Physiology

Size and form

The individual polyp is a saclike animal with a central mouth surrounded by a ring of tentacles. The opposite end is called the base, and here the polyp attaches to the substrate. The size of the individual polyp may vary considerably from less than a couple of mm's to 20 cm's in diameter. Generally the larger species are solitary soft corals, while most of the colonial stone corals are small.

An individual polyp has a body wall that is only two cell layers thick. The outside layer is called the epidermis, while the inside layer is called the gastrodermis. In between the two cell layers we find a gelatinous layer called the mesoglea. Basically the polyp consists of a gut cavity that is connected to surroundings via the mouth.

Nerves

The polyp is a relatively simple animal, which lack a brain but have a simple nervous system called a nerve net that extends from the mouth to the tentacles. The polyp can detect chemical substances in the surroundings (similar to our sense of taste and smell), which aid the animal in sensing potential prey and danger from predators.

Nematocysts

The polyp's tentacles are used to move food particles towards the mouth and for defense. The tentacles contain small stinging capsules (also called nematocysts), which contain a venom-filled thread with a minute barb at its tip. The nematocysts are triggered by physical or chemical stimulation, which results in the ejection of the thread. The barb then penetrates the victim's skin where the venom is released.

Reefbuilders

Reef-building corals are surrounded by a calcium carbonate skeleton. This skeleton is formed by the secretion of calcium carbonate on the outside of the coral polyp. The exact process by which the calcium carbonate skeleton is formed is not yet known, but it is assumed that the photosynthetic action of the zooxanthellae promotes the formation of the skeleton. The calcareous skeleton is continuously secreted, and when a polyp in a colony dies a new polyp will take its place, and build a new calcium carbonate skeleton on top of the old skeleton. This is why a coral reef, over a time span of thousands of years, can reach an impressive size (for example the Great Barrier Reef on the east-coast of Australia).

Coloring

The polyps may have various colors arising from natural pigments in their tissue and symbiotic algae (zooxanthellae) that live within the polyps (hermatypic corals). Not all corals contain zooxanthellae (ahermatypic corals). The reefbuilding corals all contain zooxanthellae, which is why the coral reefs need light to exist. The symbiosis gives benefits to both participants of the relationship: the algae receive protection, acces to light, nutrients and other excretory products from the polyp, whereas the polyp mainly receives photosynthate from the algae.

The Symbionts

The algae symbionts are unicellular autotrophic dinoflagelates. Many different species of dinoflagelates can participate in symbiotic relationships with corals, mollusks and sponges, and all symbiotic relationships have the general mutualistic benefits mentioned above. It is important to notice that these algae, although they are engaged in a symbiotic relationship, have the same general requirements as all other algae (primarily enough nutrients and enough light).