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      Several individual coral polyps. Solomon Islands.

The domination of coral reefs in today’s shallow tropical seas is due to the symbiotic relationship between scleractinian corals and unicellular algae, called zooxanthellae. This special ecological relationship allows corals to finesse their metabolic processes of respiration, metabolism, and waste product management, improving their growth rates and allowing them to supercharge their deposition of calcium carbonate. Early stony corals were small and solitary—it took millions of years before this symbiosis enhanced them into becoming true reef builders.4 However, many of the families of stony corals we see today are very old, having originated in the middle to late Jurassic period, one hundred and fifty million years ago.

      Corals are living animals, although they may not fit our preconceived notion of what defines an animal. These tiny relatives of the sea anemone and jellyfish are sessile creatures, permanently attaching themselves to the seafloor, somewhat like a plant. The living parts of the coral are very simple, soft-bodied animals called polyps. For many colonial corals, each polyp is just a few millimeters across; solitary polyps, however, such as those of mushroom corals, can sometimes be almost a foot in diameter.

      Each polyp comprises a single opening surrounded by a ring of tentacles. The tentacles are covered in specialized stinging cells, called nematocysts, which help to harpoon and trap passing food particles. Internally, most of the polyp is a simple stomach, the single opening acting for both ingestion and excretion. The living polyp sits atop a protective calcium carbonate structure, the coral’s deposit, which has been key to them becoming such prominent ecosystem engineers.

      The vast majority of a coral colony comprises the dead skeleton structure beneath, which is blanketed with a very thin layer of living tissue comprised of many individual polyps. A colony of individual polyp clones can have hundreds of thousands of polyps. They are connected to one another by a thin band of living tissue. Thousands of individual coral colonies, constituting many species, make up a reef.

      Darwin’s Paradox

While vivid blue tropical seas may draw vacation-goers in droves, the same crystal-clear waters aren’t as inviting to marine organisms. In the ocean, truly crystal clear water lacks the nutrients that can be exploited to sustain life. Nutrients in the water column, the stretch between the surface and ocean floor, are usually highlighted by the presence of plankton, which add a noticeable hue. Plankton is the term used for a diverse array of miniscule organisms that float or drift in the open ocean and are largely transported by the whim of currents. The term “Darwin’s Paradox” refers to the conundrum that Charles Darwin himself highlighted: coral reefs are oases in the desert of the blue ocean.5

      Corals are only able to flourish and grow in these nutritionally deficient waters thanks to the symbiotic relationship shared between single-celled algae, zooxanthellae, which live within their tissues. Symbiosis means that both parties benefit from the relationship; in this case there are advantages for both the coral animals and the zooxanthellae. Zooxanthellae form colonies within the safe, soft tissues and tentacles of the coral polyps, where they harness the sun’s light to photosynthesize and produce sugars. These sugars fuel the corals and allow them to sustain unparalleled growth, compared to their relatives that don’t harbor such algae. In return, the corals supply zooxanthellae with their metabolic waste products that the algae then use to fuel photosynthesis. Corals do still need to supplement the nutrition provided by the algae, so at night the polyps swell and they feed on passing plankton using their stinging tentacles.

      The meeting of two distinct coral colonies. Cenderawasih Bay, West Papua, Indonesia.

This extremely tight cycling of nutrients means that very little goes to waste and corals are able to direct significant energy into reef building. Coral polyps deposit calcium carbonate at varying rates—some of the most prolific branching species can grow seven inches in a year—although four to six inches is more common. On the other hand, the slower growing boulder-shaped forms may grow by just a few millimeters per year.6 Over thousands of years this symbiosis has been responsible for the world’s largest biogenic structure: the fourteen-hundred-mile-long Great Barrier Reef located off the coast of Queensland, Australia. Without this symbiosis, calcium carbonate deposition and coral growth in the tropical shallows would be negligible, and reefs as we know them would not exist.

      Photosynthesis above and below the waves. Solomon Islands.

      Zooxanthellae: Haves and Have Nots

      Corals have benefitted enormously from hosting intracellular zooxanthellae, and some other creatures have followed suit. Other immobile reef organisms, like sponges, sea anemones, and certain soft corals, also benefit from a relationship with these algae, as do certain mollusks. On land, we are familiar with slugs and snails, but in the oceans, mollusks are much more diverse. In addition to the tens of thousands of species of gastropods (slugs and snails), other well-known groups of ocean mollusks include cephalopods (octopuses, squids, and nautiluses), bivalves (such as clams and oysters), and chitons (unusual plated slug-like animals). Giant clams and a number of sea slugs have zooxanthellae living within their tissues.

      Waves crashing over a Red Sea reef. Egypt.

One genus of sea slugs, Phyllodesmium, has a stronger relationship with the algae than most. These slugs are covered in cerata, which are thin, finger-like protrusions that cover the back of the animal. Cerata house the digestive glands of the slug, and zooxanthellae reside within the cells of these glands. The sea slug obtains the zooxanthellae from its food: beige soft corals found in shallow water. Each species of Phyllodesmium has selective tastes, and its camouflage reflects the type of coral that it prefers to feed on, rendering them almost invisible while feeding. The slugs can sit and feed leisurely on the coral with very little risk of predation, while also supplementing their nutrition through the photosynthesis of the zooxanthellae within their cells.7 I have even spotted a particularly large species, P. longicirrum, hanging out in the open on the reef with the casual air of a sunbather soaking up the sun’s rays.

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