Coral ‘superhighway’ discovered in Indian Ocean

Amid climate change due to global warming, here’s good news for the marine ecosystem as researchers have discovered a coral ‘superhighway’ in the Indian Ocean.

The researchers have found a secret coral superhighway beneath the pristine waters of the Seychelles island connecting distant reefs scattered across the Indian Ocean.

According to a study published in ‘Nature‘, the coral superhighway has been formed due to ocean currents carrying larvae across vast distances among remote islands spanning over a million square kilometers, facilitating the formation of new coral reefs in the region.

Coral bleaching is the greatest threat to the persistence of tropical reef ecosystems and bleaching events are increasing in both frequency and severity. Though reducing greenhouse gas emissions is the only way to ensure a future for coral reefs, global coral cover has declined by half since 1950s, transforming the functionality of reefs, the study said.

Many countries with tropical reef ecosystems confront difficult decisions regarding the preservation of coral reefs amidst the challenges posed by climate change. To map coral connectivity in the Seychelles reef system, the researchers carried out a population genomic study of the Porites lutea species complex using 241 sequenced colonies from multiple islands. To identify oceanographic drivers of this connectivity and quantify variability, they further used a 2 km resolution regional ocean simulation coupled with a larval dispersal model to predict the flow of coral larvae between reef sites.

Patterns of admixture and gene flow are broadly supported by model predictions, but the realised connectivity is greater than that predicted from model simulations. Both methods detected a biogeographic dispersal barrier between the Inner and Outer Islands of Seychelles. However, this barrier is permeable and substantial larval transport is possible across Seychelles, particularly for one of two putative species found in genomic study.

A recent oceanographic model for the Western Indian Ocean predicts negligible connectivity between Aldabra Atoll and the Seychelles’ Inner Islands. However, due to the relatively coarse spatial resolution of the model and the basic parameterisation for larval behaviour, it is unclear to what extent these findings reflect actual connectivity.

The study presents the first comprehensive assessment of coral reef connectivity across the Seychelles’ reef system. The genomic analysis showed that in contrast to predictions from previous oceanographic models, genetic admixture does occur between the Inner and Outer Island groups of Seychelles, with contemporary gene flow in both directions. This connectivity is consistent with results from larval simulations using a higher-resolution oceanographic model, although the realised genetic connectivity revealed by population genomics is greater than that predicted from the model simulations.

A similar genetic study using Porites sp. to investigate coral connectivity in the Singapore reef system also uncovered three putative cryptic species among a sample of 160 colonies, despite their morphometric analysis suggesting that all samples belonged to one species.

The genetic analysis assumes that the observed genetic structure is due to gene flow between sampled populations, but there are many islands across Seychelles and the wider region that were not sampled in our genetics study. It is therefore also possible that some gene flow between distant populations, assumed by BayesAss to occur through direct dispersal, actually occurs in a stepping-stone manner via sites that were not sampled, such as much of the Amirantes and Farquhar Group or those outside Seychelles.

The most probable mechanism by which coral larvae from the Outer Islands could reach the Inner Islands is via westwards dispersal towards the east coast of Africa via the East African Coastal Current, where they would then travel north along the coast until reaching the South Equatorial Counter Current which could bring them eastwards to the Inner Islands.

Although this is a long and circuitous route, but the models show that such dispersal is a real possibility based on larval competency duration. It is therefore probable that some of the Seychelles Outer Islands, like Aldabra, are a source of larvae for the East African coast, and that, in turn, the East African reefs are a potential source of larvae for the Seychelles Inner Islands, which disperse larvae to the Outer Islands via the southwards Ekman transports, creating a clockwise ‘bus’ route for coral larvae around the region, the study analysed.

[This story is a part of ‘Punascha Pruthibi – One Earth. Unite for It’, an awareness campaign by Sambad Digital.]