Earth's Largest Living Structure
Rising out of the monotony of the open ocean, the coral reef appears like a submerged metropolis, its structure a riot of form and color.
Vast branching orchards give way to sleek plateaus. Rivulets of tiny grains wend their way into serpentine canals. Imposing monuments are capped off by every-thing from jagged turrets to rows of domes in the shape of cauliflower heads. The entire reef pulsates with the comings and goings of thousands of marine creatures. They drift past in great schools, dart furtively in and out of labyrinthine passageways, and hide out in cracks and crevices and dark, cavernous holes. Forget the idea of the ocean as a watery void – in reality, it is a finely integrated cosmos and the coral reef, more than anything, is what keeps it alive and vibrant.
Coral reefs are the planet’s largest living structures, reaching the dimensions of Australia’s 92,000 square-mile Great Barrier Reef. The raw material of this construction is the calcium that polyps take in from the sea and convert to an external limestone skeleton. One generation of coral colonies builds atop the last, creating the foundation of a vast underwater architecture.
A Symbiotic Relationship
At the core of the coral reef is a remarkable exchange of “goods and services” between two different life forms – one animal and one plant. The coral polyp, one of the simplest of all animals, is little more than a tube, capped with a mouth and fringed with stinging tentacles. The polyp has many reproductive strategies, but it mostly makes new life by cloning itself. This produces colonies of genetically identical polyps attached to one another. The polyps nourish themselves with their very own internal gardens of single-celled algae known as zooxanthellae. The algae keep the polyps healthy by converting the sun’s energy into carbohydrates and oxygen. In return, the algae reap fertilizer and food from the polyps, released in the form of carbon dioxide and other waste products. Up to a million algae can live inside a single polyp. Scientists call this cooperative arrangement a “symbiotic relationship.”
Coral reefs form rings around volcanic islands. Why this is so remained a mystery until the 19th century, when Charles Darwin proposed an explanation confirmed by scientists today. The process begins when corals encrust onto a newly cooled volcano. Reef expansion proceeds in shallow and warm water where the corals can access the sunlight they need. Over the millennia, the reef continues to build upward while the volcano slowly sinks into the sea. Eventually, this leaves only an atoll, a ring of reef enclosing a lagoon.
This constructive artistry of nature is illustrated by the way reef formations in Hawai‘i vary between the youngest and oldest ends of the chain. On the island of Hawai‘i, at the youngest end, coral colonies are still coalescing on the submerged slopes of active volcanoes. Moving northwest, more developed fringing reefs encircle the older islands of Maui, Moloka‘i, Lana‘i, O‘ahu, and Kaua‘i.
Finally, marking the spot where magma first erupted from the Earth’s molten core and heralded the existence of the mid-Pacific chain some 50 million years ago, there are the seamounts and atolls of the Northwestern Hawaiian Islands. Their embrace of blue lagoons extends 1,200 miles to Kure Atoll. Beyond Kure, the land is subsiding so fast it outpaces coral growth, and the reef is literally drowning. This end point, which paradoxically was once the beginning of the volcanic chain, is known as the Darwin Point.