Updated by: Arabinda Naik
Hard Corals
Hard corals, like elkhorn coral and staghorn coral, grow in colonies and are often referred to as “reef-building corals.” Hard corals create skeletons out of calcium carbonate, a hard substance that eventually becomes rock. Overtime, this rock builds up to form the foundation of a coral reef and provides a structure upon which baby corals can settle. Hard corals depend upon tiny algae called zooxanthellae that live inside them. Together, they share a symbiotic relationship the corals provide the zooxanthellae with shelter, and in return, the zooxanthellae provide the corals with food.
Horny corals. These belong to order Gorgonacea. They are branched and tree-like. The skeleton is formed of horny proteinaceous material, gorgonin, along with calcareous spicules which surround the polyp, e.g., Gorgonia. Sea fan: Order Gorgonacea includes plant-like colonies of sea fans. In Gorgonia, colony branches in one plane only. Its axial skeleton is made by horny proteinaceous material intermixed with calcareous spicules arranged around the polyps. Sea fan corals are characterized by their intricate, branching structures that resemble fans or trees. The branches are often thin and delicate, and they can come in a variety of colours, including red, yellow, orange, and purple. Unlike hard corals that have a rigid calcium carbonate skeleton, sea fan corals have a flexible internal skeleton made of a protein called gorgonin. This allows them to sway with the water currents. Sea fan corals are typically found in areas with moderate to strong water currents, as they rely on these currents to bring them food and nutrients. They attach themselves to hard substrates like rocks or the ocean floor. Like other corals, sea fan corals are primarily filter feeders. They extend their polyps into the water to capture plankton and other tiny organisms. The polyps have stinging cells called nematocysts, which help them immobilize prey. Sea fan corals often have a symbiotic relationship with zooxanthellae, which are photosynthetic algae. The algae live within the coral tissues and provide the corals with nutrients through photosynthesis. In return, the coral provides a protected environment for the algae. Sea fan corals, like many other coral species, are vulnerable to various threats, including climate change, coral bleaching, pollution, and physical damage from human activities. When stressed, they may expel their symbiotic algae, leading to coral bleaching and potentially death.
Red coral: In precious red coral, Corallium nobile, the branching colony has canaliferous coenenchyme of coenosarc containing dimorphic polyps. Axial skeleton consists of spicules embedded in CaC03 forming precious hard red coral which is used in jewellery. Corallium nobile, commonly known as red or noble coral, is a species of deep-sea coral that belongs to the family Coralliidae. Corallium nobile is characterized by its bright red or pinkish colour, which makes it highly valued in jewellery making. The colour is due to the presence of organic pigments called carotenoids. The coral has a branching or treelike structure, and its skeleton is composed of aragonite, a form of calcium carbonate. Noble corals are typically found at depths ranging from 100 to 400 meters. They are often associated with rocky substrates and underwater formations, such as underwater caves and ledges. Noble corals play a role in deep-sea ecosystems by providing habitat for various marine organisms. They are filter feeders, extracting plankton and organic matter from the surrounding water. These corals have been harvested for centuries for their use in jewellery and ornaments. The red or pink branches are highly prized for their aesthetic qualities. The harvesting of noble corals, however, has raised concerns about the sustainability of the practice, and efforts have been made to regulate and manage the trade to ensure the conservation of these deep-sea ecosystems.
Tubipora is a genus of soft coral that is found in tropical and subtropical waters around the world. It is also known as organ pipe coral because of its unique skeletal structure. Tubipora colonies are made up of long, thin tubes that are formed by the polyps. The tubes are interconnected by transverse platforms, which help to support the colony and provide a pathway for the polyps to move around. As you can see, the colony is made up of many vertical tubes. The tubes are composed of calcium carbonate, which is secreted by the polyps. The polyps themselves are green and retractile, meaning that they can withdraw into their tubes for protection. Each polyp has eight tentacles, which they use to capture prey. Tubipora colonies reproduce through budding. This means that new polyps form from the stolon, which is a horizontal tube that connects the polyps in adjacent tubes. The stolon is also responsible for transporting food and nutrients throughout the colony. Tubipora is an important part of coral reef ecosystems. It provides habitat for a variety of other organisms, and it also helps to protect the reef from erosion. However, Tubipora is also threatened by human activities such as pollution and overfishing.
