Coral reefs, deep gardens, and rolling rocks

 

A purple rhodolith from Fiji.

Three ways to build a reef

Check out this knobbly pinkish lump about the size of a golf ball, and another one over here about the size of my fist. It feels like a rock and looks like one. But it is alive. It has been growing, very slowly, for decades or even centuries, and while you were holding it, you may have disturbed a dozen small animals living in its cracks and tunnels.

You picked up a rhodolith, and if you really found one on a beach, you would probably not even notice it.

Some of the most important habitats in the ocean are built by organisms that turn seawater into stone. They pull calcium and carbonate from the water, lay down a hard skeleton, and leave behind structures that other organisms use for feeding, hiding, and raising young. These are ecosystem engineers that build reefs.

A reef: a bright, shallow tropical coral reef full of fish. Familiar, but that is only one way to build a reef. Herein, I want to look at three: tropical coral reefs, deep-water coral reefs, and rhodolith beds. They are built by different organisms, in different places, at different speeds. 

Coral reefs: The famous one

Tropical coral reefs are the well-known, colorful zone that requires a rum punch. They cover a small fraction of the seafloor, under one percent, but the benefit is overwhelmingly large. They support something like a quarter of all known marine species. They protect coastlines from waves, feed hundreds of millions of people, are large carbon stores, and they are awesome to snorkel in.

The reef builders are stony corals, which are animals, cnidarians, related to sea anemones and jellys. They are colonies of polyps, which are one mouth surrounded by tentacles, and each polyp sits in a cup of calcium carbonate it secretes underneath itself. Over time, generation after generation of polyps builds up a skeleton, and many colonies together build a reef. Old reefs, dead coral, might sit under the current growth, adding serious structure.

Coral uses one of the most famous partnerships: Inside the tissues of reef-building corals live single-celled algae, zooxanthellae. 

Zooxanthellae, the brown bits, inside a growing coral. 

The algae photosynthesize and pass much of the sugar they make to the coral, in many cases, most of the coral’s energy. In return, the algae get a safe home, nutrients, and a steady supply of carbon dioxide. This partnership is why tropical reefs grow so well in clear, warm water that is low in nutrients. 

Being tied to shallow waters for the light, the Zooxanthellae also expose the reef-building corals to dangers. When water gets too warm for too long, the relationship breaks down, and the coral expels the algae. Without them, the coral tissue becomes transparent as the white skeleton shows through: coral bleaching. A bleached coral is not yet dead, but it is starving, and if the heat lasts too long, it’s gone. Mass bleaching, once rare, now happens across whole ocean basins; the corals are in trouble. 

Heat is not the only problem. As the ocean absorbs carbon dioxide from the atmosphere, seawater becomes more acidic, and it gets harder for corals to build their skeletons. Nutrient runoff from land feeds seaweeds that can overgrow corals. Disease, overfishing, and coastal development add more pressure. Coral reefs are resilient in many ways, but they are being hit by many stressors at once.

Coral reefs are the reef everyone knows. The next two are the reefs almost nobody knows.

Deep-water corals: Reefs in the dark

In the 1990s and early 2000s, as more surveys hit the deep ocean floor, primarily with remotely operated vehicles, they found something unexpected. In cold, dark water, often hundreds of metres down, there were coral reefs. Not scattered colonies, but real reefs, some stretching for kilometres. One off northern Norway, the Røst Reef, is tens of kilometres long.

Deep-water coral, Desmophyllum

The most important reef builder in these places is a stony coral once known as Lophelia pertusa, now placed in the genus Desmophyllum (Desmophyllum pertusum). It builds branching, bushy colonies of white or orange polyps that can grow into large mounds. Alongside it live many other kinds of corals, including tall gorgonians (sea fans), bamboo corals (similar-looking to sea fans but have protein joints that give a striped bamboo appearance), and black corals (which have, you guessed it, a black skeleton). These corals form forests rather than reefs, which provide shelter and habitat.

