题目内容

Scientists are growing increasingly concerned that coral, which grows abundantly in the circumtropical shallow waters near bodies of land, is evincing a paling, or bleaching effect. Though experts are still at odds over what has precipitated this event, most agree that it is a stress response to changes in habitat and water quality, including temperature variations and salination percentage, and predict a loss of 95 percent of existing coral populations.An exemplary symbiotic entity, scleractinian coral lives harmoniously with vertebrates, inver-tebrates. and plants. Corals receive nutrients in two ways: by capturing planktonic organisms with nematocyst-capped tentacles and by resource-sharing and recycling with single-celled algae called zooxanthellae. These algae live within the polyps of the coral, using photosynthesis to increase (and thereby strengthen) coral calci-fication. and providing energy for coral growth. The zooxanthellae benefit from the relationship through protection from predators and a steady supply of necessary carbon dioxide. Interestingly, it is the zooxanthellae that provide coral with its brilliant coloration.When coral loses its color, it is a sign that the single-celled algae are not able to thrive. Though not necessarily a sign of mortality, a pale, wan color indicates imminent danger and is considered a stress response. The zooxanthellate invertebrates lose their concentration of pigmentation or die altogether when stressed, turning translucent and allowing the slightly darker coral skeleton to show through the decaying tissue. Whether this response stems from anthropogenic pollutions such as overharvesting coral for the exotic travel market, overfishing coral waters, and increased water temperatures due to global warming, or from natural disturbances (storms, temperature extremes, and diseases), scientists fear for the future of the radiant corals. If zooxanthellate populations continue to decrease without recovery, their host corals will eventually follow suit, triggering a cascade of unanticipated biological events.