There is a coral highway in the heart of the Indian Ocean

Scientists from the University of Oxford have investigated and discovered how coral larvae move between the islands of the Seychelles archipelago

An App to visualize the journey of corals
The simulation of the dispersal of coral larvae as displayed on the App developed by researchers at the University of Oxford (Photo: https://nvogtvincent.co.uk/coral_app.html)

Le coral reefs of the remote islands of the Seychelles are not isolated: a new study by the University of Oxford has discovered that they are connected with the reefs of the internal archipelago thanks to a network of ocean currents loaded with coral larvae, a sort of highway that runs through the heart of theIndian Ocean and which gives new hope for the protection of delicate marine ecosystems.

A genetic analysis complete study revealed that not only do gene flows travel hundreds of kilometers, but that they do so quite often: the study's results suggest that coral larvae can be transferred between different populations within just a few generations.

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The islands of Aldabra form the largest lagoon in the entire Indian Ocean: the islands, uninhabited, are a UNESCO World Heritage Site (Photo: Simisa/Wikipedia)

Seychelles, the study on the coral reefs of the outer islands

Despite being spread across over a million square kilometres, the coral reefs of the outer islands of the Seychelles are closely related to each other: the discovery is the result of research by the University of Oxford just published in "Scientific Reports”. Thanks to in-depth genetic analyses, English scientists have discovered that the ocean currents they carry a significant number of coral larvae, making them travel from one end of the archipelago to the other.

"This is a very important discovery”, explains the Doctor April Burt from the Department of Biology at the University of Oxford, first author of the study, “as a key factor for the recovery of coral reefs is the supply of larvae".

For the remote coral reefs of the outer islands, some of which are located over a thousand kilometers from Mahé, the nation's main island, this is a new hope for the success of repopulation strategies.

Even though the corals are decreased alarmingly throughout the world, especially because of the climate changes and pollution, it is in fact still possible to act to improve their health and resilience with actions on a local and national scale.

"These actions can be more effective if we better understand the connection between coral reefs,” explains Burt, “for example by prioritizing conservation efforts around barriers that act as main sources of larvae to support the resilience of regional barriers".

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Coral larvae travel hundreds of kilometers: study
Among the strategies adopted for the recovery of corals is "reef gardening", a real coral gardening which involves the breeding of anthozoans in underwater nurseries (Photo: Fascinating Universe/Wikipedia)

Coral recovery strategies in the Seychelles islands

Le outer islands of the Seychelles, also known as coral islands, are a archipelago of coral origin in the western Indian Ocean. Despite representing almost half of the total surface area of ​​the Seychelles, these atolls accommodate less than1 percent of the population.

It's the fault of the arid land and the lack of fresh water sources, unavoidable factors which over time have led to the conversion of these earthly paradises dotted with protected natural areas into atolls intended for resort tourism.

After the event of 1998 mass coral bleachingHowever, the risks have also extended to tourism. Then, due to the fatal pairing of El Niño and Indian Ocean Dipole, the coral reefs of the Seychelles archipelago have undergone a reduction which in some areas has reached up to 97 percent.

Coral recovery operations, we read in a recent report by Reef Resilience Network, they were very slow: “It took nearly 20 years to see coral cover at pre-1998 levels in most areas of the region".

A second major bleaching event occurred in 2016, affecting the entire Seychelles territory and leading to a reduction of coral cover by more than 30 percent.

Since then, projects for the recovery of the coral reefs of the African archipelago have multiplied: among these there is a huge project that aims to grow at least 50.000 corals by 2026 through the "reef gardening" method, which involves collecting small pieces of healthy coral, raising them in underwater nurseries and then transferring them to degraded sites.

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A long coral highway in the seas of East Africa

I Oxford researchers collaborated with numerous coral reef management organizations and the Seychelles government: they collected several coral samples from 19 different reef sites and performed acomplete genetic analysis.

The investigation revealed recent gene flow between all identified sites, probably occurring within a few generations, suggesting that coral larvae can travel from one population to another over rather short periods.

By associating genetic analyzes with the oceanographic modeling, scientists simulated the process of dispersion of the larvae, identifying the paths followed by small corals to move between the coral reefs of the region.

The results suggest that the dispersal of coral larvae among the Seychelles reefs is highly plausible, and that it could play a relevant role in defining the colony connectivity of anthozoans.

This oceanic connection, which acts as a sort of coral highway, travels hundreds of kilometers: for example, coral larvae reproduced in the remote Aldabra atoll could disperse westwards towards the east coast of Africa, particularly Mozambique, Kenya and Tanzania, via the coastal current ofEast Africa.

From here, they could continue their journey north, and perhaps encounter the southern equatorial countercurrent, which would take them back towards the internal islands of the archipelago.

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Coral larvae travel hundreds of kilometers: study
The new study suggests that strong connectivity within the Seychelles is established in a clockwise direction, potentially traveling between the Inner Islands and the remote Aldabra Group across the East African reefs and central reefs of the Seychelles (Photo: Noam Vogt -Vincent)

We now know which barriers will be crucial to recovery

As Dr. explains Noam Vogt-Vincent, who led oceanographic modeling, “theBroad agreement between predicted connectivity and observed genetic patterns supports the use of these larval dispersal simulations in the management of coral reef systems in the Seychelles and the wider region".

Simulations of this kind, explains the researcher, also allow us to study the regularity of these connectivity patterns over time, revealing fundamental information: "A regular supply of larvae will be essential for the coral reef recovery in the face of climate change".

Modeling data can be viewed by anyone via adedicated application, which allows you to view the journey of coral larvae from one barrier to another, also touching the coasts of East Africa. According to the researchers, this could help identify the main larval sources to be included in marine protected areas or in coral reef restoration operations.

As the Professor stated Lindsay turnbull, among the authors of the study, “this research it couldn't have come with better timing of this: the world once again watches as El Niño devastates coral reefs across the Indian Ocean".

Thanks to the discovery of coral highways, “we know which reefs will be crucial to coral recovery. This does not mean that we should relax our commitment to reduce greenhouse gas emissions and stop climate change".

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The coral reef of the Seychelles archipelago: after the very serious mass bleaching event of 1998, the local anthozoans are the subject of numerous recovery projects (Photo: Christophe Mason-Parker / University of Oxford)