food before consumption. Unlike allelopathy, nematocysts affect other organisms on a physical level, whereas allelopathy causes chemical irritation or damage.

Oxidative Stress

Oxidative stress is a reaction that occurs in coral when detrimental reactive oxygen molecules build up within coral tissue. This stress is often associated with coral bleaching, which is one of the more obvious indicators of unfavorable conditions. Other common indicators of oxidative stress include polyp recession and increased mucus production. Oxidative stress can be caused by elevated water temperatures, excessive light intensity, and other factors.

Pedal Disc

A pedal disc is the foundational structure used by many soft corals and other organisms to attach securely to a substrate. Pedal discs, sometimes referred to as “basal discs”, secure to a substrate by spreading their muscular tissue across the surface and using chemicals to anchor coral to the surface texture. Pedal discs can be intentionally detached from the selected substrate by the coral if conditions become less favorable.

Photoperiod

Photoperiod refers to the coral’s duration of light exposure in a full 24-hour cycle. The photoperiod influences the coral’s health by directly contributing to growth and metabolism. A photoperiod that is too long can cause oxidative stress that leads to bleaching, while one that is too short can delay growth due to photosynthetic limitations. Most reef aquariums are run with photoperiods of 8-12 hours, ideally with a ramp up prior to full intensity and dimming down prior to lights-off. Light intensity is just as important as duration and cycling for coral health.

Photosynthesis

Photosynthesis is a process that converts light into energy that is used by countless plants and animals. In corals, this process is carried out by symbiotic algae called zooxanthellae, which process light to support coral growth. These zooxanthellae are sensitive to unstable chemical and physical parameters, and their numbers may diminish if living conditions are not balanced. Although many corals are heterotrophic, photosynthesis through zooxanthellae is the primary source of energy that supports coral health and growth.

Polyp

A polyp is an individual head of a coral animal, which typically consists of a body, mouth, and tentacles. The body is responsible for digestion and growth, the tentacles are used for defending territory and catching prey, and the mouth is used for both feeding and expelling waste. Each polyp is capable of growing into an entirely new colony if separated from the mother colony. Individual polyps that are separated from the main colony are called coral frags.

Skeleton

A coral’s skeleton is the rigid structure composed of calcium carbonate that forms their main body. Corals produce skeletons in many different patterns depending on the metabolic needs of the coral, with some growing narrow and branching to absorb light and flow while others grow outward broadly to maximize surface area on the faces of the polyps. Corals create their skeletons through a process called calcification, which consumes carbonate and bicarbonate ions (alkalinity) in the water to build calcium carbonate structures. Calcification and skeletons are specific to stony corals, whereas soft corals do not possess skeletons.

Small Polyp Stony Coral (SPS)

Small polyp stony corals, also known as SPS corals, are corals that produce skeletal structures as they grow and possess relatively small individual polyps. SPS corals often prefer stronger flow, more robust lighting, and extremely stable water chemistry. Unstable water chemistry can cause extreme stress in SPS corals, which can lead to tissue loss and coral death. Some small polyp stony corals grow in branching patterns that provide increased surface area for flow and lighting, while others grow through plating or encrusting, which is distinguished by more lateral growth to maximize light exposure. SPS are known for their low-nutrient tolerance compared to other corals.

Soft Coral

Soft corals are one of two main subsets of corals and are distinguished by their fleshy bodies and lack of skeletal structures. Unlike their stony coral counterparts, soft corals generally grow through tissue expansion alone, which is driven by photosynthetic zooxanthellae algae and some heterotrophic behaviors. Soft corals typically use a basal foot or adhesive mucus to attach and anchor themselves to surfaces, and some are capable of detaching from their substrate if conditions become unfavorable. As with all corals, soft corals are able to regenerate their tissue if water chemistry is stable, which makes these colonies more resilient than they may appear. Soft corals are also known for being more tolerant of unstable conditions than stony corals, making them a good choice for beginning reefers.

