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
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,
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.
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
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
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
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
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
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
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
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
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
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.
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
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