REEFER MADNESS
Coral’s reproductive ritual is a sight to behold. And as so often in love, timing is everything,
At night, just after the full moon, teams of scientists dive beneath the waves to study one of the planet’s most prolific and mysterious rites of reproduction.
It’s coral behaving badly — or very nicely, depending on your point of view. Warm ocean waters suddenly teem with trillions of eggs and sperm that swirl in the currents and merge to form new life, a profligate frenzy that can leave the ocean’s surface awash in pink flotsam.
Globally, hundreds of species of coral engage in primordial rites of mass spawning tied to seasonally warming waters and the lunar cycle.
“It’s like an underwater snowstorm,” said Emma L. Hickerson, a veteran diver and research co-ordinator at the Flower Garden Banks, a coral reef about 160 kilometres off Texas in the Gulf of Mexico.
Corals are giant colonies of tiny creatures. Each small animal has a central mouth and feeding tentacles, and secretes a stony substance around its base that binds the colony together. The reefs nurture a riot of marine species and fish stocks that feed millions of people.
Studies of the procreative dance are considered vital for helping save beleaguered coral reefs around the globe, including the Great Barrier Reef off Australia, which has suffered repeated bouts of mass bleaching, mainly attributed to declining water quality and climate change. The hope is that a better understanding of coral reproduction will aid recovery, and strengthen efforts to limit coastal pollutants and sediments that can interfere with successful coral spawning.
The spectacular nature of the rite can make the research seem all the more urgent.
Alucky diver waiting for the annual event might see a coral head laden with individual sex cells that look visibly swollen and ripe for release.
“Then, all of the sudden, one goes off and, poof, they all do,” said Hickerson of Flower Garden Banks. “They all release at the same time.”
“It’s like a wave at a stadium,” she added. “You see it start at one side and go across. It’s amazing. You can’t make this stuff up.”
In nighttime dives of recent years, videographers, including Hickerson, have taken lights and cameras beneath the waves to document the natural wonder, at times zooming in so close that viewers can see the swelling and release of individual eggs. “Many people thought mass spawns did not occur in many places,” said Sally Keith, a coral ecologist at the University of Copenhagen. “It’s amazing how little is known about such a large-scale phenomenon.”
As often in romance, timing is everything. If coral shed their sex cells just minutes out of sync with neighbours, the odds of reproductive success are greatly reduced.
Indeed, scientists have discovered that the group sex can be remarkably punctual — a brain coral at Flower Garden Banks released its gametes within two minutes of its reproductive frenzy the previous year.
The liberated eggs and sperm float upward through warm ocean waters to merge near the surface and, at times, form giant pink slicks containing millions of coral embryos.
Recent studies have shown that the drifting youngsters can ride surface currents for hundreds of miles and descend to found new colonies and reefs.
While scientists have learned a lot since discovering the rite decades ago, the exact mix of environmental factors that trigger the synchronized frenzies remains unknown.
For centuries, scientists thought that stony coral reproduced mainly by brooding offspring and bringing forth live young. That dogma began to crumble when Australian graduate students followed a trail of clues to a nighttime mass spawning event on the Great Barrier Reef. In 1984, their discovery made the cover of Science magazine.
Scientists speculated that the moon, which controls tides, was integral to the ritual, but the tides during spawning events turned out to be low in some places and high in others. Scientists now say the moon most likely acts as a visual stimulus to the choreographed sex.
But how do immobile, eyeless creatures monitor the moon’s phases to determine when the time is right?
That breakthrough came after a team led by Oren Levy, a young Israeli scientist, in 2007 reported in Science that though coral don’t have eyes they do possess primitive photoreceptors, like some insects. In experiments, the team found that the photosensitive chemicals responded to moonlight as admirably as, well, human lovers.
In the Florida Keys, at relatively high latitudes, the reefs tend to go into their reproductive phase in late summer.
Last year, the big day off Key Largo turned out to be Aug. 5, five days after the full moon of July 31. Divers marvelled as staghorn coral, a spiky reef builder that can grow as much as eight inches a year, released clouds of eggs and sperm. Hickerson, of Flower Garden Banks, the marine park that houses the northernmost coral reef on the U.S. continental shelf, expects the big night to be Aug. 25.
“I always breathe a sigh of relief when I make a prediction and the coral actually spawn,” she said.
It was roughly two decades ago when Hickerson first dived beneath the waves at Flower Garden Banks to witness a mass spawning. Despite seeing it many times, she said, she still finds herself getting caught up in the primal thrill.
“To this day, I find myself screaming,” she said. “You can’t help it.”