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Coral reefs are complex mosaic of marine plants and animals. They are extremely rich in biodiversity supporting up to two million species and 25% of all marine life on the planet. As well as supporting these marine organisms, approximately 500 million people are dependent on coral reefs as a source of food, income and medicine. However, the future of coral reefs is in jeopardy as they have become threatened by human and natural disturbances which have already led to a loss of at least 19% coral reef areas since the 1950s.
Classification and anatomy
The term ‘coral reef’ is commonly used when referring to large coral structures and the ecosystems that they support. Coral organisms, called polyps, are actually tiny invertebrate animals usually ranging from 1-3mm in size.
Coral polyps are classified as anthozoans within the phylum of Cnidaria. Anthozoans represent the largest class of organisms within this phylum and are characterised by a radially symmetrical body with a single opening (which serves as a mouth and anus) surrounded by tentacles. These tentacles contain venomous cells known as nematocyst and are used by the polyp to capture prey. The mouth of the polyp leads into the gastrovascular cavity, a simple sac-like stomach, where food is digested. The gastrovascular cavity, lined by the gastrodermis (stomach lining), is divided into a number of chambers by a series of the vertical plates (septum, pl. septa). These plates support the internal stomach folds, known as mesenteries, which increase the surface area of the stomach thereby aiding the digestion of food. The gonads of polyps are also found in the mesenteries.
The top of the coral is covered with a mucus layer. The outer tissue surface of the polyp that is in contact with the water is called the epidermis. Between this surface and the gastrodermis (stomach lining) lies the mesoglea, a jelly-like connective tissue. Each coral polyp is connected to its neighbour by the coenosarc tissue. This tissue also contains gastrovascular canals which allow coral polyps to share nutrients and in some cases zooxanthelle, a type of algae hosted within the gastrodermal cells of certain species of coral. This zooxanthelle creates a mutalistic symbiotic relationship with their host corals.
Hard corals (hexacorallia or hexacorals)
The tentacles, septa and mesenteries in hard corals appear in multiples of six hence the name hexacorals. Hard corals are commonly referred to as ‘reef builders’ due to the calcium carbonate skeletons they secrete. The skeleton deposited by an individual polyp is known as a corallite. These skeletons, or corallites, are incorporated in the anatomy of coral polyps. For example, as a hard coral secretes calcium carbonate, a cup or calyx is produced, the walls of which are called theca. At the bottom of this cup is the basal plate tissue and it is upon this plate that the coral polyp sits.
A hard coral polyp (hexacorallia) b) a withdrawn hard coral polyp c) an empty coral cup or calyx.
Soft corals (Octocorrallia)
Unlike hard corals, the mesenteries and tentacles of a soft coral polyp are found in groups of eight hence the name octocorrallia or octocoral. As well as the difference in the number of tentacles and mesenteries, few soft coral polyps secrete calcium carbonate skeletons therefore their polyps do not contain septa, calyx, theca, or basal plate. Although soft corals lack the rigid calcium carbonate exoskeleton (external skeleton) of hard corals, their tissue structure has some strength and can appear rigid as it is supported by a series of calcium carbonate bodies known as spicules.
Corals and zooxanthelle: a symbiotic relationship
Some species of hard and soft corals, known as hermatypic corals, host a type of algae known as zooxanthelle within in their
gastrodermal cells. While the coral tissue protects the zooxanthelle from herbivorous grazers, the zooxanthelle photosynthesises to produce energy which they then share with their coral hosts. As the coral polyps gain extra nutrients from the zooxanthelle, the energy produced via their own feeding activity can be stored or used in high-energy activities which contribute to reef growth such as reproduction and the secretion of calcium carbonate skeletons. As well as producing energy for their host corals, the zooxanthelle use many of the polyps’ waste products such as carbon dioxides, nitrogen and phosphorus. The benefits of this relationship are felt by both the symbiont zooxanthelle and coral polyp therefore it is known as a mutualistic symbiotic relationship.
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Coral reefs are the rainforests of the sea. © Endangered Species International
Coral reefs are the rainforests of the sea. © Endangered Species International
The basic anatomy of a soft coral polyp (Adapted from http://library.thinkquest.org/26153/marine/cnida.htm)
Skeletal features of a hard coral
The basic anatomy of a soft coral polyp (Adapted from
http://library.thinkquest.org/26153/marine/cnida.htm)
Soft corals © Paddy Ryan
Acropora pingus has a strong calcium carbonate skeleton © Pierre Fidenci
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