Scientists are becoming increasingly concerned about the mysterious decline of coral reefs throughout the world and are recommending more extensive research into the potentially serious problem.
The corals are at the center of a complex food web. When they die, thousands of other species are in jeopardy. People in some developing nations are dependant on the coral reef communities for their food or livelihoods. Corals also contain toxins that offer promise for cancer research and chemical compounds that could be used to make new medicines. "But we are just in the infancy of doing those kinds of studies," said Johns Hopkins University biologist Gary K. Ostrander. Ostrander co-authored a scientific paper about the worldwide threat with biologist William J. Meehan, a doctoral student in his laboratory.
The paper, essentially an overview of declining coral reef health, will be published on April 25 in the Journal of Toxicology and Environmental Health.
"We are pointing out that these corals are dying all over the world," Ostrander said. Scientists do not know what is causing the coral reef deterioration, which has accelerated dramatically since the early 1980s. But their deaths may signal serious, as-yet unidentified environmental ills, said Ostrander, who noted in the paper that there have been few studies to detail the phenomenon at the molecular and cellular levels. Also lacking are studies aimed at uncovering possible environmental causes.
"We believe our ability to understand and possibly remedy this complex global problem will be accelerated as toxicologists begin to address these issues," the two biologists concluded in their paper.
Corals, like jellyfish and sea anemones, belong to the phylum Cnidaria. They are tiny animals that use their tentacles to catch food particles floating in the water. Certain types of corals produce calcium carbonate skeletons -- the coral reefs. Once attached to the chalky white skeleton, the tiny animals are called polyps.
The corals provide critical habitats for a multitude of other aquatic animals, and many of those animals serve vital symbiotic relationships, making up a complicated and fragile web. The most important of those symbiotic organisms is a single-celled, photosynthetic algae called zooxanthellae, which take up residence inside the clear tissue of the polyps, coloring them with various pigments. The algae collect sunlight, providing energy and nutrients for the corals. The corals, in turn, provide protection for the algae.
One of the most disturbing problems afflicting coral reefs throughout the world is referred to as "coral reef bleaching." For unknown reasons, the algae have been evacuating corals at an alarming rate.
Without the pigmented algae, the white calcium carbonate skeletons show through the clear polyps; the normally colored corals appear bleached. They then die, usually within a week or two, Ostrander said.
"We don't know what's causing the bleaching," he said. "Furthermore, the bleaching represents only one of a variety of processes contributing to the decline of coral reefs around the world."
Various diseases are ravaging the organisms, as well. "When you go to the Galapagos Islands, for example, 90 percent of the corals are gone. This is incredible."
Theories to explain what is causing the decline in coral reef health cover a wide range of possible natural and man-caused scenarios. Some scientists have implicated higher seawater temperatures, presumably from global warming. Other scientists have found evidence for just the opposite, cooler than normal seawater temperatures. Still other theories blame pollution, oil spills and sedimentation caused by construction and waste discharge.
Whatever the causes, the potential ecological and economic consequences are ominous. "There are literally thousands of species of fishes that are associated with coral reefs, and they feed off of the corals," Ostrander said. "If they all start disappearing, what's that going to do to the rest of the ecosystem?" Coral reefs are an important factor in the economies of many nations; for example, they provide key habitats for lobsters and groupers. The reefs also are a major tourist attraction. "You could imagine what Australia would be like if people didn't have the barrier reefs to go to," Ostrander noted.
The scientific paper discusses what is now known about the worldwide decline of coral reefs and portions of the paper are based on research that Ostrander and Meehan conducted while they were at Oklahoma State University; Ostrander was a professor there, and Meehan was a graduate student. They have conducted most of their studies on coral reef health in waters around San Salvador island in the Bahamas. It is one of 26 field stations in 16 nations where scientists have been using identical methods to monitor coral bleaching and the decline of species diversity around coral reefs.
An example of the rapidly declining animal populations can be seen in the shrinking number of sponges living on corals around San Salvador island. Within three years beginning in 1994, the number of sponges observed by Ostrander and his colleagues declined by about 50 percent. "We now know that San Salvador is not unique and that these sorts of trends are going on all around the world at the same time," Ostrander said.