Spectacular Conception

Deep within the Gulf of Mexico, an annual nighttime spectacle is providing scientists with new clues to coral conservation

  • Pete Taylor
  • Oct 01, 2000
SIXTY FEET beneath the Gulf of Mexico´s surface, a team of marine biologists holds a nighttime vigil. All around the wet-suited scientists, the water shimmers with bioluminescence. Below, visible in the piercing beams of the scientists´ underwater flashlights, corals overgrow the seafloor, their ridged contours polka-dotted with hundreds of pink and orange BB-sized blobs that form just once each year.

It is less than one hour past sundown on the eighth night following the August full moon. Somehow, the corals know that the time has come.

Suddenly and silently, three different species of massive brain and star corals begin releasing their colorful globules, which soar toward the sea surface like tiny balloons. Before long, the water fills with millions of these packets of eggs or sperm, the sum total of this year´s reproductive output for these colonies. Aboard the research boat, scientists notice a faint, sweet odor as the water´s surface becomes strewn with floating gametes.

The show´s first act continues for two hours before other species of corals take over. Mountainous star corals measuring 15 feet across emit their vast quantities of egg-and-sperm bundles. Nearby, another species of star coral releases its delicate eggs and sperm separately like champagne bubbles. At the ocean surface, the slick formed by the gametes transforms from pink to orange with the new arrivals.

Invisible to the scientists but crucial to this annual reproductive bonanza, microscopic sperm wriggle throughout the water in a blind quest for eggs. Each fertilized egg may develop into a tiny larva that spends weeks drifting with the ocean currents, possibly traveling great distances. Eventually, if it escapes fish and other predators and encounters a suitable clean spot on the seafloor, the offspring will attach itself to a hard surface and metamorphose into a single, immobile coral polyp--the beginnings of a new coral colony.

Here at the Flower Garden Banks National Marine Sanctuary, a pair of underwater mountains 123 miles south of the Texas-Louisiana border that rise from surrounding depths of 500 feet, coral spawning reaches its peak in late summer when water temperature is at its highest. The awesome phenomenon of mass coral spawning, which involves numerous species releasing their eggs and sperm nearly synchronously into the water, was unknown to science until the 1980s, when marine biologists happened upon the event on Australia´s Great Barrier Reef. In the Atlantic, mass spawning went undocumented until recreational divers witnessed it by chance during a 1990 night dive at the Flower Garden Banks, which was designated two years later as a marine sanctuary.

Now, a decade later, the Flower Garden event boasts a reputation as the most spectacular and predictable mass spawning in the western Atlantic. Each August, teams of biologists who seek the thrill of observing this natural wonder make a pilgrimage there to study the phenomenon, joined by dozens of recreational divers. Meanwhile, numerous other marine scientists also are studying coral spawning at reefs throughout U.S. waters. As these researchers learn more about this crucial event in the coral life cycle, they are developing new technologies that one day could help restore damaged reefs.

Along with the Florida Keys and Hawaii, U.S. coral reefs encircle Puerto Rico, the U.S. Virgin Islands, Guam, American Samoa and numerous remote atolls of the Pacific. Far removed from polluted coastal waters and most human activity, the Flower Garden Banks may be the most pristine of them all. So named by fishermen who discovered the banks a century ago and often pulled up colorful flowerlike chunks of corals in their nets, the Flower Garden inhabits relatively deep water, buffered from variations in sea temperature and the atmosphere that seem to harm many other reefs.

"I consider the Flower Garden Banks to be the baseline of what reefs used to be like. It´s a place where you can see how things should be in a healthy ecosystem," says Steve Gittings, the science coordinator for NOAA´s National Marine Sanctuary Program.

Small colonial animals, corals create large formations that resemble apartment buildings, housing countless individual polyps that look like tiny sea anemones. The polyps use tentacles to snag bits of food from the water but also receive energy from minute algae, called zooxanthellae, that live symbiotically within their tissues. While living polyps are found only on the exterior surface of a coral formation, the limestone skeletons of previous generations form the foundation of the colony.

Until the discovery of mass spawning, experts believed that most corals reproduced by brooding larvae inside the polyps and then releasing these larvae onto the surrounding reef. Now they know that many corals reproduce by broadcasting eggs and sperm into the water for external fertilization. In some coral species, individual colonies are either male or female. (But other species release packets that include both eggs and sperm.) Once these buoyant gamete bundles reach the sea surface, they break apart to allow the sperm to fertilize eggs from other colonies.

Mass spawning is especially spectacular at the Flower Garden because corals there cover nearly 50 percent of the seafloor, compared to other reefs that typically range from 2 to 25 percent cover. Moreover, the seven coral species that ultimately participate in mass spawning at the Flower Garden account for some 90 percent of the banks´ 350 coral-covered acres. "So everywhere you look you´ll see spawning," says Gittings. "Compare that to the Florida Keys where coral cover is generally lower and diversity is higher, which means that to see colonies that are spawning you may have to swim far and wide."