There is no sunlight at these depths, so there are no symbiotic algae. Deep-water corals have to make a living entirely by catching stuff from the water: tiny animals, bits of organic material sinking from above, and particles carried by currents. Because of this, they tend to grow in strong currents where food is reliably delivered: the edges of continental shelves, on seamounts, and in submarine canyons. Also, because of this, they grow slowly.

Life in the cold and dark is slow. Deep-water corals typically grow millimetres per year, and some reefs have been building since the end of the last ice age, roughly ten thousand years ago. Colonies of some deep species are thousands of years old. Some of the oldest living animals.

Habitat, yes, I know I stated that already, but habitat is key; diverse habitat is better. These reefs are crowded with life: Fish hang among the branches, sponges, sea stars, crabs, brittle stars, and many species that are still being described. Commercially important fish use them too, and the major disaster that follows is atrocious. Bottom trawling, where heavy nets are dragged across the seafloor, destroys in minutes what took thousands of years. 

Surveys in Norway and elsewhere found that a large share of known reefs had been damaged by trawling before anyone thought to protect them.

Bottom trawling should be outlawed, everywhere.

Bottom trawling scars

The Northwest Atlantic has its own deep corals, in places like the Gulf of Maine, the Scotian Shelf, and the canyons along the edge of the continental shelf. Canada and the United States have set aside protected areas for some of them, but the protected areas are too small, and protections need to be extended to much more of the deep; much of the deep seafloor has never been surveyed. Let’s protect it now so there is something left to discover.

Deep-water corals also face a quieter threat. Cold water holds more carbon dioxide than warm water, and deep, cold water is often already close to the point where aragonite (the crystalline form of calcium carbonate that corals make their skeleton out of) starts to dissolve.

Deep-water corals show that you do not need sunlight or warm water to build a reef. Rhodolith beds show that you do not even need to be an animal.

Rhodolith beds: The reef nobody noticed

Rhodoliths are not coral. They are red algae, a group of seaweeds, and specifically coralline red algae, which deposit calcium carbonate in their cell walls. Most coralline algae grow as pink crusts on rocks. Anyone who has looked into a tide pool in the Gulf of Maine has seen them, painting the rocks a chalky pink. Rhodoliths are coralline algae that grow free, unattached to anything, as loose nodules lying on the seafloor. When enough of them accumulate in one place, they form a rhodolith bed. In Europe, these beds are called maerl.

Types of Coralline algae.

Rhodolith beds are found from the tropics to the Arctic, wherever there is enough light. Some are enormous. One of the largest known stretches across a big area of the Brazilian continental shelf. But they have been easily overlooked, partly because from a distance they look like gravel.

Some of the best-studied cold-water beds are in Newfoundland. Patrick Gagnon and his students at Memorial University stumbled on the first one during an early dive off Portugal Cove-St. Philip’s, where he found the seafloor covered in red stony lumps. The rhodoliths there, and in nearby beds, are made almost entirely of a single species, Lithothamnion glaciale. Some are unusually large, among the biggest ever measured.

Rhodoliths grow extremely slowly. Some Newfoundland rhodoliths grow only a few hundred microns a year (200 microns is a fifth of a mm, so in a few years they might grow the length of a line made with a thick pencil). A large rhodolith is old, very old. A bed made of thousands of them represents a massive amount of time.

What have they done with that time? They build habitat. 

The surface of a Newfoundland rhodolith is riddled with tunnels; tunnels filled with living things.

More than a hundred kinds of animals live with rhodoliths, including brittle stars, chitons, clams, snails, worms, sea urchins, and sea stars. Hollow rhodoliths have been found packed with fish eggs: The beds may serve as nurseries.

The beds are also carbonate factories. Each rhodolith is slowly adding calcium carbonate to the seafloor. Estimates state 200 grams of calcium carbonate per square metre per year.

