Biology and Ecology of Coral Reefs

A series of flashcards focusing on the key vocabulary and concepts related to the biology and ecology of coral reefs, including coral anatomy, types of corals, their symbiotic relationships, and reef structures.

Coral Reefs

Structures that cover less than 1% of the ocean floor but support 25% of marine biodiversity.

Cnidaria

Phylum that includes corals, characterized by radial symmetry and stinging cells.

Radial Symmetry

Symmetry around a central axis, with no distinct left or right sides.

Polyp

A body form of some cnidarians, typically sessile and cylindrical.

Medusa

A jellyfish-like body form of cnidarians that is often free-swimming.

Zooxanthellae

Photosynthetic algae that live in coral tissue and provide energy from sunlight.

Hermatypic corals

Hard, reef-building corals that typically contain zooxanthellae.

Ahermatypic corals

Soft corals that do not contribute to reef building and may lack zooxanthellae.

Colony Formation

The manner in which individual coral polyps aggregate to form larger structures.

Coral Bleaching

A phenomenon where corals lose their symbiotic algae, leading to a loss of color and vitality.

Symbiosis

A close ecological relationship between organisms of different species, such as corals and their algae.

Calcium Carbonate

The compound that forms the skeletons of stony corals.

Suspension Feeding

A method used by corals to capture tiny food particles from the water.

Polyp Expansion

The process by which corals open their mouths and tentacles to feed, typically occurring at night.

Fringing Reef

A type of reef that is directly attached to the mainland or island and is the most common reef type.

Barrier Reef

A reef that is separated from the mainland by a lagoon and is further offshore.

Atoll

A ring-shaped coral reef that encircles a lagoon, often formed from volcanic islands.

Coralline Algae

A type of algae that contributes to reef formation by cementing coral structures together.

Nematocyst

A specialized cell in cnidarians that contains a stinging mechanism for capturing prey.

Cnidocytes

Stinging cells found in cnidarians used for defense and capturing prey.

Calcareous Structures

Structures made of calcium carbonate, formed by various marine organisms including corals.

Coral Reefs

Magnificent and highly diverse underwater ecosystems constructed primarily by colonial marine invertebrates called stony corals. These structures are built from the continuous deposition of calcium carbonate (CaCO_3) skeletons over millennia. Despite covering less than 0.1% of the ocean floor, coral reefs support an estimated 25% to 33% of all marine species, acting as vital nurseries, feeding grounds, and protective barriers for coastlines against waves and storms.

Cnidaria

A diverse phylum of aquatic, mostly marine, invertebrate animals characterized by a sac-like body plan, radial symmetry, and the presence of specialized stinging cells called cnidocytes (which contain nematocysts) for prey capture and defense. This phylum includes corals, sea anemones, jellyfish, and hydras, exhibiting two main body forms: the sessile polyp and the free-swimming medusa, often alternating between them in their life cycle.

Radial Symmetry

A fundamental body plan in which body parts are arranged concentrically around a central oral-aboral axis, like spokes on a wheel. Organisms with radial symmetry, such as cnidarians and echinoderms, possess multiple planes of symmetry and lack a distinct left or right side, though they do have discernible oral (mouth) and aboral (opposite the mouth) surfaces. This body arrangement is advantageous for sessile or slow-moving animals that encounter their environment from all directions.

Polyp

One of the two basic body forms in Cnidaria. A coral polyp is typically sessile and cylindrical, with a mouth surrounded by a ring of tentacles at its free (oral) end, facing upwards into the water column. Its basal end secrete a calcium carbonate cup (corallite) for attachment. Coral polyps are colonial, reproducing asexually to form vast, interconnected structures that collectively build coral reefs.

Medusa

The other primary body form in Cnidaria, commonly known as a jellyfish. It is typically free-swimming, bell-shaped or umbrella-shaped, with the mouth located on the underside of the bell (subumbrellar surface) and tentacles extending downwards from the bell's margins. Medusae are pelagic and are often involved in the sexual reproduction and dispersal stages of cnidarian life cycles.

