It is of vital importance to, on occasion, consider how little we know about the spinning rock we all live on. Take coral reefs, for example. Given how much they've been studied, you'd think we've learned just about everything about them by now. But talk to a marine biologist, and they will quickly disabuse you of this notion.
"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 is the lead author of a new study that took decades to complete and changes something fundamental about our understanding of the diversity and distribution of corals.
Key points:
- An international team of scientists have completed the world's second and largest survey of soft corals, using more than 4,400 museum specimens collected over the past two decades.
- The results show that soft corals have two primary centers of diversity, one of which was unexpected.
- The distribution of soft coral diversity is likely influenced by their limited dispersal ability and their formerly extensive range throughout the Tethys Sea, which once separated Europe and Africa.
- Soft corals have comparatively low dispersal rates and are often restricted to a limited area. This puts them at an increased risk of extinction from global warming and other human pressures. This is especially true of soft corals in the newly discovered biodiversity hotspot.
Soft and stony corals locked in a feud millions of years in the making
Corals come in a dizzying kaleidoscope of forms, and their lineage is incredibly ancient. Anthozoa — the group that contains all corals and sea anemones, plus a hodgepodge of animals with grabby tentacles called gorgonians, zoanthids, sea pens and others— originated roughly 771 million years ago. Soon after Anthozoa split in two, one group — the Hexacorallia — would evolve into stony corals and their many relatives (black corals, various sea anemones). The other, Octocorallia, evolved into soft corals and their respective relatives (gorgonians, organ pipes, sea pens, etc.). This means that what we typically refer to as corals are made from two vastly different lineages that are less related to each other than lands plants are to algae.
Of the two, scientists have focused most of their prodigious efforts on stony corals, which secrete a hard skeleton. This rigid structure is dimpled with caldera-shaped holes inhabited by individual coral polyps, sort of like a high-rise condo in a metropolitan reefscape. Stony coral species can often be distinguished by the shape of their polyps and their unique skeletal architecture. This has made it relatively easy to tell one species apart from another.
Soft corals are a different story. They lack a solid skeleton, which according to study co-author Gustav Paulay, curator of invertebrate zoology at the Florida Museum of Natural History, makes them a taxonomic nightmare. "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," he said.
Soft corals perform the trick of staying upright by pumping water through internal cavities, which allows them to grow and shrink in response to environmental conditions. This liquid, or hydro-, skeleton is further augmented by tiny calcium deposits in the shape of needles, plates, nodules and dumbbells called sclerites, which are loosely held together to provide support and protection.
"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."
Sclerites are so variable within — and similar among — species that they provide little in the way of resolution for the purposes of identification. But for many years, that's all scientists had to go on, so they made do. Then DNA sequencing was invented, and it quickly became apparent just how insufficient sclerites were.
"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."
Therein lies the issue McFadden set out to resolve in her new study. Scientists have only a tenuous grasp of how many soft coral species there are; McFadden and her colleagues went to great lengths to get a better grip.
Scientists discover secret soft coral biodiversity hotspot
They collected tissue from more than 4,400 soft corals throughout the Pacific and Indian Oceans on multiple biodiversity surveys that took place between 1998 and 2023. These specimens are now curated in natural history museums around the world, including the Florida Museum of Natural History. Next, they sequenced DNA from the preserved museum specimens and used it to determine where soft corals were most diverse and to derive a baseline estimate for how many species live in different regions.
Finally, they compared their results to those from similar studies on the better-known stony corals. If they expected anything, it was to see the same overall pattern in both soft and stony corals. After all, both groups mostly inhabit the same environments, to the extent that their proximity is often too close for comfort. As is fitting for ancient relatives, soft and stony corals have been engaged in a long-running family feud over the limited space available to them on reefs. Soft corals can emit a noxious brew of terpenes to kill their stony coral cousins and inhibit others from gaining a foothold. Stony corals retaliate by deploying sweeper tentacles, long whip-like tendrils with a bouquet of stinging cells at the end, which they use to whack their enemies with. Others simply eat their competitors by enveloping them in a web of digestive tissue called mesenterial filaments. It gets ugly out there.