Blue Corals: Heliopora: coenothecalia includes a single genus heliopora, commonly known as blue coral. Its massive calcareous, skeletonor corallium is secreted by polyps living in large, erect, cylindrical solenialtubes on the surface of skeleton. Heliopora corals are known for their blue skeletons, which are made up of a unique calcium carbonate mineral. Heliopora corals are found in shallow, tropical waters of the Indo-Pacific region.
Octocorallian corals (Soft Corals)
Octocorallian corals, also known as Alcyonaria, are a diverse and fascinating group of marine invertebrates that play an important role in coral reef ecosystems. They are characterized by their eight-fold radial symmetry, which is evident in the arrangement of their tentacles, mesenteries, and reproductive structures. Octocorals are colonial organisms, meaning that they are made up of many individual polyps that live together in a common colony. The polyps are embedded in a soft, fleshy matrix that forms the visible structure of the colony.
Sea pens: Pennatulacea, also known as sea pens, are a distinctive group of octocorals that are characterized by their elongated, feather-like bodies. Sea pens are found in a variety of habitats, from shallow, sheltered bays to deep, open ocean. They are filter feeders and use their tentacles to capture plankton and other small particles from the water. Octocorallian corals play an important role in coral reef ecosystems by providing food and shelter for a variety of marine animals, and by helping to protect coral reefs from erosion. They are also a valuable source of natural products with potential medical and pharmaceutical applications.
Alcyonacea includes marine, colonial, and soft corals. A well-known genus is alcyonium, popular as “dead man’s fingers” because of its resemblance to a human hand. It has an endoskeleton of separate calcareous spicules embedded in a massive mesoglea or coenenchyme. Alcyonacea, also known as soft corals, are the most diverse group of octocorals, with over 2,000 species. They are characterized by their flexible, fleshy bodies, which can be a variety of shapes and colours. Soft corals are often found in shallow, tropical waters, where they provide food and shelter for a variety of marine animals.
Hexacorallia Corals (True Coral): Order madrepora contains stone corals or genuine corals. Polyps secrete a calcareous, hefty exoskeleton. They are either solitary or colony in nature. Madrepora (stone coral or stag horn coral), Favia, Oculina, Acropora, and Meandrina are examples of colonial corals (Brain coral). Fungia (mushroom coral), Flabellum, and Caryophylla are solitary corals.
Solitary corals. Fungia, Flabellum, Caryophylla, etc., are the solitary corals or cup corals. The corallite is disc-like, cup-like or mushroom-shaped in form and measures 5 mm to 25 cm across. It is often without a theca.
Colonial corals.
Madrepora: corals are found in tropical and subtropical waters around the world. They are most common in shallow, clear water, but they can also be found at depths of up to 100 meters. Madrepora corals are important contributors to coral reefs. They provide food and shelter for a variety of marine life, including fish, invertebrates, and algae. Their skeletons also help to protect reefs from erosion. Madrepora corals play an important role in coral reef ecosystems. They provide food and shelter for a variety of marine life, and their skeletons help to protect reefs from erosion. Madrepora corals are also a source of food for humans. They are harvested for their calcareous skeletons, which are used in a variety of products, including jewellery, building materials, and even toothpaste.
Most of stony corals are colonial with plate-like, cup-like, spherical, or vase-shaped skeleton (coralium). Polyps live at the surface of the calcareous skeleton. Typical examples of colonial madreporarian corals are Acropora, Oculina, Favia, Madrepora, Meandrina, etc. Some of the colonies are branched. In stag- horn coral, Acropora, there is always a primary polyp at the top of colony with lateral branches on either side. In some corals, like Oculina, polyps remain widely separated, each occupying a separate theca. In others, like Favia and Astraea, thecae are so close together as to have common walls. In the brain-coral, Meandrina, polyps as well as thecae become confluent, occupying valleys separated by ridges, on the surface of coralium.