Zooxanthellae

Microscopic, single-celled, golden-brown dinoflagellates (algae of the genus Symbiodinium and closely related genera) that live in a crucial endosymbiotic relationship within the gastrodermal tissues of many corals and other marine invertebrates. Through photosynthesis, they provide the host coral with up to 90% of its nutritional requirements (primarily carbohydrates, glycerol, and amino acids), receiving in return a protected environment and essential compounds like carbon dioxide (CO_2) and metabolic wastes. They are also responsible for the vibrant coloration of corals.

Hermatypic corals

Often referred to as stony or reef-building corals, these are hard corals that primarily form the rigid framework of coral reefs. They are characterized by their ability to precipitate and secrete large amounts of calcium carbonate (CaCO_3) as aragonite to construct robust, rigid exoskeletons (corallites). Hermatypic corals derive a significant portion of their energy from their symbiotic zooxanthellae, thus requiring clear, warm, shallow, and sunlit waters for optimal growth and calcification.

Ahermatypic corals

Also known as soft corals or non-reef-building corals, these species do not significantly contribute to the structural framework of coral reefs. Many ahermatypic corals do not possess symbiotic zooxanthellae and therefore do not rely on sunlight for nutrition. They predominantly feed on plankton and detritus, allowing them to inhabit a wider range of habitats, including deep, cold, or turbid waters where sunlight is scarce or absent. Examples include octocorals like gorgonians (sea fans) and black corals.

Colony Formation

The fundamental biological process by which individual coral polyps, typically through various forms of asexual reproduction (e.g., budding or fission), generate new polyps that remain physically connected to the parent polyp. This asexual proliferation leads to the establishment and growth of a larger, genetically identical aggregate known as a coral colony. Over time, the collective calcification by millions of interconnected polyps within a colony builds the extensive and intricate structures characteristic of coral reefs.

Coral Bleaching

A severe physiological stress response in corals where they expel their symbiotic zooxanthellae from their gastrodermal tissues. This phenomenon is primarily triggered by environmental stressors such as elevated sea surface temperatures, ocean acidification, pollution, or disease. The loss of these pigmented algae causes the coral tissue to become transparent, revealing the underlying white calcium carbonate skeleton, hence the term "bleaching." Bleached corals lose their primary source of energy, making them highly vulnerable to disease, starvation, and often leading to widespread mortality if stressful conditions persist.

Symbiosis

A close and often long-term ecological interaction between individuals of different biological species. These relationships can be classified into several types:

• Mutualism: Both species benefit from the interaction (e.g., corals and zooxanthellae, cleaner shrimp and fish).

• Commensalism: One species benefits, while the other is neither significantly harmed nor helped (e.g., barnacles on whales).

• Parasitism: One species (the parasite) benefits at the expense of the other (the host) (e.g., tapeworms in intestines).

  Symbiotic relationships are critical drivers of biodiversity and ecosystem function, particularly evident in the intricate food webs and interdependencies found within coral reefs.

Calcium Carbonate

A chemical compound with the formula CaCO_3. In marine biology, it is the fundamental mineral secreted by hermatypic (stony) corals in the form of aragonite crystals to form their hard, protective exoskeletons, known as corallites. This continuous biomineralization by countless coral polyps over geological timescales is responsible for the massive structural framework and growth of coral reefs. Calcium carbonate is also a key component of mollusk shells, echinoderm skeletons, and the calcified structures of many algae.

Suspension Feeding

A widespread feeding strategy employed by many aquatic animals, including coral polyps. In this method, organisms filter or capture tiny food particles, such as plankton (zooplankton and phytoplankton), detritus, and marine snow, that are suspended in the water column. Corals extend their tentacles, often armed with stinging nematocysts, to ensnare these particles and bring them to their mouths for digestion, thereby supplementing the energy they receive from their symbiotic zooxanthellae. This process is essential for coral nutrition, especially during periods of low light or stress.

