Spawn-taneous Coral
By: Nadine Slimak
For the first hour, the night dive on Looe Key reef in the Florida Keys was pretty quiet: Fish tucked in for the night among coral heads and spines, sleeping; tiny crabs and sea stars cautiously taking the opportunity to come out of hiding for a snack.
Then all of a sudden, in a hailstorm of Bazooka bubble-gum colored pink, it happened: Heads of mountainous star corals started “popping off.” Everywhere flashlight beams fell, coral polyps were releasing eggs in reproductive synchronicity.
For millions of years, corals have been reproducing in the middle of the night during just the right time in the lunar cycle by releasing eggs and sperm into the water column where they mix and fertilize. If all goes well, the “planula” — coral babies — eventually find a safe place to settle on the sea floor to begin new coral reefs. The process sounds a lot simpler than it really is, and with reefs under all kinds of stress caused by man-made or natural damage, diseases and climate change, coral reproduction is one hotly watched phenomenon.
In August, nearly two dozen scientists, students and observers spent eight nights diving from the NOAA Research Vessel Dante Fascell at Looe Key reef in the Florida Keys National Marine Sanctuary. The goal was to learn more about this annual event and use that knowledge to help save coral reefs.
A lunar prediction
Preparation began six months before the actual trip. Logistics had to be worked out to accommodate about a dozen gear-laden divers each night, and studies had to be planned. Dr. Kim Ritchie, Mote’s Marine Microbiology Program manager, was the lead science investigator on the trip, and Lauri MacLaughlin, resource manager at the Florida Keys National Marine Sanctuary, handled the details that made the trip run smoothly.
MacLaughlin, the trip’s veteran coral spawn watcher, gave a nightly briefing for the newcomers on what they could expect to see and what they should look out for.
A night dive on the reef is different from a dive during the day.
Before the spawn, the water has an oily coating and fishy smell at the surface and the fish below start darting around, as if they, too, are watching all the corals and trying to decide which would start spawning. The coral polyps themselves are full and bloated-looking.
“There are a lot of different animals active on the reef at night,” MacLaughlin said. “And spawning just adds another little dimension to that. There’s extra excitement on the reef when the coral spawn, and there are a number of organisms that partake of the event — having a little snack. It’s a crazy, darting mass of mayhem.”
If the fish feeding on the eggs and sperm released by the corals create one kind of mayhem, the cheers and shouts and grabbing of dive and scientific gear on the boat’s topside represent another. A coral spawn prediction, after all, is just that. No one really understands why corals release their gametes in a synchronous dance, although scientists have gotten better at pinpointing annual spawning events on coral reefs in the Keys and elsewhere. Corals spawn according to a lunar cycle, and, in the Keys, spawning usually begins three to five days after the August full moon, about two hours after sunset.
For the trip to be successful, the scientists had to be there at just the right moment. They also had to figure out a way to capture coral gametes and keep them alive for the hour-long trip back to the lab.
Bringing home baby
“People have used queen-sized pantyhose to capture the eggs,” Ritchie said. “That’s OK for analysis, but the eggs don’t survive; you can’t reproduce the corals in the lab.”
So, under Ritchie’s direction, Mote interns Carly Kenkel and Jon Onufryk engineered coral spawn catchers using muslin, clear plastic jars and a lot of ingenuity. They cut out swaths of muslin, sewed them into various shapes and sizes with holes in the middle for the jars and floated them like tents over top several species of coral — including the threatened elkhorn. If it worked, the muslin would be a chute that funneled eggs into the jar floating in the middle of the tent.
Scientists also planned to use syringes to suck in gametes and bring them back to the boat.
Would the tents work? Like expectant parents, scientists could only wait and see.
The first two nights of coral watching were a bust. Scientists saw evidence that a few corals had spawned, but it wasn’t the massive event they were hoping for.
On the expedition’s third night, divers geared up and jumped in the water to check on the corals just after sunset. The first dive of the night produced more of the same — that is, not much more than some cool night diving — and most divers got back on the boat to wait. MacLaughlin, not one to give up easily, and a few others continued watching the corals.
About 20 minutes later, she surfaced and her voice rang out: The corals were spawning! The corals were spawning!
Divers rigged up and hit the water to watch the elkhorn coral spawn. Others manned their stations on the boat. At one station, Dr. Andreas Heyland of the University of Florida’s Whitney Lab, waited for the divers to bring back eggs and sperm so he could mix them in a large bucket of sea water so they would fertilize.
Many corals are hermaphroditic and produce eggs and sperm, which are often released as packets that float to the water’s surface and burst — if they’re not eaten by fish on the way up. Once sperm and eggs are separated, they float in a freefall, waiting to meet up with members of a different colony. Somehow, eggs and sperm produced by the same or a closely related coral colony know not to fertilize each other.
