The reef is a living shore

A coastline built by animals

At Mapu ʻa Vaea, on the southwest coast of Tongatapu, the sea offers a demonstration of what a reef can become. A swell enters openings in the limestone platform and sends water up through holes in the rock. The show depends on the shape of the coast and the sea that day. Its raw material, limestone, is a clue to a much slower event: generations of animals building a structure that outlives them.

The architects of a living shallow reef are stony coral polyps. Each small animal makes a skeleton from calcium carbonate. Many live in colonies, and their skeletons build on earlier generations. Microscopic algae live in the tissue of many reef-building corals and supply some of their food through photosynthesis. Light therefore matters down to the scale of a single polyp. It is one reason so much reef-building activity takes place in clear shallow water.

A shelter with many entrances

A branching coral can give a small fish a place to retreat where larger mouths cannot follow. Crevices under the reef offer places for nocturnal animals to wait out the daylight. Crabs, sea urchins, sea cucumbers and clams inhabit or feed around the structures other species make. Coral, algae, fish and invertebrates all change the shape and function of the place as they live in it.

Many reef fish use a greater area than a visitor sees from above. Some move between reef and adjacent seagrass or mangrove habitat as they grow or seek food. Along Tongatapu, assessments of marine reserves document mangroves and eelgrass alongside the reef and its fisheries. That variety matters. A stand of mangroves, a shallow meadow and a coral slope can support different parts of an animal’s life within a short swim of one another.

Even the sand has a story. Parrotfish graze algae from reef surfaces, taking in bits of carbonate as they feed. The hard material is ground up and passes out as grains. On coral coasts, some of what ends up underfoot has passed through a living fish.

Why a clear sea can still be in trouble

A clear, blue sea makes good conditions for looking into water. It says little by itself about the health of the animals below. During prolonged heat, corals can lose the algae that live in their tissues. The pale skeleton shows through in an event called bleaching. Some corals can recover when conditions improve. Long or repeated stress makes that harder. Storms can break colonies, while sediment or pollution can affect how much light reaches the reef.

Monitoring teams look at changes that an afternoon snorkel cannot reveal. The Tonga chapter of the Global Coral Reef Monitoring Network assessment describes surveys at several sites. Repeated observations can distinguish a particular disturbance from a longer change in how much live coral covers the reef. Village fisheries add another scale of observation. At ʻAtata, a local fish reserve formed part of a wider area managed with the community before the tsunami.

From a shallow platform to a deep trench

The same ocean contains some of Earth’s sharpest changes in relief. East of the islands the Pacific Plate descends at the Tonga Trench. West of the main island ridge lies an active volcanic arc, the setting of Hunga Tonga–Hunga Haʻapai. Parts of ʻEua expose much older volcanic rocks below layers of limestone. Tongatapu’s surface geology is dominated by limestone and younger cover. The islands’ volcanic foundations and their coral-built coastlines record different episodes of the same long history.

There is a third scale, the living ocean beyond the reefs. Every austral winter, humpbacks use Tongan waters as part of a migration linking warmer breeding areas with much colder feeding grounds. The animals travel distances that put the twelve kilometres from ʻAtata to the capital into perspective. Following one whale takes the story from a reef and its village to an ocean basin.