Stony corals and the many dazzling fish species that live in or around reefs are most diverse in the central Indo-Pacific Ocean, whose borders enclose Australia, the Malay Archipelago and a portion of mainland Southeast Asia where ample shoreline and warm, shallow water create ideal conditions for coral reefs.
But the results of this study show that soft corals have not one but two centers of diversity. The first overlaps with stony corals in the central Indo-Pacific, which is unsurprising. Even without counting the Asian mainland or Australia's Great Barrier Reef, the island nations of Indonesia, the Philippines, Papua New Guinea and the Solomon Islands straddle the equator for thousands of miles. This region is often referred to as the Coral Triangle , which — despite only accounting for 7% of the world's oceans — is home to more than 70% of coral diversity.
Charles Darwin, who wrote a book on reef formation, was singularly moved by the coral reefs of the nearby Cocos Islands in the Indo-Pacific, writing in his journal while aboard the HMS Beagle, "We feel surprised when travelers relate accounts of the vast…extent of ancient ruins; but how insignificant are the greatest of these, when compared to the matter here accumulated by various small animals. Throughout the whole group of islands, every single atom, even from the most minute particle to the large fragments of rocks, bear the stamp of once having been subjected to the power of organic arrangement."
There is 3,000 miles of open water west of the Coral Triangle, with hardly an island in sight. The few corals here are those that can live in deep-sea environments. The Indian Ocean ends in the outline of Africa, where coral diversity picks up again.
According to the study, the coasts of Madagascar and Southeast Africa are especially important for soft corals, which are just as diverse there as they are in the Coral Triangle, if not more so. This contrasts sharply with stony corals, which have notably fewer species in East Africa than they do in Southeast Asia.
Soft coral diversity drops off below the arid horn of Africa, and though the Red Sea is wrapped in a garland of reefs, it has significantly fewer soft coral species. This means the reefs of Madagascar and Southeast Africa are essentially isolated, with only a tenuous connection to other centers of coral biodiversity.
However, this wasn't always the case.
Closing of the Tethys Sea altered the distribution of corals
Until about 20 million years ago, Earth's northern and southern continents were separated by a vast and biologically diverse body of water called the Tethys Sea. Had there been any humans around to make the journey, it would have been possible to circumnavigate the globe by sailing west from Tibet (back when Tibet had a shoreline), past the northern dome of Africa, between North and South America and across the Pacific back to Tibet. This journey can still be accomplished today from India and back again, but only because the Suez and Panama Canals now artificially recreate a small slither of what was once a giant seaway.
The shallow waters south of Europe and north of Africa were once covered in coral reefs. But this changed as the two continents slowly collided. Caught between an incoming rock and a stationary hard place, these reefs were gradually hemmed in and ultimately pushed out. The corals present today off the coasts of Madagascar and parts of Africa represent what was once the southern spur of a vast system of reefs that has since been broken up into isolated centers of diversity.
At about the same time that the Mediterranean was being closed off, the Australian continental plate was cozying up with Asia, creating a shallow water system of islands that mimicked the environmental conditions that were in the process of disappearing between Europe and Africa.
"As the Indonesian archipelago formed, reefs ended up colonizing that, and the diversity shifted there," Paulay said.
This explains why the Malay Archipelago is a modern hotbed of marine diversity, but not why soft corals in the far western Indian Ocean are more diverse than those of the stony variety.
There are a few possible answers, McFadden said. It could simply be an artifact from the way in which data were collected. "We sampled that area more thoroughly than the Coral Triangle, so there may be sampling bias," she said. The study authors used robust analytical measures to account for any potential discrepancies that might result from an uneven distribution of data, but scientists are scrupulously cautious people when it comes to drawing conclusions, and McFadden said it's too soon to know for sure whether it's a legitimate signal or a statistical phantom.
Stony corals have conversely been moderately well-studied in the Coral Triangle but have received less attention off the coasts of central Africa and Madagascar. It may turn out that they, like soft corals, are equally diverse in both areas. The only way for scientists to get a definitive answer is to collect more data.
