Coral
From Envirowiki
Global warming is a major contributor to the phenomenon of coral bleaching. Increased sea temperatures kill corals. Bleaching occurs either when the corals die, leaving behind the white calcium exoskeletal cells which once protected the living polyps, or when they lose their colour.
Coral bleaching is an early sign of environmental stress, and it is happening with increasing frequency.
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[edit] What is coral?
Although they often look like plants, corals are animals which live in colonies of many individuals, new generations building on old. They belong to the Cnidarian group (class Anthozoa) and are closely related to jellyfish, anemones and bluebottles, all animals with a central, tube-like body cavity. Their structure includes a stomach and a mouth surrounded by anemone-like stinging tentacles which catch passing plankton. The two coral types are distinguished by the number of tentacles. Soft corals, octocorallia, have eight, while scleractinians, the hard corals, bear tentacles in multiples of six. The hard, or stony, corals are the main reef-builder (hermatypic) corals, and the most common.
Hard corals broadcast their male and female gametes, or spawn. A fertilised egg develops into an embryo within twenty-four hours and this planula (or larva) drifts for about five days until the baby coral drops, looking for a surface to anchor to. If successful the planula grows into a polyp. Polyps of hard coral secrete theca to form limestone cups called corallites, and the corallites join to form the skeleton of the colony. In these safe, miniature caves the polyps develop into small organisms like anemones. The soft, colourful tentacles with which they catch their prey flower at each opening.
Soft corals are generally brooders; instead of broadcasting they retain their egg cells, releasing only the sperm gametes. Once fertilized the planula drifts or crawls from the mother polyp’s opening, looking for the anchor-point needed to start another colony. An established planula grows into a polyp, then divides in two and secretes a new corallite cup forming colonies by asexual budding.
The protective calcium skeleton grows upwards and outwards as the polyps bud or divide, creating limestone structures unique to each polyp type, including delicate finger, fan and lettuce corals, star corals and staghorns and the eerily familiar brain corals. Boulder corals, or coral heads, increase annually by one or two centimetres, more under favourable conditions, and become more stable as they grow. Branching corals like elkhorns, which grow around ten centimetres a year, are more vulnerable to strong wave or anchor damage. Sometimes the broken pieces can stabilise and form the basis of a new colony.
[edit] Why do corals bleach?
While some cold water corals have been founds at depths up to 3000 m, most live no deeper than 60m because they share their gut with single cell algae called zooxanthellae which need sunlight to photosynthesise. The zooxanthellae are miniature recycling experts which keep the polyp clean by consuming its nitrogenous wastes, and fuel themselves by converting sunlight into the sugars the polyps need for ninety-eight percent of their food requirement. And the zooxanthellae create the brilliant colours.
A rise in sea temperature can prevent the zooxanthellae from photosynthesizing and the result is a build up of poisons within the polyp. The polyp, to survive, spits the zooxanthellae out, losing along with some of its own tissue, the characteristic brilliant colour. The colourless limestone skeleton looks bleached. Without the zooxanthellae the coral will starve. However if the sea temperature re-adjusts to an acceptable level, for instance after a seasonal spike, the zooxanthellae may reestablish, returning colour to the surviving coral.
[edit] Can the coral reefs be saved?
Australian scientists are investigating the possibility of harvesting gametes during spawning and incubating them in floating ponds. These are then towed to denuded areas and when the planulae are ready to drop are released to seed the bleached corals. Others are investigating corals which contain fluorescent pigments, which seem to protect the zooxanthellae from damage by sunlight at high temperatures, giving them a better chance of surviving bleaching events.
But global warming is altering the biosphere. Corals are very sensitive to environmental change. They can die if salinity levels drop or a rise or fall in water temperatures is sustained more than a couple of degrees beyond the normal range. Atmospheric warming and the melting of the polar ice-caps means more rain, raising the volume of runoff from landforms. Acidity and turbidity will increase, inhibiting the zooxanthellae from photosynthesizing, while salinity will decrease. The nutrients in the sea will intensify causing an overproduction of algae. Any one of these factors will kill coral.
Scientists are predicting the destruction of more than 50% of the world’s coral reefs by the year 2030.
