Wanted: Reef Cleaners

For millennia, hordes of delicate, nocturnal sea urchins grazed the Caribbean sea

floor. By dining on shag carpets known as turf algae, these spiny herbivorous

urchins, often referred to as lawn mowers, kept most area corals clean as a

whistle.

That all ended in 1983 with the arrival of a mysterious plague.

Over the course of 13 months or so, a still-unidentified germ swept through the

Caribbean basin, beginning at the Atlantic side of the Panama Canal. It proved

lethal to just one species–Diadema antillarum, the corals’ primary housekeeper.

Infected urchins lost spines, grew lethargic, and exuded mucus. Any reef hit by

the epidemic would be devoid of living Diadema within 2 weeks.

Hammering the entire Caribbean and tropical West Atlantic–more than 3.5 million

square kilometers–this die-off was “the most extensive ever reported for any

marine animal,” notes Haris Lessios of the Smithsonian Tropical Research Institute

in Balboa, Panama. Overall, more than 97 percent of the area’s Diadema perished,

he says.

The loss triggered an immediate and drastic change in the regional

environment–from a diverse coral culture to one increasingly dominated by algae.

Now, 18 years later, largely unmowed blankets of greenery cover most of the

Caribbean’s hard surfaces, including its largely dead corals. Unless this dense

algal cover is cropped, larval corals can’t resettle and reclaim reefs built by

their dead and dying progenitors.

That’s why things look grim.

The abundance of living coral in the Caribbean appears to be at its lowest point

since the epoch when these creatures originally colonized the area, according to

reef ecologists. As things stand, “within another 10 years, there won’t be any

corals left to recover,” Lessios maintains.

With prognoses getting ever more dire, a bold, month-old pilot program to save the

reefs is attracting attention. In the Florida Keys, scientists have reintroduced

several hundred lab-raised Diadema to patches of reef overgrown with algae. The

goal of this long-shot experiment is to gauge the possibility of reestablishing

lost urchin populations to recover healthy ecological balances.

This follows on the heels of a report earlier this spring of patchy natural

recovery of Diadema and young coral.

Yet even a rebound of Diadema populations, many scientists worry, would amount to

a band-aid gesture. The urchin die-off and the choking algal communities that

resulted are symptoms of more fundamental environmental stresses, these

researchers argue. The most injurious of these, they say, is the centuries-long

practice of overfishing the finned vegetarians that had once helped keep the

area’s greenery in check.

Wall-to-wall urchins

Before its die-off, Diadema had been ubiquitous throughout its range. In any

square meter, one might encounter close to a dozen specimens, some as much as 30

centimeters across. Plenty of spots even harbored smaller, wall-to-wall urchins–up

to 70 per square meter, notes Terence P. Hughes of James Cook University in

Townsville, Australia.

At those densities, the urchins were practically starving, he notes. In search of

algae, they scraped clean any available surface, often to the point of eroding

reefs. Their thoroughness even killed off some larval corals.

Several biologists have speculated that this urchin’s dominance reflected an

overfishing not only of other reef grazers–predominantly parrot fish and surgeon

fish–but also of Diadema‘s many predators, which included toad fish and queen

triggerfish.

Yet until the 1983 die-off, no one appreciated how dependent the ecosystem had

become upon just one species of urchins.

There were other urchins, most notably Echinometra viridis. It’s also a black-

spined urchin that to the untrained eye, looks just like Diadema, says Gary K.

Ostrander of Johns Hopkins University in Baltimore. But there are important

differences. Not only does Diadema have spines that are more needlelike, but it

actively roves in search of algae. In contrast, Echinometra anchors itself to a

rock.

Although marine ecologists had suspected that Echinometra and other grazers would

fill any void left by Diadema, that hasn’t happened. Since the epidemic, no algae-

eating species expanded its population.

Nor has Diadema staged much of a comeback.

