Most sponges sit on top of the reef. Boring sponges live inside it. They tunnel into anything made of calcium carbonate, honeycombing coral heads, rocks and shells with chambers and galleries while showing only a few small coloured patches at the surface. They are easy to miss on a dive, but they are among the most important forces shaping coral reefs, slowly breaking down the reef framework that corals build. In a healthy reef, that erosion is balanced by coral growth. When reefs are stressed, the balance can tip.

As a coral farmer and reef keeper, I’ve spent decades thinking about calcium carbonate: dosing calcium and alkalinity so corals can build skeletons, and working with live rock, which is reef limestone riddled with holes. Many of those holes are made by boring organisms, including sponges. Every piece of live rock is a record of this hidden tunnelling.

How a sponge drills through stone

Boring sponges excavate by a combination of chemistry and mechanics, using specialised cells to cut out tiny chips of carbonate.

The key players are etching cells, amoeboid cells that press against the carbonate surface and release chemicals that dissolve a thin groove around a small patch of material. Once the groove is complete, the patch, a tiny chip often only a few hundredths of a millimetre across, comes free. The sponge carries the chip away through its canal system and expels it from its oscula. Studies of Cliona suggest that most of the excavated material, the large majority by weight, leaves as chips, with only a small fraction actually dissolved. The process is slow on a human scale but relentless: over years, a boring sponge can riddle a coral head or shell with interconnected chambers. The chips, sometimes called sponge chips, have a distinctive cupped shape and are a recognisable part of reef sediments.

What boring sponges look like

Most of a boring sponge is hidden, so you usually see only its surface openings.

On rock or dead coral, many species show as clusters of small, raised, pore-covered bumps called papillae, often bright yellow or orange, poking up through the surface like tiny volcanoes. Cliona celata, sometimes called the sulphur sponge, shows yellow papillae on oyster shells and rocks in the Atlantic and Mediterranean. Some species grow beyond the boring stage into an encrusting or massive form that spreads over the surface. The red boring sponge, Cliona delitrix, forms bright red-orange encrusting patches with prominent oscula on Caribbean corals, often surrounded by a white band of dead coral where it is advancing. The brown variable sponge, Cliona varians, can grow from a boring form into a thick brown crust. In the Indo-Pacific, Cliona orientalis spreads over corals and contains symbiotic algae. Our guides to yellow sea sponges and what sponges look like underwater include examples.

Boring sponges on coral reefs

On coral reefs, boring sponges are among the most important bioeroders, the organisms that break down reef limestone.

A coral reef is a balance between construction and destruction. Corals and coralline algae deposit calcium carbonate; parrotfish, sea urchins, worms, molluscs, microbes and boring sponges remove it. In many Caribbean reefs, sponges are among the largest contributors to internal bioerosion, weakening coral heads from within and making them more likely to break in storms. Some species go further and attack living coral. The red boring sponge, for example, undermines and kills living coral tissue as it spreads, and on stressed reefs it can expand at the expense of corals. Our page on sponges and coral covers the wider relationship between the two, which ranges from partnership to open competition.

This erosion is not all bad. It recycles carbonate, creates crevices and caves that shelter fish and invertebrates, and produces fine sediment that fills spaces in the reef. On a healthy reef, boring sponges are part of a natural cycle, and the cavities they leave behind become homes for small fish, shrimp, crabs and worms.

Pollution, warming and acidification

Several studies suggest boring sponges do well when reefs are under stress, which can tip reefs toward net erosion.

In the Florida Keys, researchers found that the red boring sponge was more abundant and larger near sources of sewage pollution, suggesting that nutrient enrichment favours it, probably by increasing the food and organic matter available. Experiments with the Indo-Pacific species Cliona orientalis found that its boring rates increased in more acidic, warmer water like that expected in the future, while coral growth slows under the same conditions. Boring sponges can also take advantage of corals weakened by bleaching. Put together, these findings suggest that some reefs could shift from net growth to net erosion, with their framework dissolving faster than corals can rebuild it. As someone who has watched how quickly water chemistry problems show up in a closed reef system, I find that a worrying prospect for wild reefs.

Boring sponges and oysters

For oyster growers and oyster reefs, boring sponges are a familiar problem.

Cliona species bore into oyster shells, riddling them with holes. Lightly infested shells survive, but heavy infestations make shells brittle and force oysters to spend energy repairing them, which can slow growth and make oysters more vulnerable to predators such as crabs and to damage during harvesting. Infested shells are easy to recognise by their many small holes and yellow sponge tissue, and beachcombers often find old shells peppered with these tiny tunnels. Boring sponges generally cannot tolerate low salinity, so oysters in fresher parts of estuaries are often less affected, while those in saltier water suffer more. That pattern matters for the restoration of oyster reefs, which are among the most threatened marine habitats in the world.

Boring sponges in live rock and aquariums

Reef keepers meet boring sponges more often than they realise, usually as small coloured spots on live rock.

Live rock collected from reefs or aquacultured in the sea is full of borings made by sponges, worms, bivalves and microbes. Small yellow or orange patches and tiny raised papillae on rock in a tank are often boring sponges that came in with the rock. In most aquariums they stay small and harmless, filtering water like other sponges, and they add to the diversity of life on the rock. They are rarely a problem for corals in a tank, because conditions and food supply are different from a polluted wild reef. If you see one, the best course is usually to leave it alone, and avoid lifting the rock out of the water, since air trapped in a sponge’s canals can kill it and foul the water. Our page on sponges in reef tanks covers what to expect.

How scientists measure bioerosion

Measuring how fast a sponge eats a reef takes patience and some ingenuity.

One common method is to place blocks of coral skeleton or limestone of known weight on a reef, leave them for months or years, then recover them, clean them and weigh or scan them to see how much material has been removed. Micro-CT scanning can reveal the full network of borings inside a block in three dimensions. In the lab, researchers place pieces of sponge-infested carbonate in tanks and measure the chips the sponge releases over time, or track changes in water chemistry. Combining these measurements with surveys of how much reef area boring sponges cover lets scientists estimate their contribution to a reef’s carbonate budget, the balance between how much limestone is added and how much is removed each year.

An ancient way of life

Sponges have been boring into carbonate for hundreds of millions of years.

The distinctive networks of chambers left by boring sponges are preserved in fossil shells, corals and limestones, where paleontologists classify them as trace fossils under the name Entobia. Such traces are found in rocks from many periods, showing that bioerosion by sponges has shaped reefs and shelly seafloors through much of animal history. Our page on sponge fossils covers the broader record. Today, boring sponges remain a reminder that reefs are not permanent structures but dynamic systems, constantly built up and broken down. For other sponges that interact with corals, see our sponges hub.