Here is the part that I find most fun. The standard explanation for why rhodolith beds exist where they do was water motion. The idea was that rhodoliths need regular waves or currents to roll them around, so they do not get buried in sediment and smothered. It made sense, but nobody had really tested it in these cold-water beds. When Gagnon’s group did, they found that the waves and currents in the Newfoundland bed were not strong enough to move the rhodoliths at all. Instead, the animals living in the bed were doing the work. Brittle stars and small sea stars climbing over the rhodoliths knocked sediment off. The inhabitants of the reef were keeping their home clean.

That does not mean physical forces do not matter; intense storms can rework them. That makes shallow beds vulnerable if storms become stronger and more frequent. Because they grow so slowly, a damaged bed takes a very long time to recover; rhodolith beds are essentially non-renewable.

Rhodolith beds from Brazil. Bulleri et al. 2024.

These three worlds 

These habitat builders build at different speeds. Tropical corals are no speed demons, but they are fast by comparison. Deep-water corals are slow. Rhodoliths are slower still. The slower the builder, the longer recovery takes.

And their mineral is slightly different. Corals build with aragonite. Coralline algae build with a form of calcite that has a lot of magnesium, which actually makes it more soluble than aragonite. That may make rhodoliths especially sensitive to changes in ocean chemistry.

But they also share a lot. All three take calcium carbonate from seawater and turn it into habitat. All three support far more life than the seafloor around them.

We know much more about tropical coral reefs, and they get most of the attention. They are beautiful, they are accessible, and you can go see them with a snorkel. Deep-water reefs require ships and expensive equipment to reach. Rhodolith beds are reachable by divers but look, at first glance, like gravel. Nobody puts a photo of gravel on a travel poster.

This matters, because we tend to protect what we know. Deep-water reefs were damaged long before most people knew they existed. Where are all the Rhodolith beds? We are just now getting a clue.

Lots of work to do, lots of marvels to discover; get in the water.

Sources and Further Readings:

Images, in order: Wikipedia. Modified from osf.co.uk. Ocean Explorer, NOAA. Skytruth.org. Jardim et al. 2025. Bulleri et al. 2024.

Arnold, Cassidy & Bélanger, David & Gagnon, Patrick. 2022. Growth Resilience of Subarctic Rhodoliths (Lithothamnion glaciale, Rhodophyta) to Chronic Low Sea Temperature and irradiance. Journal of Phycology 58. 251-266.

Bulleri, Fabio & Schubert, Nadine & Hall-Spencer, Jason & Basso, Daniela & Burdett, Heidi & Francini-Filho, Ronaldo & Grall, Jacques & Horta, Paulo & Kamenos, Nicholas & Martin, Sophie & Nannini, Matteo & Neves, Pedro & Olivé, Irene & Peña, Viviana & Ragazzola, Federica & Ribeiro, Cláudia & Rinde, E. & Sissini, Marina & Tuya, Fernando & Silva, João. 2024. Positive species interactions structure rhodolith bed communities at a global scale. Biological Reviews 100. 428-444.

Jardim V, Grall J, Barros-Barreto M, Bizien  A, Benoit T, Braga J, Brodie J, Burel T, Cabrito A, Diaz-Pulido G, Gagnon P, Hall-Spencer J, Helias M, Horta P, Joshi S, Kamenos N, Kolzenburg R, Krieger E, Legrand E, Page T, Peña V, Ragazzola F, Rasmusson L, Rendina F, Schubert N, Silva J, Tâmega F, Tauran A, and Burdett H. 2025. A Common Terminology to Unify Research and Conservation of Coralline Algae and the Habitats They Create. Aquatic Conserv: Marine and Freshwater Ecosystems 35: e70121. 

Hicks A, Millar K, and Gagnon P. 2024. High Vulnerability of Rhodolith Bed Frameworks and Underlying Sediment to Ongoing Ocean Climate Change. Aquatic Conserv: Marine and Freshwater Ecosystems 34: e70001. 

Macaya, Erasmo & Riosmena-Rodriguez, Rafael & Melzer, Roland & Meyer, Roland & Häussermann, Verena. (2014). Rhodolith beds in the South-East Pacific. Marine Biodiversity 45. 153-154.