Polyp Expansion

A behavioral process where coral polyps extend their soft bodies and tentacles fully into the surrounding water column. This activity is typically synchronized within a coral colony and often occurs predominantly at night. Polyp expansion maximizes the surface area available for capturing microscopic zooplankton and other organic particles through suspension feeding, supplementing the energy acquired from symbiotic zooxanthellae. It also facilitates respiration and waste removal.

Fringing Reef

The most common and developmentally simplest type of coral reef, which grows directly seaward from the coastline of a mainland or island, or separated by only a very narrow, shallow lagoon, if any. Fringing reefs typically grow seaward from the shore and consist of a shallow reef flat (closest to shore), a crest, and a slopping reef front. They are highly susceptible to terrestrial runoff, sedimentation, and human activities from the adjacent landmass.

Barrier Reef

An extensive type of coral reef that grows parallel to the coastline but is separated from it by a wider, deeper body of water known as a lagoon. Barrier reefs can stretch for hundreds or even thousands of kilometers, acting as natural breakwaters that protect the calm, shallow waters of the lagoon and the adjacent coastline from strong ocean waves and storms. The Great Barrier Reef off the coast of Australia is the largest and most famous example, exemplifying its critical ecological and physical roles.

Atoll

A distinctive ring-shaped coral reef, or a chain of coral islets, that partially or completely encloses a central lagoon. Atolls typically form in tropical oceanic regions from the gradual subsidence of a volcanic island. As the island slowly sinks, constant upward and outward coral growth maintains the reef structure at the sea surface, eventually forming a reef ring around a submerged island, often with only small land patches (motus) above sea level. This formation usually follows Darwin's theory of reef development (fringing to barrier to atoll).

Coralline Algae

A diverse group of red algae (Rhodophyta) characterized by their unique ability to precipitate calcium carbonate (CaCO_3) within their cell walls, forming hard, crust-like or branching structures. These calcifying algae play a crucial role in maintaining reef integrity by acting as a 'natural cement,' binding together coral fragments, rubble, and dead coral skeletons. This cementing action helps to consolidate the reef structure, making it more robust and resilient to physical erosion from waves and storms, and significantly contributing to overall reef accretion.

Nematocyst

A specialized, microscopic, harpoon-like stinging organelle contained within a cnidocyte (stinging cell) of cnidarians. When triggered by mechanical or chemical stimuli (e.g., touch from prey), the nematocyst rapidly discharges an inverted barbed, hollow, venomous thread with extreme speed and pressure. This powerful ejection can penetrate, entangle, or adhere to prey items or potential predators, enabling food capture and defense.

Cnidocytes

The highly specialized stinging cells found exclusively in animals belonging to the phylum Cnidaria. These cells are predominantly located on the tentacles and oral arms of corals, jellyfish, and sea anemones. Each cnidocyte contains one specialized intracellular capsule called a nematocyst. Upon proper stimulation, the nematocyst explosively discharges its coiled, harpoon-like barbed thread, injecting venom to paralyze or capture prey, and to deter predators. The presence of cnidocytes is a defining morphological characteristic of all cnidarians.

Calcareous Structures

Biological formations or components composed primarily of calcium carbonate (CaCO_3). In marine ecosystems, these structures are metabolically diverse and critically important. Examples include the rigid exoskeletons (corallites) of hermatypic corals (which form the foundation of reefs), the shells of mollusks (e.g., bivalves, gastropods), the hard tests of echinoderms (e.g., sea urchins, starfish), and the calcified secretions of coralline algae. These structures provide essential support, protection, and contribute significantly to biogenic sediment formation and geological formations like limestone.

Calcification

The biological process by which organisms precipitate calcium carbonate (CaCO_3) from seawater to form hard, mineralized structures. In corals, this involves converting dissolved calcium ions (Ca^{2+}) and bicarbonate ions (HCO_3^-) into solid aragonite (CaCO_3) crystals, which are then used to build their exoskeletons. This process is highly sensitive to ocean chemistry, particularly ocean acidification.