One reason, Hughes suspects, is that most surviving members of this species now

live too far apart. Adults release their eggs or sperm–perhaps millions of

gametes–into the water. If the spawners are more than a meter apart, Hughes notes,

fertilization doesn’t occur.

Though Diadema hasn’t gone extinct, it remains rare throughout most of its range.

“I can [scuba] dive 16 times in a week and maybe see just three,” Ostrander told

Science News.

Coral health

The prevailing rarity of Diadema has had grave repercussions. Hughes monitored

coral health along some 250 kilometers of Jamaican coastlines throughout the

decade ending in 1993. During that period, local corals suffered a major bleaching

caused largely by a stretch of unseasonably warm water there. In response, many of

the overheated corals expelled the symbiotic algae that had not only given them

color but also helped them to survive.

Ordinarily, some bleached reef heads would have recovered as larval corals

reseeded them. But without masses of Diadema present, algae moved in first. They

blanketed the bleached reefs and preventing larval coral from getting access to

the hard surfaces that they need to set up housekeeping.

In several instances, Hughes witnessed “a smothering of established [live] corals”

by large, weedy algal blooms in Diadema-free regions. He now suspects that

hurricane damage and the runoff of nutrient-rich pollution from land fostered

these blooms.

In the end, live-coral cover at the sites Hughes studied declined from about 60

percent of the reef area to just 5 percent. Since many of these species grow

slowly, “it may take a century for these corals to come back,” he notes.

Last year, Ostrander and his colleagues reported a similar trend at sites they had

studied since the mid-1990s in relatively pristine waters off San Salvador Island

in the Bahamas.

In 1994, the seafloor and reef were covered by about equal areas of live corals

and algae. Then came a January 1995 bleaching event. It launched “a rapid decline

in coral abundance and a significant increase in [large seaweed],” the researchers

reported in the May 9, 2000 Proceedings of the National Academy of Sciences.

By 1998, live coral accounted for just 5.2 percent of the studied area. That was

about a third of the live coverage seen 4 years earlier. Meanwhile, algal cover

climbed to nearly 45 percent–more than 2.5 times the territory it swathed when the

study began.

The ecological story was the same: Once a coral died, algal squatters rushed to

cover it, all but eliminating any chance of natural recovery through the

recolonization of reefs by larval coral.

When Ostrander returned to his study sites about 10 weeks ago, he says, “things

appeared to have gotten a little worse.”

An ecological rebound

Two new studies are injecting some hope, however, that an ecological rebound may

be possible.

Earlier this year, Peter J. Edmunds and Robert C. Carpenter of California State

University in Northridge reported signs of localized Diadema recovery. They saw a

concomitant reduction in local algal cover and an increase in the success with

which juvenile corals reinfiltrate reefs.

The two scientists tallied the abundance of Diadema and other pivotal species at

five shallow reefs off the north coast of Jamaica. In some zones at each site,

they found a rebound in Diadema numbers. Though these averaged about five of the

urchins per square meter, some sites hosted up to 12 per square meter. In the

rebound zones, the researchers found just 10 percent as much algal cover as in

areas where Diadema was rare. Moreover, juvenile-coral densities were 11 times

higher in urchin-grazed zones than in areas nearly devoid of Diadema.

After more than a decade of reproductive failure, corals and Diadema appear to be

successfully repopulating limited areas, Edmunds and Carpenter find. Although

these observations are heartening, “our results should not be construed to mean

that reef recovery is inevitable throughout the western Atlantic,” they cautioned

in the April 24 Proceedings of the National Academy of Sciences.

Alina M. Szmant is hoping to generate equally heartening news. A marine biologist

at the University of North Carolina at Wilmington, she headed a team that on July

27 released 200 lab-raised juvenile Diadema on plots in the Florida Keys.

This was no small feat, observes her colleague Thomas Capo of the University of

Miami in Key Biscayne. Figuring out how to raise the young urchins proved

devilishly difficult, he notes. During their unusually long larval phase, they

must be nurtured while they float in water for 3 months–a deceptively tough task.

