Part II

Rock, Sand, and Reef

Jules Verne provided the world its first glimpse of life under the waves in his brilliant and lovingly detailed novel 20,000 Leagues Under the Sea. Few people on Earth had the slightest conception, when the watery tale was published in 1870, of what lay beneath the surface of the sea. Verne himself had never pierced the briny depths, but his formidable imagination was kindled when he beheld the world’s first mechanical submarine, the Plongeur, unveiled at the 1867 International Exposition in Paris. The 140-foot steel vessel, cylindrical in shape with a rapier-like bow built for ramming enemy ships, inspired him to concoct the now-famous story of Captain Nemo and his own craft, the Nautilus. Verne was an avid amateur naturalist, and his meticulous research of sea life provided rich detail for the spectacular chapter modestly titled “A Walk On The Bottom Of The Sea.”¹

Having been captured by Nemo, the protagonist Professor Pierre Aronnax is introduced to the wonders of undersea life on an excursion across the ocean floor led by the captain himself. The party don diving suits and bell helmets, sip air recycled by rebreathers, and set their weighted shoes on the bottom. “And now, how can I retrace the impression left upon me by that walk under the waters,” begins the Professor. “Words are impotent to relate such wonders!” But relate them Verne does, majestically and reverentially, as his underwater party explores a diversity of seafloor habitats in breathtaking detail. First they stroll on “fine sand … sown with the impalpable dust of shells.” Approaching some rocky outcrops, “hung with a tapestry of zoöphytes of the most beautiful kind,” the colors and range of forms arrest the Professor’s gaze. “It was then ten in the morning; the rays of the sun struck the surface of the waves at rather an oblique angle, and at the touch of their light, decomposed by refraction as through a prism, flowers, rocks, plants, shells, and polypi were shaded at the edges by the seven solar colors. It was marvellous, a feast for the eyes, this complication of colored tints, a perfect kaleidoscope of green, yellow, orange, violet, indigo, and blue; in one word, the whole palette of an enthusiastic colorist!”

Verne’s enthusiasm was fanned by painstaking zoological research, and his potent imagination made organisms that he could have known only as sallow and shrunken specimens in jars come brilliantly alive. Coral, anemones, and sea stars are gorgeously depicted, as are giant basket stars that remind the author of “fine lace embroidered by the hands of naiads, whose festoons were waved by the gentle undulations caused by our walk.” In the space of ten pages, the expedition visits a half-dozen distinct marine habitats, including kelp forests and underwater canyons cleaved through stone; they even take a leisurely nap on the seafloor, to recover from their exertions. Verne flashes his genius by staging this first marine exploration on foot, a means of travel immediately familiar to anyone who has ever strolled through a forest or prairie. It is an invitation to the reader to open the eyes of their imagination and see the undersea world where it is most similar to land: on the ocean floor.

When a submarine dives beneath the surface, it enters a vast void, a three-dimensional volume filled only by water. It is the equivalent of stepping outside a space station into the great empty vacuum of the universe. Of course, the aquatic void is not empty, it is freckled with life, from phytoplankton to whale sharks that dot the sea like stars in the cosmos. But when that submarine reaches the seafloor, it encounters something new: physical structure. The bottom is decorated by a panoply of monuments and edifices. Some are the product of geology, like the great stone blocks around which Nemo’s party trudged, while others are engineered painstakingly by living organisms. Towers and pedestals soar over archways and tunnels; crevices and canyons are crenellated with filigree scrollwork that would satisfy the most flamboyant baroque architect. It is this structure that sets the seafloor apart, making it so fundamentally different from the open ocean.

All manner of marine organisms collaborate to build these structures, in convoluted and dynamic processes that unfold over millennia. Sponges decorate rocks, form small blobs, or blossom into enormous towers and barrels that can dwarf a person. Corals of a thousand varieties cohabitate with photosynthetic algae and put their combined energies to work building palaces, ramparts, and cathedrals of calcium which link to form immense reefs. Leafy fronds of algae festoon underwater surfaces, but algae can also be rock-hard and encrusting, coralline varieties that build their own structures and glue together loose rocks and boulders. Barnacles, mussels, oysters, and other shellfish also secure and accrete loose materials in a constant struggle against the destructive energy of waves, tides, and cyclones. Collectively, these living organisms build on the seafloor an intricate architecture that is wholly lacking in the open seas.