Planula Larva

The sexually produced, free-swimming larval stage of corals and other cnidarians. Planulae are typically elongated and ciliated, allowing them to swim in the water column for periods ranging from days to weeks. They eventually settle on a suitable hard substrate, metamorphose into a single coral polyp, and begin asexual budding to form a new coral colony. This larval stage is crucial for coral dispersal and recruitment.

Broadcast Spawning

A common method of sexual reproduction in many marine invertebrates, including most stony corals. In broadcast spawning events, male and female corals simultaneously release their gametes (eggs and sperm) into the water column, where fertilization occurs externally. These mass spawning events, often synchronized by lunar cycles and water temperature, facilitate genetic mixing and dispersal over wide areas, but are also vulnerable to environmental conditions affecting fertilization success.

Brooding (corals)

A method of sexual reproduction where corals undergo internal fertilization, retaining the eggs within the polyp while they are fertilized by sperm collected from the water. The eggs then develop internally into planula larvae, which are subsequently released into the water column. Brooding typically results in fewer but larger larvae, which may be more developed and settle closer to the parent colony, often exhibiting higher initial survival rates compared to broadcast spawned larvae.

Gastrovascular Cavity

The central body cavity in cnidarians, serving as both a digestive system and a circulatory system. This sac-like cavity has a single opening that functions as both mouth and anus. Food is digested extracellularly and intracellularly within this cavity, and nutrients are circulated to the cells lining its walls. In colonial corals, gastrovascular cavities of individual polyps are often interconnected via the coenosarc.

Coenosarc

The thin layer of living tissue that connects individual polyps within a colonial coral. It extends over the skeleton between the polyps, containing the interconnected gastrovascular cavities of adjacent polyps. The coenosarc allows for communication, nutrient sharing, and the coordinated growth and defense of the entire coral colony.

Septa (coral skeleton)

Radiating, blade-like calcareous structures found within the corallite (the cup-shaped skeleton of an individual coral polyp). These vertical partitions extend inwards from the corallite wall towards the center, providing a rigid framework of attachment for the polyp's mesenteries (internal soft tissue folds) and increasing the surface area for digestion and support. The arrangement and number of septa are important taxonomic features for coral identification.

Theca (coral skeleton)

The outer, cup-shaped wall of an individual coral polyp's calcium carbonate skeleton, also known as the corallite wall. It encircles and protects the soft body of the polyp and is the primary structure from which the septa radiate inwards. The strength and morphology of the theca are crucial for the structural integrity of both the individual polyp and the entire coral colony.

Columella (coral skeleton)

A central, pillar-like calcareous structure found in the base of the corallite, typically formed by the fusion or twisting of the inner ends of the septa. Not all corals possess a columella, but when present, it adds additional structural support to the coral polyp and helps anchor the base of the polyp within its skeleton.

Reef Crest

The shallowest and most exposed part of a coral reef, often experiencing intense wave action during storms. This zone is characterized by robust, calcified corals and coralline algae that can withstand high energy. It often forms the wave-breaking zone, protecting the inner reef and lagoon from the full force of ocean swells, and is a site of high biological activity and primary production.

Fore-reef

Also known as the reef slope or seaward slope, this zone extends downwards from the reef crest towards deeper waters. It is characterized by diverse coral growth, often hosting larger, more plate-like or branching corals in shallower areas, transitioning to more delicate and light-sensitive forms in deeper sections. The fore-reef typically receives less wave energy than the crest and is influenced by light penetration and depth.

Back-reef

The zone located behind the reef crest, situated between the crest and the shoreline or lagoon. This area is typically shallower and calmer than the fore-reef, sheltered from strong ocean waves. It often has a patchwork of coral growth, sand flats, and seagrass beds, and can experience higher fluctuations in temperature and salinity due to its more enclosed nature.

Lagoon (reef context)

A relatively shallow body of seawater separated from the open ocean by a coral reef (e.g., a barrier reef or atoll), or connected by channels. Reef lagoons are typically calm, protected environments with sandy bottoms, seagrass beds, and scattered patch reefs. They serve as valuable habitats for juvenile fish, invertebrates, and provide important foraging grounds, often influenced by restricted water circulation.