“We’re also the first to get these animals to actively spawn under captive

conditions–which they’ve been doing for almost a year now,” Capo says.

To date, he has raised at least 4,000 larvae through metamorphosis, during which

larvae leave their buoyant life in the water to become round, spiny bottom

dwellers.

Because the researchers wanted robust animals to release on the reefs, they kept

their urchins for 4 months after metamorphosis. By that time, the creatures had

reached about 5 centimeters in diameter, as measured from tips of opposing spines.

“No one can afford to give us enough money to repopulate urchins throughout the

Caribbean–or even the Florida Keys,” Szmant acknowledges. “But if we can create

high-density pockets of the urchins, then hopefully their fertilization success

will improve,” and their populations will rebuild naturally.

In the first experiment, the juveniles were placed in two areas–one caged, the

other open. These populations will be watched to see which set-up is better at

both surviving predation and removing algae.

Szmant’s team also rounded up some wild adults and transplanted them as a group.

The adults now may be close enough to each other to successfully reproduce. Over

the next month or so, her group will also collect larval coral to eventually

replant on urchin-cleaned reefs.

The longer-term plan, Szmant says, is to tailor bigger releases according to the

lessons learned in these small-scale trials.

“Reintroducing sea urchins to start clearing off coral-reef rock is radical and

exciting,” says Elliott Norse of the Marine Conservation Biology Institute in

Redmond, Wash. Short of sending in squads of divers to scrape corals free of

algae, there are few other options available right now to reverse the continuing

decline of corals, he says.

Though the amount of algal pruning necessary to counter nearly 2 decades of

neglect is daunting, Lessios says, “you have to start somewhere.”

Hughes adds a caution to this iota of optimism. Even if robust new populations of

Diadema could be reestablished, it’s only a partial solution, Hughes argues. “It’s

far from ideal because [algal pruning] should be shared among lots of species,” he

says.

Norse agrees. He favors the siting of new marine reserves that are off-limits to

fishing (SN: 4/28/01, p. 264) so they would enable recovery of the parrot fish and

surgeon fish that formerly backed up urchins in algae management.

Crisis management

The growing need for crisis management in the Caribbean and other marine

environments should serve as “a wake-up call” that there can be high costs of

paring down the food web, says marine ecologist Jeremy B.C. Jackson of the Scripps

Institution of Oceanography in La Jolla, Calif. Underlying many marine plagues

today, he argues, has been the common practice of mining aquatic systems of

desirable fauna until only one species remains to hold the food web together. When

something knocks this species out, Jackson notes, “the whole system inevitably

collapses.”

In the July 27 Science, he and 17 colleagues review many dozens of studies and

documents that provide a historical backdrop on a host of ecosystems devastated by

human activities that rendered them vulnerable to catastrophes–including the

Caribbean’s Diadema epidemic.

For instance, Chesapeake Bay oysters were once so large and numerous that they

could filter the entire estuary every 3 days, Jackson observes. After a century of

overfishing, the depleted oyster populations takes “a lot more than a year” to do

the same thing.

After dredging of the bay to

harvest deepwater oyster populations began in the 1870s, the species’ numbers

diminished precipitously. Within a few decades, Jackson notes, the Chesapeake

became vulnerable to the toxic effects of pollution and eutrophication–in which an

overabundance of nutrients initiates a cascade of algal blooms, episodes of oxygen

deprivation, and fish kills.

Too many urchins can be as big a problem as too few.

Overfishing of North Atlantic cod, Jackson notes, knocked out a major predator of

Gulf of Maine sea urchins, species that don’t include Diadema. As the cod numbers

diminished, populations of the algae-chomping urchins grew explosively. That, in

turn, ultimately “transformed kelp forests into bare rock bottom” and erased the

protective environment that had sheltered other bottom-dwelling animals, he says.

The lesson is clear, Jackson and his colleagues say. Humanity can hope to keep the

species it cherishes only if it nurtures a diverse, integrated community of their

neighbors.