Exploring the depths: a dive into the evolution of scleractinian corals

Scleractinian corals, known as stony or hard corals, are cosmopolitan. While we often associate them with tropical species like the brain or staghorn coral, it turns out that the first corals were deep-sea inhabitants! The emergence of zooxanthellae and coloniality gave rise to tropical coral reefs.
Published in Ecology & Evolution
Exploring the depths: a dive into the evolution of scleractinian corals
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The mere act of going to the beach, taking a stroll, and contemplating the ocean makes you feel very small next to its vastness. Just donning scuba diving gear and diving a few meters underwater is enough to be amazed by the diversity that exists, beyond our day-to-day activities and our imagination. Ongoing advancements in ocean exploration technologies continually reveal how little we know about the world we live in. It's exciting to witness this diversity and, at the same time, almost inevitable to try to organize it in our minds. Finding patterns in what exists in different locations, at different depths, and how this diversity responds to environmental variables such as temperature, the amount of organic matter, light, oxygen, or ocean currents. And there's a history as well. Every living being is partly who they are because of their ancestors, and partly where they are because of where these ancestors were. This has been the starting point of our work, a phylogeny that tells us no more or less than the evolutionary relationships between species, when they originated, and how the speciation process has occurred. Additionally, we decided to study hard corals due to their widespread distribution, distinctive features—which I will elaborate on—and their significance in both shallow and deep ecosystems.

In the phylogeny tips, we encounter all species that have originated over approximately 415 million years since the first coral emerged in the world. This initial coral differed from the brain coral or staghorn coral. Unlike them, it was a solitary coral, meaning it did not reproduce asexually to form colonies and, consequently, did not create coral reefs. Furthermore, it lacked zooxanthellae, single-celled algae that live symbiotically in the coral endodermal tissue. This association is essential for tropical coral reefs, as these waters are very clear with minimal organic matter, and the coral primarily feeds on photosynthetic products of their algal guests. In times of stress, if the algae leave the coral tissue, the coral eventually dies—a process known as coral bleaching, which is increasingly concerning due to rising temperatures. As we can observe, these two traits are crucial in defining where and how a coral can thrive. An azooxanthellate coral requires organic matter in the water for nourishment, whereas a colonial coral forms a skeleton of aragonite—a crystalline form of calcium carbonate—that is larger than that of a solitary coral and, therefore, necessitates waters more saturated in this material. In general, the availability of aragonite decreases with depth.

Desmophyllum dianthus (Huinay, Chilean Patagonia), an example of an azooxanthellate and solitary scleractinian coral
Desmophyllum dianthus (Huinay, Chilean Patagonia), an example of an azooxanthellate and solitary scleractinian coral

Using a phylogeny for this group of corals, we have been able to trace the journey that corals have taken in depth from that solitary and azooxanthellate ancestor to colonize all the seas and oceans of the world with the diversity of forms and ecological strategies that we see today. Thus, with the current depth of each species, ranging from the ocean surface to over 6,000 m, the ancestor of all stony corals lived below 200 m in depth, in what we know as the deep sea. Specifically, it inhabited depths between 229 and 2,287 m. At the beginning of its evolutionary history, this ancestral coral slowly expanded its distribution, giving rise to other species through speciation events. These early corals were still solitary and azooxanthellate. However, when the first colonial corals appeared, they began to colonize shallower depths more rapidly. In fact, azooxanthellate and colonial corals have proven to be the fastest in colonizing new depths. The species that exist today with these traits are truly impressive. Imagine a reef at 200 m depth, in darkness, spanning kilometers, and dominated by a coral species, such as Lophelia pertusa, teeming with life around. And you don't even need to go so deep; diving, you can find azooxanthellate and colonial corals that are breathtaking. My favorite is Dendrophyllia ramea, which appears as the poster image of this post.

Hypotheses (H1-4) for the origin of scleractinian bathymetric distribution

The first corals with zooxanthellae didn't appear until about 273 million years ago in shallow waters, and it wasn't until just 142 million years ago that the first zooxanthellate and colonial corals emerged. Therefore, our well-known tropical coral reefs have not been present for a significant portion of the group's evolutionary history. The origin of these traits has occurred several times, and specifically, the origin of symbiosis has always been associated with significant depth changes, always towards shallower waters, albeit very slowly.

The issue of colonization speed is crucial because an increase in background speed indicates an opportunity. If I'm at a concert trying to move forward through the crowd, and suddenly people disperse, I'll take advantage to quickly move forward. In the natural world, this can occur due to an extinction event that creates space for new colonizations or a change in environmental conditions, as seen with invasive species rapidly invading a new environment. In the case of corals, the emergence of traits more favorable for colonizing and succeeding at shallower depths, such as coloniality and symbiosis with zooxanthellae, has favored this increase in speed. We observe this in species with an intermediate phenotype, the colonial and azooxanthellate corals. Therefore, the overall result of this, when we consider the big picture, is that scleractinian corals have exhibited an evolutionary trend toward shallower waters, whether at higher or lower velocities (rates of evolution). However, as we delve deeper, we enter a more stable and slow-paced world for azooxanthellate corals. Here, events of faster colonization are nearly absent, and, as you can imagine, any disturbance lasts for a very long time before recovery occurs.

Considering that the majority of the ocean is deep sea, and much of it remains unexplored, it is not far-fetched to think that many groups may have originated in the depths and later colonized shallower waters. Of course, there are many factors at play to consider. This has been an open question for many years, and we are still beginning to understand which historical processes have shaped the diversity we see today. It is bewildering to step out of our day-to-day, week-to-week lives and ponder how millions of years of evolution have allowed different groups to emerge, and certain adaptations to prevail over others, creating the beauty and complexity of which we are both spectators and a part. After such a long history, surviving major mass extinctions, and still expressing themselves in such extraordinary ways, it is disheartening to see how corals, for example, are so threatened today due to climate change and other anthropogenic pressures. Interestingly, these factors influence not only tropical reefs but also deep-sea species. For instance, they directly impact the depth to which the available aragonite still allows the coral skeleton to form. The truth is that we are in a race against time. Can we even imagine a world without corals, considering they have been here for over 400 million years, while we only arrived some 200,000 years ago?

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