Patch Reef

Isolated, often circular or irregular coral reef formations that grow within lagoons or on the shelf floor, distinct from the main barrier or fringing reef. Patch reefs vary widely in size and demonstrate diverse coral communities, often providing local hotspots of biodiversity and important foraging or refuge sites within otherwise less complex sandy or seagrass habitats.

Bioerosion

The process by which marine organisms mechanically or chemically erode, degrade, or break down hard substrates, such as coral skeletons and reef rock. Common bioeroders include parrotfish (grazing on coral surfaces), sea urchins (rasping algae and rock), boring sponges, bivalves, and worms. While bioerosion can contribute to sediment formation, excessive rates due to environmental stress or species imbalance can weaken reef structures and impede reef accretion, altering reef architecture.

Ocean Acidification

The ongoing decrease in the pH of the Earth's oceans, accompanied by a reduction in the availability of carbonate ions (CO_3^{2-}), primarily caused by the absorption of excess atmospheric carbon dioxide (CO_2) from human activities. When CO_2 dissolves in seawater, it forms carbonic acid (H_2CO_3), increasing acidity. This change profoundly impacts calcifying organisms like corals, making it more difficult for them to build and maintain their calcium carbonate (CaCO_3) skeletons, thus threatening reef growth and stability.

Sedimentation

The process by which solid particles (sediments), such as sand, silt, and clay, are transported by water or wind and deposited onto the seabed. In coral reef environments, excessive sedimentation, often caused by coastal development, deforestation, or dredging, can physically smother corals, block sunlight for zooxanthellae, and hinder feeding mechanisms. This stress greatly reduces coral growth, increases coral mortality, and impairs larval settlement, leading to reef degradation.

Nutrient Runoff

The flow of excess nutrients, primarily nitrogen and phosphorus, from land into coastal waters. These nutrients originate from agricultural fertilizers, sewage, and industrial discharges. While some nutrients are natural, excessive runoff can lead to eutrophication in reef environments, promoting the overgrowth of macroalgae that can outcompete and smother corals, and altering the delicate balance of reef ecosystems.

Marine Protected Areas (MPAs)

Clearly defined geographical areas in the marine environment dedicated through legal or other effective means to achieve the long-term conservation of nature with associated ecosystem services and cultural values. MPAs play a crucial role in coral reef conservation by reducing direct human impacts (e.g., overfishing, destructive fishing, anchoring), allowing ecosystems to recover, and enhancing resilience to broader threats like climate change.

Crown-of-Thorns Starfish (COTS)

A large, multi-armed starfish (Acanthaster planci) that preys specifically on hard corals by everting its stomach over coral polyps and digesting their tissues. While naturally occurring, periodic outbreaks of COTS, often linked to nutrient runoff and removal of their natural predators (e.g., Triton's trumpet snail), can cause extensive and rapid destruction of vast areas of coral reefs, severely impacting reef health and biodiversity.

Coral Disease

Pathological conditions affecting corals, often leading to tissue loss, skeleton degradation, or mortality. Coral diseases are caused by various pathogens (e.g., bacteria, fungi, viruses) and can be exacerbated by environmental stressors such as elevated water temperatures, pollution, and increased turbidity, which weaken coral immune systems. Diseases like White Band Disease or Black Band Disease have caused significant coral mortality and reef decline globally.

Reef Succession

The predictable, gradual process of change in species composition and community structure over time, following a disturbance or during the initial colonization of a new substrate, in a coral reef ecosystem. This ecological process describes how a reef community develops from initial colonizers (e.g., pioneer corals, algae) to a more mature and complex climax community, involving species interactions, competition, and environmental filtering.

Primary Productivity (reef)

The rate at which energy is converted by photosynthetic organisms (e.g., zooxanthellae within corals, symbiotic algae, phytoplankton, macroalgae) into organic substances. Coral reefs are among the most productive ecosystems globally, largely due to the highly efficient symbiosis between corals and zooxanthellae, which allows these nutrient-poor waters to support extraordinary biodiversity by fixing solar energy into organic matter.

