Colorful corals are a mainstay in tropical reefs around the world, but warming waters are causing many corals to bleach. Researchers recently evaluated how coral larvae flow between different reef sites across the Seychelles archipelago off the eastern coast of Africa. The team found greater connectivity than predicted by models and several unexpected dispersal routes. Understanding how coral communities are connected could inform future reef restoration efforts, the researchers suggested.
Tiny Country, Many Reefs
The Republic of Seychelles is among the world’s smallest countries—its total landmass is equivalent in area to just 10 Manhattan Islands. But Seychelles’ watery resources are myriad, and coral reefs lie scattered around the nation’s 115 islands. Many of those reef ecosystems are suffering as water temperatures rise, said April Burt, a marine biologist at the University of Oxford in the United Kingdom and a consultant for the Seychelles Islands Foundation. “The entire reef system in Seychelles has already been heavily impacted by coral bleaching.”
Coral bleaching occurs when corals expel the symbiotic algae living within their tissues. The ghostly white corals that result aren’t necessarily dead, but bleaching does render corals more susceptible to subsequent stresses, said Noam Vogt-Vincent, an oceanographer at the Hawai‘i Institute of Marine Biology in Kaneohe. “The real problem is when the bleaching is prolonged,” Vogt-Vincent said. Coral bleaching can be caused by changes in ocean temperature, increased pollution, or extreme levels of sunlight.
Wandering Larvae
With an eye toward informing coral conservation efforts, Burt, Vogt-Vincent, and their colleagues recently investigated the genetic similarity of different coral colonies in Seychelles believed to all belong to the same species. The goal was to better understand how coral larvae—which ride ocean currents en route to establishing new coral colonies—disperse among the various reefs of the Seychelles archipelago. That’s important for identifying which reefs are important sources of larvae to other reefs, Burt said.
Such “donor” reefs could be prioritized when it comes to allocating limited conservation resources, she added, given their outsized importance to supplying the next generation of corals. “We can use our results to identify quantitatively which reefs in Seychelles are the biggest suppliers of coral larvae for other reefs and then channel management resources to safeguard these reefs.”
“The western Indian Ocean, largely because it’s surrounded by some of the poorest countries in the world, is comparatively unstudied.”
Though there are many coral reefs worldwide, studying those in Seychelles is particularly important, Vogt-Vincent said. That’s because relatively little is known about the reef systems there, he said. “The western Indian Ocean, largely because it’s surrounded by some of the poorest countries in the world, is comparatively unstudied.”
In 2018 and 2019, Burt spent several months collecting samples of coral from reefs in Seychelles. She hitchhiked rides on boats to 12 different islands, donned scuba gear, and dove to depths of 5–10 meters (16–33 feet). Burt then used a chisel to extract roughly thumbnail-sized pieces of one species of coral, Porites lutea.
Back in a laboratory, she and her colleagues extracted DNA from each sample. A biomedical firm sequenced the genetic material, and the researchers then zoomed in on small genetic variations—known as single nucleotide polymorphisms—to evaluate the genetic similarity of more than 200 Porites lutea samples.
All Mixed Up
The researchers found that the samples could be segregated into two distinct subpopulations. In one of those so-called clades, genetic mixing was prevalent, the team noted: Nearly 75% of the samples showed mixed ancestry and therefore likely mixing between different reef sites. In the other clade, only about 20% of the samples exhibited mixed ancestry.
Burt and her colleagues estimated that a few percent of all the corals they sampled were born in a different reef. “That’s quite a bit higher than I would have expected,” Vogt-Vincent said.
That’s also more mixing than is predicted by ocean current models, the team noted. And unlike most oceanographic models, the genetics that Burt and her colleagues uncovered suggest that coral larvae can be effectively transported between Seychelles’ Inner and Outer Islands, which are separated by well over 1,000 kilometers (620 miles).
On the basis of prevailing ocean currents, the researchers hypothesize that larvae might take a circuitous route: They likely travel from the Outer Islands, which are located farther to the east, to mainland Africa before traveling west again to the Inner Islands.
Zooming in on Donor Reefs
Sites such as Aldabra Atoll and Assumption Island in the Outer Islands are particularly valuable conservation targets, the team found. Those sites seem to be donating large numbers of coral larvae over large distances, Vogt-Vincent said. “They are connected to a massive number of coral reefs all across East Africa.”
These results were published in Scientific Reports, and the researchers also designed the Coral Connectivity app to visualize their results.
“We tend to have this idea that coral reefs are isolated systems.”
These findings highlight that coral reefs don’t exist in isolation, said Luisa Fontoura, an oceanographer at Macquarie University in Sydney, Australia, not involved in the research. “We tend to have this idea that coral reefs are isolated systems.” The study also reveals the complex interconnectedness of Seychelles’—and mainland Africa’s—coral reefs, she said. “It’s not necessarily one source and sink.” As temperatures rise, it’ll also be important to study how coral larvae potentially disperse in different ways as a result of, for instance, shifting ocean currents and changes in larval mortality rates, Fontoura said.
In the future, Burt and her collaborators plan to expand their analysis to a wider swath of the western Indian Ocean to include reefs in such places as the Chagos Archipelago and the Scattered Islands.
—Katherine Kornei (@KatherineKornei), Science Writer