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.
Detail of a giant clam’s mantle. Great Barrier Reef, Australia.
Not all corals contain zooxanthellae, and it tends to be simple to tell which ones do. Zooxanthellae typically have a beige coloration, so corals containing these algae tend to be beige. Non-zooxanthellate corals (those corals without intracellular algae) tend to be much brighter in color. Most of the bright colors you might associate with coral reefs come from organisms that do not have symbiotic algae. These animals obviously do not benefit from contributions of a symbiont; however, as they do not require access to sunlight (a key ingredient in Darwin’s Paradox), these corals are not as limited in where they can grow. They will often proliferate inside large overhangs or caves that receive little natural light.
Detail of non-zooxanthellate soft coral polyps. Raja Ampat, West Papua, Indonesia.
Rather than deriving nutrition from their symbionts, non-zooxanthellate corals mostly filter feed and usually situate around parts of the reef where nutrient-rich currents hit. Nutrient-rich water tends to be rather murky, which explains why—in that setting—the vibrant colors of soft corals, sponges, and other filter feeders truly come alive.
At 100 feet beneath the surface, filter-feeding organisms proliferate. Komodo Island, Indonesia.
Triton Bay in West Papua, Indonesia, experiences localized seasonal upwellings of cool, nutrient-rich water that turn the existing water into a green soup. The first time I dived there, descending with a little hesitation into the eerie water, I wasn’t sure what to expect. When I reached the seafloor I was taken aback by the profusion of growth and riot of color. Without having to compete with light-loving corals, other organisms had been able to proliferate. Soft corals in reds, pinks, yellows, and purples covered every inch of the reef. In an illogical contradiction, most color exists in the gloom.
In plankton-rich waters, invertebrate life grows in profusion. Triton Bay, West Papua, Indonesia.
Soft corals do not produce a ridged calcium carbonate skeleton like hard corals do; rather their structure is based on a mesh of tiny calcareous rods, known as “spicules.” During the parts of the day when currents aren’t flowing, soft corals shrink, forming small spiky balls. As currents begin to pick up, they swell and expose their polyps, which can then begin to trap plankton from the passing water. These corals, which live without zooxanthellae, are also found in the deep, lightless sea where individual colonies grow extremely slowly for thousands of years. Twenty thousand feet below the surface, in water of 30 degrees Fahrenheit, life continues at a slower pace. Without the volatile climatic conditions experienced in shallower waters, the corals here have created rich but fragile ecosystems that are home to their own unique fauna.
Where Corals Grow
With the symbiotic coral-algal powerhouse facilitating growth of corals in clear tropical seas, few other organisms in this habitat are able to compete. However, corals do have specific environmental requirements; where these are not met they can be outcompeted by other organisms that are more tolerant of the particular conditions. On the whole, corals have a few basic ecological requirements to maintain meaningful reef building growth.
Warm water is fundamental to coral growth. Corals generally have a tolerance for waters between 70 and almost 90 degrees Fahrenheit; in winter, they can briefly endure temperature dips to 65 degrees. Because of this, most reefs fall within the latitudes of 30 degrees north and south; however, in some parts of the ocean the reefs might extend slightly farther toward the poles. Certain globally important currents assist in extending the reach of warmer waters beyond these latitudes, for example the major Pacific equatorial currents that flow separately in the northern and southern hemispheres. In the southern hemisphere, the South Equatorial Current is driven by winds across the central Pacific and strikes the Great Barrier Reef off the coast of Queensland, Australia; it then flows from there in a southerly direction as the East Australian Current. As the East Australian Current flows southward, it carries warm waters from the equator that can ultimately reach as far as Tasmania. This was the current that Nemo used to travel from the Great Barrier Reef in the north to Sydney in the south in Finding Nemo. Without the East Australian Current, the waters off Sydney, for example, would be significantly cooler and many organisms would have their southern ranges much farther north. Over recent decades, this current has been strengthening and extending farther south, pushing the geographic ranges of some organisms in a southerly direction.8
I have seen firsthand how this extension of the warm East Australian Current has had an unfortunate impact on the historically cool waters of southern Australia. In 2011, I visited the Tasman Peninsula in southeast Tasmania to dive the giant kelp beds. Much like the famed kelp beds of California, these giant algae form huge dense and towering forests. These kelp beds were known as being similarly impressive as their northern Pacific counterparts, and it was the only time I have had the chance to experience this amazing ecosystem. The kelp beds were previously so dense and widespread as to allow commercial harvesting but have gradually dwindled over the years. I dived one of their last real strongholds, Waterfall Bay, in southeast Tasmania—where colorful weedy seadragons and a wide variety of other unique creatures exist.
Robust coral growth enduring crashing waves. Egyptian Red Sea.
Huge and elaborate coral growth can take many decades or even centuries to grow. Egyptian Red Sea.
Having been so amazed by this aquatic jungle, I returned six years later, eager to share the spectacle with some friends. To my utter dismay, I was told that kelp had disappeared almost entirely from Tasmania since my last visit, due to the extension of the East Australian Current. Although Tasmania may seem like a remote wilderness, the area is suffering some of the most extreme impacts of climate change on the planet.9 It is among the top 10 percent of places on Earth for extreme ocean warming, heating at four times the global average, with no sign of this deadly trend abating.10 The reason for the kelp’s disappearance