Undersea structure offers a nearly endless variety of benefits to marine fishes. Hiding places abound in rocks and corals, into which a fish can slip to escape predators. Conversely, hunters conceal themselves in holes and crevices, then burst out to attack a passerby. Some fishes sleep in caverns or beneath shadowy overhangs, or loiter in those dim spaces until night falls on the reef. When the siren song of mating beckons, fishes use undersea structure as courtship parlors, then bury eggs amid pebbles or glue them to rock faces. Juveniles seek safety amid seagrasses and mangrove roots, intricately woven refuges from predators. Whereas nearly all open ocean fishes are built to chase and attack smaller fish swimming in front of them, a host of adaptations have sprung up in reef fishes enabling them to feast on the diverse life encrusting the bottom’s structures. Parrotfish chew on coral, butterflyfish pluck invertebrates from stone walls, and triggerfish blast divots in sandy bottoms to expose buried worms and crustaceans. Industrious damselfish even tend tiny, leafy crops of algae on rock outcrops, then pugnaciously defend their harvest from marauding schools of herbivores looking to steal their lunch, like schoolyard bullies.

Out in the high seas, plankton and tiny larvae floating near the surface are the foundation of the food webs that sustain sardine, tuna, whale sharks, and the other denizens of the great blue. But down on the ocean floor, that foundation is much more expansive. Where light reaches the bottom, coral and algae and even photosynthetic sponges convert solar energy into food, like a vegetable garden. Near land, nutrients flush from rivers into the sea, providing an endless source of food. And everywhere, even in the ocean’s deepest places, edible material falls like snow from above. Referred to unromantically as particulate organic matter, everything from fish scales to feces, expired plankton to jellyfish tentacles, even dead whales, all this edible material tumbles from the surface as a veritable rain of food morsels. In most of the abyssal depths of the ocean, where light never penetrates, this organic snow represents the only source of food for a multitude of deep-dwelling organisms.

In those great depths, the waters are inky black and icy cold. Some 500 feet below the surface, all light fades and a transition is passed beyond which the temperature hovers just a few degrees above freezing: you have crossed beneath the thermocline. In these depths, photosynthesis is impossible, oxygen is scarce, muscles are frigid, and the cost of movement is exorbitant. The animals who live below this threshold have adapted to a life so different from ours that they might as well inhabit the surface of the moon. Indeed, the seafloor in these great depths often resembles the moon, a featureless landscape of fine silt, only infrequently interrupted by rocky outcrops. But great challenges breed evolutionary innovation, and among the strangest and most entrancing of creatures to inhabit the planet are the fishes of the abyss.

Between the surface and the abyss, one finds a cold water world in the so-called twilight zone, where light filters only dimly but nutrients and structures sustain surprising productivity. Fishes in this watery gloom rely on built-in flashlights to find prey and mates, but they also sniff the water for chemical messages of danger and opportunity. In the twilight zone, the seafloor supports little algae or coral but is rich with crustaceans, sponges, anemones, and other invertebrates that thrive on organic snow. Daily vertical movements of plankton add to this plethora of food. These middle depths may be home to more fishes (by weight) than in all the rest of the oceans,² and the diverse banquet supports massive schools of some of the most important commercial fish on Earth, like pollock and haddock and cod.

But let us begin like Professor Aronnax and embark on a tour of the sunlit seafloor to visit one of the most biodiverse and ancient places on the globe: a coral reef. In this oasis of abundance, bright tropical sunlight boosts coral and algae into levels of productivity that rival a rainforest. Plentiful food blended with complex physical structure is a world-class recipe for biodiversity, and evidence suggests that coral reefs have been hotbeds of diversity for millions of years. They have even survived planetary shifts in climate, like the one that confronts Earth today, and served as refuges for fish diversity through those challenging times.³ The result, today, is a magical location teeming with life, festooned with color and texture, and boasting some of the most unique, extraordinary, and downright bizarre fishes in the sea.