Indo-Pacific corals might end up dominating reefs just by being 'less picky' with their food, study shows

Corals are some of the most alien-looking species you'll find on Earth. But oh boy, are they stunning and captivating! The world’s oceans host thousands of colourful coral species that dwell anywhere from warm shallow coastline waters to the sinister frigid waters of the ocean seafloor.

Unfortunately, warming oceans have exerted a tremendous toll on these aquatic creatures, leading to massive die-offs worldwide. Australia's Great Barrier Reef is a famous example of this, where multiple bleaching events led to the structure losing half of its corals since 1995.

While similar deaths are being observed in virtually most coral reefs, scientists have discovered that some colonies might be more resilient than their peers.

But first, what exactly is bleaching, and why is there so much hoo-ha around it?
Corals: the spoiled and the grave
As beautiful as corals are, they are incredibly spoiled creatures (speaking figuratively, of course). These marine species act like living housing arrangements for some symbiotic algaes that they are excruciatingly dependent on.

These photosynthetic algae virtually serve as butlers, manufacturing energy and nutrients for their coral masters and infusing them with their fabulous colours. In return, the corals emit waste products such as ammonium that the algae can munch on.

Tragically (and predictably), climate change has been throwing a spanner into this happy family's work. Warming waters stress the corals, which in turn expel the algae from within as a stress response. Naturally, this cuts off their food supply, and eventually, they start losing colour — what we term bleaching.

Corals can survive these bleaching events, but the monumental stress they endure in the process can lead to mortality, especially if they depend on very specific algae, termed ‘symbionts’ by the study's authors.
The algae difference
According to study author Todd LaJeunesse, while coral species themselves have been studied in detail, the same love is deathly missing for their butler symbionts. In fact, the field is so criminally understudied that we don't even have names for many of the species.

Thus, the team studied many symbiont species in the Indian and Pacific Oceans, including reefs from Palau, Thailand, Zanzibar of Tanzania, the Phoenix Islands, New Caledonia and even the Great Barrier Reef of Australia.

They could identify and describe five species of the butler algae that were so diverse and flexible, they could associate with a variety of host coral species. The team believes that as oceans warm, these thermally tolerant "generalists" could hightail to other coral communities instead, helping them thrive and survive better in the increasingly torturous environment.

The researchers also found that in contrast to Indo-Pacific species, Caribbean corals are like spoiled rich brats that rely agonisingly on very specific symbionts to provide them with the yummy food they crave. This lack of flexibility makes them much more sensitive to environmental changes and that much more susceptible to subsequent bleaching episodes.

Furthermore, the study has indicated that in the coming years, Cladocopium (the genus that the five aforementioned flexible species belong to) might bring a complete overhaul to the algae status quo. According to LaJeunesse, these species may come to dominate coral communities as Earth's oceans warm and more sensitive symbionts die out.

Whatever transpires, it is clear that we need to fill in existing research gaps and better recognise these distinct species to understand how corals might fare in the near future.

The findings of this research have been published in the Journal of Phycology and can be accessed here.

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