The fact that corals broadcast their eggs and sperm and that their offspring drift with the ocean currents has important implications for coral conservation. "What this means is that different reefs may be very interconnected," says Peter Vize, a biologist from the University of Texas who studies coral spawning at the Flower Garden Banks. "If you damage any part of larval supply between different coral reefs, it could have catastrophic consequences."

Previous studies have shown that water currents flow northward from the Caribbean Sea between Mexico and Cuba before turning eastward to the Florida Keys. Biologists suspect that Florida´s reefs are sustained by larvae that drift on these currents from points west. However, it is impossible to track larvae directly over these vast distances because of their small size, so the exact linkages between reefs remain unknown. Now Vize and University of Texas graduate student Derek Hagman are using sensitive genetic techniques to characterize the DNA fingerprints of coral populations on the Flower Garden Banks and elsewhere in the western Atlantic in hopes of learning how the populations are connected.

"Reefs at the Flower Garden, which are 400 miles from any other reefs, probably depend on larval input from Mexico´s Yucatan Peninsula," says Vize, "so we´re very interested in how damage to Mexican coral reefs could affect the health of American coral reefs."

Worldwide, threats to coral reefs range from anchor damage, ship groundings and overharvesting to water pollution and global warming, which is the suspected cause of so-called "bleaching" events that occur when temperature-stressed corals expel their zooxanthellae and turn bone white. So far, restoration efforts for coral reefs mainly have succeeded on a highly localized scale through mechanical repair of broken corals. But now, scientists are seeking ways to help nature actually increase the number of corals.

Biologist Aileen Morse of the University of California at Santa Barbara, for example, has discovered that drifting coral larvae respond to a specific chemical cue present in certain types of encrusting algae. This cue tells the larvae that they have encountered a good place to settle.

Now Morse has developed a novel material, which she likens to flypaper, that incorporates this chemical cue. She proposes that this "flypaper" could be placed on damaged reefs to encourage coral larvae to settle there, ultimately helping the reefs to recover. Morse´s research team has already demonstrated that the flypaper approach facilitates culturing of coral larvae in laboratory conditions.

At the Flower Garden, Vize and Hagman have experimented with a related approach that also augments natural reproduction. They collect gametes during the mass spawning event, let the eggs become fertilized and then enclose the larvae inside an underwater fine-mesh housing with terra cotta floor tiles. After the normal development period of a few days, the larvae settle onto the tiles, which the scientists can then attach to the reef. Although Vize, Hagman and other researchers have achieved success with this method on a limited scale, they foresee challenges in trying to apply it to large areas of damaged reef. "When you have hundreds of those tiles, it becomes difficult to deal with them all in an efficient way," says Hagman.

Another technology currently being developed sidesteps those challenges and may play an important role in future reef restoration. Marine biologists in Australia have devised a system for collecting thousands of coral gametes during the spawn, allowing the larvae to develop en masse in a floating aquaculture apparatus until they are nearly ready to metamorphose. Then, the researchers use a hose to squirt the larvae into an underwater "tent" where they can settle naturally onto the seafloor.

Alina Szmant, an expert on coral reproduction based at the University of North Carolina at Wilmington, has conducted small tests of this procedure at the reefs off Key Largo, Florida, and plans to experiment with it at larger scales within the next year. Eventually this tool might become a option for increasing the number of settling corals on a damaged reef.

With all of these state-of-the-art restoration methods, however, scientists face one inescapable fact: It can take two decades for a grapefruit-sized coral head to develop from an initial polyp. "Corals simply take a long time to grow," says Vize. "There´s just no rapid way to replace them once they have been destroyed."

During this year´s mass spawning event at the Flower Garden, Vize and Hagman began investigating a newly recognized aspect of coral reproduction: the ways in which separate coral colonies of the same species might communicate to coordinate their release of gametes. "We´ve noticed over the years that in species with separate sexes the males tend to start spawning a little bit earlier than females," says Hagman. "Presumably something released by the males triggers the females to start spawning."

In Guam, a similar phenomenon involving soft corals recently was discovered by University of Mississippi researcher Marc Slattery. In this case, however, the females release their gametes before the males, apparently because the sperm can remain functional for only half an hour, whereas the eggs can last for much longer.

"What´s fascinating is that when you swim above the upstream end of the reef at night, just before the spawn, you start seeing these soft coral colonies leaking eggs," says Slattery. "And if you just let the water movement carry you downstream, you start seeing the colonies underneath you spawn right about the time you get there. It seems like some sort of a chemical cue is being carried by the water current downstream and telling the colonies when to fire their gametes into the water."

Indeed, Slattery found that levels of a female hormone called estradiol increase in the water when a colony releases its eggs and he suspects that this cues other colonies to join in. Might a similar hormone or some different chemical be playing a key role in coral reproduction at the Flower Garden Banks? So far, the answer remains one of Nature´s best-kept secrets.

Formerly an editor at Islands magazine, Maine journalist Pete Taylor has a graduate degree in marine ecology.

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