Editor’s note: As a student-driven publication, Reporting Texas strives for the advancement of innovative, multimedia journalism. The site also works to reveal the hidden stories of Texas. The following is the product of a joint effort with partner publication, the Austin American-Statesman, Austin’s largest daily metro newspaper.
By Tara Haelle
For Reporting Texas and the Austin American-Statesman
Imagine looking at children’s hair colors to predict whether they will stick around the hometown or head farther afield, perhaps across the country. Such a prediction isn’t far-fetched if the children are young coral.
Biologists at the University of Texas have found that redder larvae of a species of staghorn coral are less likely than green larvae to settle down and develop into coral polyps, the first step in building a new colony. The study brings scientists a step closer to understanding the function of coral’s fluorescent colors.
The laboratory findings, published in the Proceedings of the Royal Society B by assistant biology professor Mikhail Matz and graduate student Carly Kenkel, could improve reef management and determine how coral species might adapt to global warming.
If redder larvae that aren’t settling down are swimming off to colonize another reef, this behavior could give the coral an evolutionary advantage. Coral is susceptible to sea surface temperature changes of even a few degrees, so being able to gradually shift to cooler latitudes could be a coral species’ best chance at survival.
“Coral needs to be able to repopulate its home reef, which is a good strategy when the environment is stable, so you need certain amount of individual larvae who settle right away,” Matz says. “But also you need to plan for the future if the environment changes and you have to move. Then you need to produce the long rangers to give the coral population an ability to move and colonize a new reef.”
Though the long-range larvae have a higher risk of never settling down, coral that sends out millions of larvae have a better chance of surviving in the long run — shifting to new locations over a couple generations — if the home reef is threatened.
The paper advises caution in generalizing too much from the research because the results came from only seven adult corals.
Seventy-five percent of the world’s coral reefs are under threat from climate change, pollution, fishing practices and other factors, according to an analysis issued last month by the United Nations and other organizations.
Coral reefs are the bedrock of tropical marine ecosystems and home to countless creatures that humans rely on for food, tourism and even certain medicines.
“As you remove certain portions of the coral reef environment, the rippling effect starts occurring, and before long, some species, whether we like them on our dinner table or in our aquarium, or simply left in their natural environment, will start disappearing,” says Billy Causey, southeast regional director of NOAA Office of Marine Sanctuaries.
Causey says Matz’s work could provide clues to how corals are distributed.
“This helps us add one more thread to this cloth of knowledge that gives us more information to protect and manage coral reefs,” Causey says.
Matz has made his name working on fluorescence in coral, helping to identify fluorescent proteins that scientists attach to genes and then track them with “fluorescent labels.” While coral fluorescence has been helpful in biotechnology, the reason coral boasts such brilliant reds, greens, yellows and cyans in the first place has eluded scientists.
“Every coral has them, and we have very little understanding what these fluorescent proteins mean for the biology of the animal,” Matz says. “The range of hypotheses is extremely broad, so we were trying to explore all routes to put us on a path to explore the biology of fluorescence.”
Among a half-dozen theories are ones suggesting that fluorescence may help regulate photosynthesis, attract algae that coral consume or help coral communicate with fish, much as flowers communicate to bees that their pollen is open for business.
Matz traveled to Australia’s Magnetic Island during coral spawning season and collected eggs and sperm from three red adults, three green adults and one adult with muted fluorescence. The Matz team crossed-fertilized the eggs to produce larvae whose fluorescence ran the spectrum from green to red. Then they introduced the larvae to ground-up red algae comprised partly of calcium carbonate, a chalky substance also found in coral skeletons. This calcareous red algae is a catalyst known to cause larvae to settle down and metamorphose into the tiny polyps that start a coral colony.
The researchers reported that 56.2 percent of the settlement variation among the larvae was attributed to color, though genetics appeared to play a part as well.
“The larval stage is very important because that’s the only stage of the life cycle when coral is mobile,” Matz says. “Once they decide to settle down, there’s no turning back.”
Kenkel says larvae will appear to “sniff” the algae and then seems to “decide” whether to stay.
Once larvae change into polyps, circular organisms with rings of tentacles, they begin laying down a skeleton by excreting calcium carbonate, and then growing into a colony of polyps.
“If it’s a bad decision — if the temperature changes or there are other competitors in the region or there’s algae that overgrows them before they’re large enough — then they can’t get out of it because they have physically cemented themselves to the reef. It pays to be choosy,” she says.
Matz and Kenkel also discovered a correlation between a larva’s parents and its settlement decision. Nearly 50 percent of the variation between siblings who settle or swim could be explained by genetics, though genetics only accounted for about 18 percent of the red-green color variation among the larvae.
The inherited color variation did not correlate to the parent’s color — a red parent didn’t necessarily yield a red larva — but sibling larvae had a greater likelihood of sharing similar colors. Matz says this finding might mean the genes coding fluorescence on larvae are different from the genes coding it on adults.
The next steps are for Matz and his colleagues to validate the results on a natural reef, and to follow individual offspring to see whether each, whether red or green or something in-between, settles down.
The research also raises several additional issues.
“The question is why there are so many poorly settling ones,” Matz says. “Apparently, not responding well to the settlement cues has some advantages.”
Another question is whether the color of the larvae causes them to settle, or the gene that codes fluorescence is just physically close on the chromosome to the gene that controls the decision to settle down. If they’re close on the chromosome, both traits could be inherited together.
“It could be that one really has nothing to do with another, but it’s a really nice marker for indicating settlement likelihood anyway,” Kenkel says. “That’s what we’re thinking.”
Matz says the study offers a good lead to a connection between fluorescence and a biological function.
“Right now, we have this interesting observation that color can tell us something about larvae and about what it will do,” he says.