HELPING URCHINS MAY BENEFIT CORALS

MAY 7, 1999

VIRGINIA KEY, FL--In 1983 a massive die off of the long-spined black sea urchin, Diadema antillarum occurred in the Atlantic-Caribbean region. Within a few years, billions of the urchins, a major algal consumer, disappeared from the Caribbean basin. The die off, which was caused by an unknown water-borne pathogen, spread with the circulatory patterns of ocean currents to the Florida Keys. Scientists estimate that about 90 percent of the individuals of this species have been wiped out, causing an ecosystem imbalance that currently threatens corals throughout the Caribbean and the Keys.

"As a result, many reefs have changed from coral reefs to algal reefs," says Alina Szmant, associate professor of marine biology at the University of Miami's Rosenstiel School of Marine and Atmospheric Science. "Corals are very slow-growing creatures compared to the rapidly-growing algae."

Algae is important to reef productivity and it's a natural component to the health of reefs. But when there are not enough grazers to maintain the algae population at proper levels, it can smother the corals and cover up the substrates where juvenile corals would normally settle and colonize.

During the 1950s, sixties, and seventies, researchers discovered that the abundance of the long-spined black sea urchin had increased dramatically in the Caribbean due to overharvesting of the various fishes that eat either algae or sea urchins. Without natural enemies and competition from other grazers, the urchins became overpopulated, but they held the algae population in check until the die off in the early 1980s.

"It is not known whether the extreme high densities of the voracious grazer Diadema were masking an undetected nutrification problem or whether the algal problem is directly related to the reduced grazing," Szmant says. She calls Diadema "the lawn mowers of the reef."

In an attempt to address these problems, Szmant and other Rosenstiel School researchers have recently made two important breakthroughs toward the possible restoration of Florida's coral reefs.

Tom Capo, manager of the Rosenstiel School's Experimental Hatchery, has begun to culture and raise the long-spined black sea urchin. "We thought that the possible reintroduction of Diadema might help remediate some of the negative impacts of overfishing by bringing the algae problem under control," Szmant says. "The coral larvae could then have a better chance to settle and grow, and we might then lose fewer corals to algal overgrowth. Tom started raising urchins and was successful at getting several to grow to a late juvenile stage. We hope that eventually we can get the funds that would allow us to optimize the culture of these creatures and scale-up the process so we can raise thousands of Diadema. We would like to experimentally reseed a few reef areas to see if this helps improve the quality of the reefs by keeping algae down. This may be a way to help the species make a recovery wherever they have been slow in returning."

A limited reintroduction would allow scientists to compare the effects of nutrification to grazing pressure and help to resolve the intensity of nutrification problems. "The only accurate way to determine if the overpopulation of algae on the reefs is due to nutrification or to the loss of algal grazers is to remove one of the variables," Capo says. "By controlled reintroduction of the sea urchin, we can gain new insight into the problem."

By using careful laboratory techniques, Capo succeeded in raising more than 250 of the urchins from the larval stage through metamorphosis to a late juvenile stage in about 65 days. He was able to document the previously poorly known early larval phase of the species. The study proved that a seed population of Diadema could be raised for a feasibility study or for a clear and carefully planned eventual restocking of Diadema on South Florida's reefs.

A second success is Szmant's ability to promote the fertilization, development, and subsequent settlement of coral larvae from several species of mountainous star coral and elkhorn coral, the major reef-building corals of the Florida reefs. For the first time, these corals have been successfully grown in semi-natural field conditions.

"We've been able to collect the spawn from these reef builders and have succeeded in getting them to settle on natural and artificial substrates. We want to know why there has been such poor recruitment and what we can do to improve the success rate of these reef builders," Szmant says. "In Florida, for example, we are not getting much rebuilding or reefs after damage from storms or ship groundings."

Szmant's group first learned to predict the exact time of spawning of these coral species, then learned how to raise their larvae and grow them to settlement on aged substrates in the lab and at select reef sites in Biscayne National Park. "We're also trying to learn about the recruitment stages of coral life," Szmant says. "We've been working on coral reproduction since the 1980s, and in the last couple of years have made breakthroughs in being able to predict the spawning times of some major reef-building species. There are only about a dozen coral species that grow big enough and produce enough calcium carbonate to create the substrate mass that is associated with reefs.

"What we are accomplishing is the first step toward enhancing settlement rates of these corals. If we can get them to settle and keep them healthy through the early stages when they are vulnerable to predation and overgrowth of algae, then we may be able to enhance the recruitment and regrowth." But Szmant says that the bigger issue of resource management needs to be addressed. "The root cause of the problems we are facing is overfishing in the Caribbean," she says. "There are many reef areas that are completely fished out. It is likely that the absence of predators and other reef fish contributed to the overpopulation of algae and their grazer, Diadema. An imbalance was created that has led to the serious algal problems we are witnessing today."