“By avoiding self-fertilization, they’re able to increase their genetic diversity,” Heyland said. “In corals, multiple sperm fertilize the same egg. It’s a very good reproduction mechanism.”
While Heyland waited on the boat, Ritchie, Erich Bartels, manager of Mote’s Coral Reef Science and Monitoring Program, Cory Walter, Mote’s BleachWatch Program coordinator, and Billy Causey, National Marine Sanctuaries Regional Superintendent, Southeast Atlantic, Gulf of Mexico and Caribbean, were on the reef setting up an extra tent over a spawning coral head. The three positioned the tent, then hovered nearby. Success: Eggs floated free of the coral and right into the jar.
“All right! We have coral spawn,” Causey cheered when they returned to the Dante. The group quickly shed their gear, and, using Causey’s personal boat, sped back to Big Pine Key to hand some of the fertilized eggs off to Dave Lackland, staff biologist, so he could take them back to Mote’s Tropical Research Laboratory on Summerland Key.
Bringing up baby
“Billy handed off the eggs at about 12:30,” Lackland said. “It was like this relay going on, with a mad dash from Big Pine Key to the lab on Summerland to get these guys in the tanks.”
At Mote’s Tropical Research Laboratory, Lackland had put small limestone tiles in several small aquariums. One tank was sterile and others held tiles that had been exposed to seawater for different periods of time. This would allow Lackland to test which medium would produce the most coral settlement — a key element for growing corals in the lab. “It’s hard to work with a species that’s threatened,” he said. “It would be really great if we could create a new colony in here with the spawn from Looe Key.”
Dr. David Vaughan, Director of Mote’s Center for Coral Reef Research, said the studies could help scientists better understand coral lifecycles and use that knowledge to help rebuild coral populations in the wild. “If we can capture the live gametes (reproductive cells), get them to grow in the lab, then we can see all the components of the life cycle and see what makes a coral successful. Not much work has been done with this species (Acropora palmata) and because they’re threatened, it’s more important to understand them.”
Growing gametes in the lab is just the first step, though. Vaughan’s goal is to get this species of coral to spawn in the lab, too. “That way, instead of growing them from fragments broken off from another part of the reef and replacing them a few at a time, we can propagate them into the tens of thousands,” he said.
While Lackland dutifully watched over his coral babies — changing their water every four hours, closely monitoring water temperature and chemistry — Ritchie and the rest of the team returned to the boat. She and co-investigators Heyland, Dr. Max Teplitski, of the University of Florida, and Dr. Mikhail Matz, of the University of Texas at Austin, needed additional samples to study for genetics, bacterial and antibiotic properties.
Bacterial blend
Corals might seem like plants — and they are plant-like in their hermaphroditic abilities — but they’re really animals, called polyps, that secrete calcium carbonate to build the reef skeleton. Coral polyps also house another type of organism called zooxanthellae (zoh-zan-THEL-ee), tiny single-celled algae. The zooxanthellae give corals their color and provide food for the polyps in a symbiotic relationship.
Ritchie thinks there’s a third part to the equation: bacteria.
As a microbiologist, Ritchie’s work at Mote has focused on the kinds of bacteria that live on coral and the role they play in keeping coral healthy, or alternatively, allowing them to get sick.
So far, Ritchie has isolated different strains of bacteria living on adult corals that perform different functions. Some keep corals healthy, using antibiotic properties that inoculate corals from disease by killing bad bacteria and protecting corals from invaders. Ritchie’s studies, featured recently in the journal Marine Ecology Progress Series, found that bacteria in the mucus of healthy Acropora palmata, inhibits the growth of potentially invasive microbes by 10-fold.
But just what is that bacterial-coral-zooxanthellae relationship? Do coral planula choose to settle in places where the “right” bacteria are already living? If so, how do they know the good guys from the bad?
“We know that biofilms — communities of bacteria — form on things under the water, like the hulls of ships,” Ritchie said. “We also know that these bacteria signal each other to form.”
These biofilms are tiny microbial “cities” that have intricate architecture, complete with pillars, towers and mushroom-like domes, Teplitski said. “These microbial communities behave as one organism, rather than a myriad of individual cells,” he said.
Biofilms also form on the bottom of the seafloor — where coral babies might eventually settle and grow to form new reefs. And when biofilms reach a critical mass, some become bioluminescent or produce chemical cues that might help attract coral babies.
“It could be that the bacteria are signaling the coral to settle,” Ritchie said. “If we can figure out how to unlock these different signals, we might be able to use bacterial biofilms to improve coral settlement and help save the reefs.”
Funding for the coral reef spawning expedition was provided by the Florida Keys National Marine Sanctuary.
Learn more about: Coral Reefs
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Mote Marine Laboratory has been a leader in marine research since it was founded in 1955. Today, we incorporate public outreach as a key part of our mission. Mote is an independent nonprofit organization and has seven centers for marine research, the public Mote Aquarium and an Education Division specializing in public programs for all ages.
