- Written by Nadia O'Carroll
Corals are animals, which belong to the class Anthozoa. The individual coral polyps are radially symmetrical invertebrates, with soft tubular bodies, which range in size from 3mm to 56mm. They have a central mouth, which both ingests food into the stomach, and expels waste after digestion. The mouth is surrounded by tentacles, which are used to catch prey and for defence. The tentacles contain nematocysts (stinging cells); each nematocyst cell consists of a coiled thread under pressure, with a barbed end and a sac of toxin. A touch will trigger the ejection of the barbed coil, and toxin will be injected to kill or paralyse prey, or to discourage predators.
Reef building Coral polyps extract calcium carbonate from sea water to build a calciferous case around themselves. As they grow they build upon this base, and when they die their case forms the foundation for the next polyp.
Communication Although polyps are individuals they live in a coordinated colony, which utilises hormones, electrochemical discharges and light to form a sophisticated web of communication.
Reproduction About 25% of corals are male or female and may live in single sex colonies, but the majority of corals are hermaphrodite (both male and female)
Coral can reproduce asexually, in which case a polyp divides buds or fragments to form another individual.
Corals, which sexually reproduce, fall into one of two categories – broadcasters or brooders. The broadcasters release millions of eggs and sperm in a mass spawning. To be successful this must be synchronised between colonies, and it is thought that water temperature, day length and the lunar cycle play a part in this timing process. The eggs and sperm float to the surface, and once fertilisation occurs the fertilised eggs hatch and the larvae drift with plankton until they settle and begins a new colony. Brooders harbour, then release fertilised eggs, which sink and settle rather than float.
Zooxanthellae some corals have a mutually beneficial relationship with algae. The algae use sunlight to create sugars and oxygen through photosynthesis, and this provides food for the coral polyps and influences their colouration. The corals that rely on this mutualistic relationship require warm (18 to 33 degrees C), clear water above 60 metres deep.
Deep cold sea corals do not rely on Zooxanthellae; they are suspension feeders, which collect food from the strong deep-sea currents. They can be found in water temperatures of 4 degrees C, in complete darkness, at depths in excess of 2000 metres. The largest deep cold sea coral reef is off the coast of Norway.
Corals provide a diverse marine habitat rich in biodiversity however both warm and cold sea corals are under threat as a result of human activity.