Scientists discover hidden hotspot where soft corals are thriving

Soft corals have been part of coral reef ecosystems for millions of years, yet scientists are only beginning to grasp how many kinds exist and where they live. Even after decades of research, large parts of reef life remain a mystery.

“A majority of the things that are out there on coral reefs are simply unknown to us or haven’t been formally described or named,” said Catherine McFadden, a professor of life sciences at Harvey Mudd College.

McFadden leads a new study with an international team from Harvey Mudd College and the Florida Museum of Natural History.

It’s the largest-ever genetic survey of soft corals, built from more than 4,400 museum specimens collected over two decades. And it has fundamentally changed how scientists understand coral diversity and where it occurs.

New center of coral life

Coral reefs are typically richest in species in the central Indo-Pacific. That’s the Coral Triangle – Indonesia, the Philippines, Papua New Guinea, and surrounding islands – a region known for its warm, shallow waters and unmatched biodiversity.

But this new study found that soft corals break that rule. They don’t have just one center of diversity – they have two.

The first overlaps with the Coral Triangle, as expected. But the second hotspot? It’s thousands of miles west, along the coasts of Madagascar and Southeast Africa. Here, soft corals are just as diverse – maybe more so – than in the better-known Indo-Pacific. That’s not the case for their harder cousins.

Stony corals, which build reefs from calcium carbonate, are less common and less varied in East Africa. Soft corals are different. And this difference may hold clues to how coral ecosystems evolved, and how they’ll survive in the future.

Ancient coral branches split

Stony and soft corals both evolved from the same ancient group: Anthozoa. This group originated around 771 million years ago. Shortly after, it split into two major branches – Hexacorallia and Octocorallia. The former gave us stony corals. The latter gave us soft corals.

Their differences go far beyond skeletons. Stony corals have hard structures that make them easier to identify by shape. Soft corals lack these features, making them notoriously hard to tell apart.

“It is really challenging to identify them in the field. It’s a colony of very similar polyps, often with similar growth forms, and that’s about it,” said Gustav Paulay, co-author of the study and curator at the Florida Museum of Natural History.

Looks deceive in soft corals

Soft corals use water pressure to keep their shape and rely on tiny calcium structures called sclerites for support. These structures are beautiful under a microscope, but unreliable when it comes to classification.

“We call them snowflakes because every single sclerite is different,” McFadden said. “They have intricate forms. They’re hard to quantify. And we don’t have a good understanding of what makes them different.”

Once scientists began using DNA to study soft corals, it exposed major gaps in how these species had been classified. Visual identification alone had masked both hidden diversity and false differences.

“There are many cases where people thought there was a widespread species, but when we do the genetic work, it turns out it’s six or eight different things that in some cases aren’t even closely related to each other,” McFadden said.

“We have the opposite problem as well where corals take a different growth form in some environments, but genetically, we’re not finding any differences between them.”

A constant struggle on the reef

Despite their different lineages, soft and stony corals often compete for the same space. The battle can get nasty.

Soft corals release toxic chemicals to keep others at bay. Stony corals strike back with stinging tentacles or even smother competitors with digestive tissue. Coral reefs may look peaceful, but turf wars are constant.

And yet, both types of coral have adapted to reef life in their own ways—until you get to the western Indian Ocean, where soft corals seem to be pulling ahead.

Reefs stranded by tectonics

This unexpected diversity in the Indian Ocean has roots in ancient geography. Twenty million years ago, a vast seaway called the Tethys Sea connected oceans from Asia to the Atlantic. It was full of coral reefs -until continental drift closed it off.

As Africa and Europe moved together, coral systems were squeezed and isolated. Some were cut off entirely.

“As the Indonesian archipelago formed, reefs ended up colonizing that, and the diversity shifted there,” Paulay said.

The reefs of Madagascar and East Africa are remnants of what once stretched across a much wider region. This isolation might help explain their unique mix of species today.

Why soft corals don’t travel well

One big reason soft corals have two diversity centers comes down to how they reproduce.

Corals reproduce both asexually and sexually. In sexual reproduction, some corals “broadcast” their eggs and sperm into the ocean. Others, called brooders, keep fertilized larvae inside until they’re ready to release them.

Broadcasting allows coral larvae to travel far – sometimes hundreds of miles. Brooding larvae, on the other hand, tend to sink near their parents. They don’t go far, and that limits their range.

Soft corals, especially brooders, aren’t good travelers. This helps explain why they’re largely missing from places like the tropical Atlantic and parts of the Pacific. It also explains why so many are unique to a single region.

The study found that endemic species – those found only in one area – were most common in the western Indian Ocean. And most of those were brooders.

Warming threatens rare corals

Endemic species are vulnerable. If their habitat disappears, they have nowhere else to go. The soft corals of Madagascar and East Africa face serious threats from warming oceans, overfishing, and pollution.

“When bleaching occurs with stony corals, they can often recover, depending on the severity and length of time they bleach,” McFadden said. “That’s not been documented with soft corals. They bleach, and they’re gone.”

If soft corals are more sensitive to bleaching – and more geographically restricted – the risks are compounded. But this also creates opportunity. Knowing where biodiversity is concentrated helps scientists know where to look for new species and where conservation efforts can do the most good.

“If you look at the number of studies that have actually identified species reliably across broad geographic ranges, it’s really focused on just maybe a half dozen groups of organisms,” McFadden said.

Most of those studies target species that humans use or consume. Soft corals don’t fall into that category. But they matter – ecologically, biologically, and as part of the reefs that support life in the ocean.

This new research gives us a clearer picture of coral diversity. More importantly, it reminds us how much we still don’t know – and how much there is left to protect.

The full study was published in the journal Scientific Reports.

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