Epipelagic Zone

The uppermost layer of the ocean, extending from the surface down to approximately 200 meters (656 feet). This zone, also known as the euphotic or "sunlight" zone, is where sufficient sunlight penetrates to allow photosynthesis. Nearly all coral reefs, particularly hermatypic ones, thrive exclusively within the epipelagic zone to provide their symbiotic zooxanthellae with the light necessary for photosynthesis.

Aragonite

A specific crystalline form of calcium carbonate (CaCO_3), which is the primary mineral secreted by stony corals to build their hard skeletons. Aragonite is less stable than calcite (another form of CaCO_3) under oceanic conditions but is the preferred crystalline structure for biomineralization by most corals. Its formation is highly sensitive to the aragonite saturation state of seawater.

Calcium Carbonate Saturation State (Omega)

A measure of the thermodynamic favorability for the formation or dissolution of calcium carbonate (CaCO_3) in seawater. Specifically, the aragonite saturation state (\Omega ext{aragonite}) indicates whether seawater is supersaturated (\Omega > 1) or undersaturated (\Omega < 1) with respect to aragonite. Corals require supersaturated conditions (\Omega > 1) for efficient calcification; declining saturation states due to ocean acidification make it energetically more difficult for corals to build and maintain their skeletons.

Scleractinia

An order of marine animals within the class Anthozoa, commonly known as stony corals or hard corals. This order encompasses all hermatypic (reef-building) corals and many ahermatypic corals. Scleractinian corals are characterized by their ability to produce a rigid, external calcium carbonate skeleton and are the primary architects of coral reef ecosystems.

Octocorallia

A subclass of cnidarians within the class Anthozoa (which also includes Scleractinia). Octocorals are commonly known as soft corals, sea pens, and gorgonians (sea fans). They are distinguished by having polyps with eight pinnate (feathery) tentacles and eight mesenteries. Unlike scleractinians, octocorals typically have internal skeletons of protein and calcareous spicules, and generally do not form rigid reef structures, though they contribute significantly to reef biodiversity.

Asexual Reproduction (corals)

A mode of reproduction in corals that does not involve the fusion of gametes, resulting in genetically identical offspring (clones). Common methods include budding (a new polyp growing from an existing one), fission (a polyp splitting into two or more), fragmentation (pieces breaking off and reattaching), and bailout (individual polyps detaching). Asexual reproduction is crucial for colony growth, reef expansion, and recovery from disturbances.

Sexual Reproduction (corals)

A mode of reproduction involving the fusion of male and female gametes, leading to genetically diverse offspring. In corals, this typically involves broadcast spawning (releasing eggs and sperm into water) or brooding (internal fertilization and larval development). Sexual reproduction is essential for genetic recombination, adaptation to changing environments, and long-distance dispersal of coral populations via planula larvae.

Light Penetration (for zooxanthellae)

The depth to which sufficient sunlight can penetrate the water column to support photosynthesis by symbiotic zooxanthellae within coral tissues. This factor is critical for the survival and growth of hermatypic corals. Water clarity (turbidity) and depth are major determinants, limiting reef growth to shallow, euphotic zones where light intensity is adequate.

Turbidity (affecting corals)

The cloudiness or haziness of a fluid caused by large numbers of individual particles (suspended solids) that are generally invisible to the naked eye. High turbidity in coral reef waters, often caused by land runoff, dredging, or strong storms, reduces light penetration (harming zooxanthellae), physically smothers corals, and can clog their feeding mechanisms, leading to stress, reduced growth, and mortality.

Ecosystem Services (of reefs)

The numerous direct and indirect benefits that humans receive from healthy coral reef ecosystems. These include provisioning services (e.g., food, medicine, raw materials), regulating services (e.g., coastal protection from storms, waste detoxification), cultural services (e.g., recreation, tourism, aesthetic value), and supporting services (e.g., habitat provision, nutrient cycling, primary production). The degradation of coral reefs thus leads to significant economic and social losses.