Corals can exist as individual polyps, or in colonies and communities that contain hundreds to hundreds of thousands of polyps. For example, brain corals are some of the most recognizable coral species. These corals are colonies of many individual polyps; the individual polyps average 1-3 mm in diameter. However, some corals, such as Fungia plate corals, are solitary and have single polyps that can grow as large as 25 cm in diameter. In comparison, the head of a typical straight pin is 1.5 mm in diameter and the diameter of a US penny is 1.9 cm. [a] In addition, some colonial deep-sea corals form communities that resemble forests.
Anatomy of a Coral Polyp
Coral polyps are multicellular and their cells exhibit specialization to perform various functions. Unlike other Cnidarians, corals exhibit very limited organ development. They share two anatomical features with other Cnidarians: a gastrovascular cavity (simple stomach) that opens only on one end, and a ring of tentacles. Corals have no central nervous system. Many corals, while appearing to be a single organism, are actually a colony of many individual, yet genetically identical, coral polyps.
Hard corals, also known as scleractinian and stony coral, produce a rigid skeleton made of calcium carbonate (CaCO3) in crystal form called aragonite. Anatomic structures such as septa, tentacles, and mesenteries are found in sets of six, so hard corals are also often termed hexacorals.[b]
Hard corals are the primary reef-building corals. Colonial hard corals consisting of hundreds to hundreds of thousands of individual polyps are cemented together by the calcium carbonate 'skeletons' they secrete. Living coral grow on top of the skeletons of their dead predecessors. A layer of tissue covers the skeleton and connects coral polyps, allowing for distribution of nutrients as well as communication amongst individuals of the colony. [b] Hard corals that form reefs are called hermatypic coral.
Other stony coral species are incapable of producing sufficient quantities of calcium carbonate to form reefs. Many of these corals do not rely on the algal metabolites produced by zooxanthellae, and live in deeper and/or colder waters beyond the geographic range of most reef systems. [a] Some of these deep-sea stony corals may form community structures that resemble forests, but do not build reefs. Other stony deep-sea corals, Oculina and Lophelia species, for example, do build reefs.
The rate at which a stony coral colony lays down calcium carbonate depends on the species, but some of the branching species can increase in height or length by as much as 10 cm a year (about the same rate at which human hair grows). Other corals, like the dome and plate species are more bulky and may only grow 0.3 to 2 cm per year. [c]
Hard Coral Anatomy
The diagram at right shows the typical structure of an individual hard coral polyp. Polyps are usually only a few millimeters in diameter. The polyps of hard corals sit within a cup, or calyx, produced as the coral secretes calcium carbonate at its base. This secretion is how reefs are formed; the process is covered in more detail below. The walls surrounding the calyx are called the theca and the bottom of the 'cup' is the basal plate. The basal plate is a calciferous ring with six supporting radial ridges. The ridges grow vertically and project into the base of the polyp. Their anatomy also includes tabulae, which are horizontal partitions that allow for upward growth of a polyp by isolating the surface from the underlying calcium carbonate skeleton.
The stomach opens at the center of the polyp and is surrounded by tentacles. This single opening is used both to ingest food and to expel waste. The tissue lining the stomach is called the gastrodermis. Between the epidermis and the gastrodermis is a jellylike tissue called mesoglea. Septa, or vertical plates, are also present within the gastrovascular cavity; they support the internal folds, or mesenteries, of the stomach. The mesenteries increase the surface area of the stomach cavity and also contain the reproductive cells. In hard coral, the edges of the mesenteries support long mobile filaments that can protrude through the mouth and capture food or assist the coral colony in competing with its neighboring organisms for space to grow. At the bottom of the stomach is the basal plate.
The individual polyps are connected via a system of gastrovascular canals that allow for the sharing of nutrients and symbiotic algae. These canals run through the coenosarc, which is tissue that connects individual polyps at the surface of a coral colony. The outer tissue of the polyp is the epithelium or epidermis.
Tentacles for this group of corals are typically smooth and occur in sets of six. [b] They are used mainly for defense and to capture and pass food into the mouth. Tentacles may be retracted when physically stressed to protect them from predators and the elements. Some species also retract their tentacles during the day, but extend them at night, primarily to feed on plankton. The tentacles contain stinging cells, called nematocysts or cnidae.
These cells are also found on the epidermis and are used for both incapacitating prey and defense from predators. The nematocysts inject poison into the coral's prey, immobilizing or killing them. While effective against small prey such as plankton, and in discouraging predators, the sting of most corals has no harmful effect on humans. A notable exception would be fire corals, whose stings can be extremely painful for up to two weeks. Nematocysts are also one of the means by which coral competes for space to grow.
Polyps are radially symmetrical, meaning that if one cut a polyp like a piece of pie, each slice would look virtually identical. All hard corals could be divided into pie slices in multiples of six, since tentacles, septae, and mesenteries occur in groups of six. [b]
While most reef-building coral gain their yellow to brown shades of color from the symbiotic algae that live within their tissue, other corals contain protective pigments that give them bright colors. Exposure to ultraviolet light (UVA and UVB) can destroy DNA, so some coral species dwelling in shallow waters have evolved protective pigments to reduce the negative effects of ultraviolet light. These pigments are often blue, purple, or pink and account for the bright colors found in some corals. [d]
In addition, observations of coral color can be misleading as corals may appear to have different coloration above and below the water's surface. As water depth increases, the number of visible colors in the light spectrum decreases. Exposure to artificial light, such as a camera flash, adds light that is lost at depth, capturing the true colors of many corals.
Soft corals, such as sea fans, appear to be colorful underwater plants, bending and swaying with the ocean waves, but they too are coral animals..
Soft coral, also known as Alcyonacea and ahermatypic coral, do not produce a rigid calcium carbonate skeleton and do not form reefs, though they may be present in a reef ecosystem. Anatomic structures such as tentacles and mesenteries are found in sets of eight, so soft corals are often called octocorals. [b] Soft corals are also mostly colonial; what appears to be a single large organism is actually a colony of individual polyps combined to form a larger structure. [e] Though they don't secrete calcium carbonate as prolifically as hard corals, they do contain spiny skeletal elements called sclerites in the cells on the outside of the colony. Sclerites are found in the jelly-like tissue, called coenenchyme, between polyps. Sclerites are made of protein and calcium carbonate and give soft coral support to allow them to achieve their vertical structures. The sclerites also give their surfaces a spiky or grainy texture. The inner core of the colony consists of gorgonin; a flexible, fibrous wood-like protein. This gives the soft coral colony the ability to flex with the ocean waves and currents.
Visually, soft coral colonies tend to resemble trees, bushes, fans, whips, and grasses. Colonies are usually attached to the substrate at a single point at the base of a stem and most species have some level of branching in their structure. Some are also encrusting.[f]
A soft coral colony has the growth potential of two to four cm per year.
Soft Coral Anatomy
Soft coral polyps are anatomically similar to their hard coral counterparts, with a few notable exceptions. As previously stated, these coral species do not secrete calcium carbonate skeletons so their polyps do not contain calyx, theca, tabulae, septae, or a basal plate. Tentacles and mesenteries are present in groups of eight, rather than six. In addition, their tentacles are often fringed and many soft corals do not contain zooxanthellae. [b] In some soft corals, polyps specialize to perform specific functions for the colony, such as forming a supportive core at the center of the main stalk of the colony. [e]
Deep-sea coral species include both hard corals and soft corals and follow the general anatomy described above for both types of coral. However, unlike the shallow hard corals, most deep-sea corals do not contain zooxanthellae within their cells because they live too deep to rely upon sunlight. [g]