Space Competition

Space competition refers to the territorial balance that all species engage in across the substrates within a reef environment. Everything from bacteria to coral requires basic needs for growth, and one of these requirements is the space in which to grow. Organisms like coralline algae not only contribute to a healthy ecosystem by their effects on water chemistry, but also occupy real estate on their substrate that non-beneficial bacteria would eventually colonize. Because reef ecosystems contain countless organisms, one of the best defenses against pests is building up a healthy ecosystem of organisms that compete with nuisance organisms and prevent them from gaining a foothold. Space competition is most apparent in reef systems when discussing the microbiome, which is a complex collection of microorganisms that form a balanced ecosystem to process chemicals and stabilize water chemistry.

Stalk

The stalk is the part of a coral below the polyp or colony that elevates it above the base of the coral. Stalks are particularly common in soft corals, but they are present in stony corals as well. The stalk often contains digestive organs and zooxanthellae algae that use photosynthesis to provide energy to the coral. The stalk typically connects the basal foot or base of the coral to the base of the tentacles or polyp.

Stomach (Gastrovascular Cavity)

The stomach of a coral, also known as the gastrovascular cavity, is the organ responsible for the digestion of food and nutrients. The stomach is located beneath the polyp and processes the nutrients ingested by the mouth. Some corals can expel their stomachs to externally digest nearby organisms or compete for space. As colonial animals, coral polyps each possess their own stomach to process nutrients.

Stony Coral

Stony corals are one of two main subsets of corals and are distinguished by the calcium carbonate skeletal structures they produce as they grow. Stony corals pull alkalinity from the water to produce these aragonite-based skeletal structures, which occur in numerous growth patterns. Stony corals are known to grow in branching, plating, or encrusting patterns depending on the species, all of which require high levels of calcium and carbonate hardness in the water. Stony corals can be further broken down into small polyp stony corals (SPS) and large polyp stony corals (LPS). Stony corals are generally more demanding than soft corals in terms of water chemistry stability, lighting, and flow.

Stolon

A stolon is a tissue extension from the base of the coral that stretches across the surface of a substrate as the coral grows. This stolon anchors the coral to the substrate and forms new polyps as the coral expands horizontally. Stolons are typically possessed by soft corals like waving hand anthelia, and can be detached if conditions become less favorable for the coral. Each polyp will form its own stolon if separated from the colony to use for substrate attachment and regrowth.

Symbiosis

Symbiosis is a mutually beneficial relationship between two separate species. There are many that exist in reef settings, including zooxanthellae algae and coral polyps, clownfish and anemones, goby fish and pistol shrimp, boxer crabs and anemones, and more. Zooxanthellae algae perform the photosynthesis that sustains corals, and clownfish provide nutrients to anemones in exchange for shelter. Pistol shrimps, which are nearly blind, dig burrows for protection, and goby fish use their sharp eyesight to guard this shared burrow. Boxer crabs, sometimes called “pom pom” crabs, carry little stinging anemones with them to help them catch food. The anemones benefit from absorbing additional nutrients, and the crabs benefit from the stinging power possessed by the anemones. Symbiosis is the natural concept of mutually assured success.

Tendril

A tendril is a slender appendage extending from the coral tissue used to attach or expand the colonial mass. Tendrils are often used to anchor the coral to the surface of a substrate before calcium carbonate is deposited to form skeletal mass. Unlike tentacles, tendrils are primarily used for the expansion of the colony by serving as the soft precursor to secure anchoring, rather than absorbing nutrients or defending the colony.

Tentacles

Tentacles are sensory appendages surrounding the mouth of the coral that are used to capture food, harvest nutrients from the surrounding water, and defend the colony. Unlike tendrils, tentacles are not used in anchoring or supporting expansion. Defense occurs through stinging cells called nematocysts. These nematocysts aid in the capture of food and prevent competing creatures from establishing dominance. Corals also use tentacles to explore the environment around them because of their sensory perception capabilities.

Zooxanthellae

Zooxanthellae are a kind of algae that form a highly symbiotic relationship with corals. These algae live within coral tissue and use photosynthesis to provide metabolic energy for the coral, enabling growth, attachment, and overall survival. The presence and health of zooxanthellae is typically observed through coral coloration, which is often rich and evenly toned in healthy specimens. However, when coral is stressed or otherwise unhealthy, signs of bleaching and discoloration are often the first indicators that the zooxanthellae within the coral are stressed or dissipating. Because corals derive most of their energy through photosynthesis, these algae are essential for coral survival.