If the pattern holds, however, there's a more tantalizing explanation, one that has to do with the way corals reproduce.
Broadcasters go the distance, brooders shirk the swell
Corals have multiple options when it comes to reproduction. The simplest of these, called asexual propagation, is for one coral to simply split in two, thereby creating a clone of itself. Sexual reproduction is a little trickier, given that corals are primarily sedentary animals. To do it successfully, male and female corals of the same species have to get the timing just right. They'll wait for the moon to enter a particular phase and for the water to reach a certain temperature, then release their eggs and sperm into the water all at once.
For some corals, this is the beginning and end of their parental responsibilities. This mode of reproduction is called broadcasting. When it works, it results in numerous, tiny coral larvae that get swept up in ocean currents and carried far from home. Many come provisioned with algal cells passed down by their parents, which they use to manufacture food on the go from the raw ingredients of water and carbon, along with a few filtered rays of sunlight to mix the two. Those that survive can end up hundreds of miles from their place of birth, but survival is far from guaranteed. The larvae of various marine creatures, collectively called plankton, are at the very bottom of the food chain. A juvenile coral in the water column is more likely to get swallowed by some random whale or suffer a similar gastrointestinal fate than it is to become successfully established on a reef.
Other corals take a less dangerous approach by brooding their young. In such cases, the females do not release their eggs during sexual reproduction. Instead, fertilization and larval development occur inside the coral. Some species keep their larvae in nutrient-packed pods called brooding pouches. When the larvae do eventually strike out on their own, they don't go far. Many travel a short way but rarely cover the kinds of distances traversed by broadcasters. Some are even negatively buoyant, causing them to sink and settle just below the parent coral.
There are broadcasting and brooding species in both soft and stony corals, but these reproductive strategies seem to have resulted in differing geographic patterns between the two groups. With the exception of a few invasive species — some likely introduced through the aquarium trade — there are no soft coral species in the tropical Pacific east of Southeast Polynesia or anywhere in the tropical Atlantic. The ailing reefs in Florida and the Caribbean, for example, are made mostly of stony corals. There are also plenty of soft coral relatives, such as gorgonians and sea whips, but actual soft corals are absent. Nor are they common in places like Hawaii or the many distantly scattered islands of the southern Pacific. The diversity of stony corals declines in this region as well, but not by nearly as much.
For reasons that remain unclear, it seems that soft corals aren't as good at getting around as stony corals, and the soft corals that brood their larvae are worst of all.
Paulay suspects this is the primary reason why soft corals have two centers of diversity. The results of this study show that, with few exceptions, the greatest number of endemic species — those that can be found in one region and nowhere else — are located in the western Indian Ocean. The results also show that among brooders and broadcasters, the former are more likely to be endemic.
Thus, the most likely explanation for why soft corals diversified in the western Indian Ocean is because they've been there for a long time. They spread into the Coral Triangle as well, but because of their low dispersal rates, this has been a slow process, and diversification has taken longer.
This has several crucial implications. The discovery of a biodiversity hotspot shows scientists the most promising areas to look for new species and which regions to prioritize for conservation.
In our age of information, there's a general feeling among people that all the world's terra incognita has been entirely explored and mapped. But while it's true that humans have done a thoroughly good job at placing landmarks along a thin crust of Earth, we still know very little about the organisms that inhabit it. According to McFadden, the work of delineating the world's most biologically active regions is still very much ongoing, even for some of the most easily recognizable and ecologically important groups.
"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," she said, further noting that most often, these groups are of commercial value.
The high percentage of endemism in the western Indian Ocean also makes this region highly susceptible to extinction events. Species that have widespread distributions can afford to lose a few populations here and there, but endemic species are constantly treading a fine line between existence and the only alternative.
There's more bad news. It's no secret that coral reefs are dwindling because of climate change. If the planet keeps warming at its current rate, coral reefs as we know them will likely be gone within the next century, but soft corals might go even sooner than that. When ocean water gets too warm for corals, they undergo a process called bleaching in which they spit out all the photosynthetic algae cells from which they derive energy.
"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."
The authors published their study in the journal Scientific Reports.
