Divers often mistake sponges for plants or even for coral, but they are animals — and some of the most important ones on the reef. On many Caribbean reefs, sponges now cover as much of the sea floor as living coral, and in places more. They clean the water, feed other animals, shelter shrimp and brittle stars, and supply chemists with some of the most promising natural compounds known. This guide explains how sponges work, introduces the main types found on coral reefs and shows why these simple animals matter. For the wider picture, see coral reef invertebrates.

What is a sponge?

An animal built around a water-pumping system, with no organs at all.

A sponge is a multicellular animal organised around a system of water channels. Water enters through many tiny pores (ostia) in the body wall, flows through canals and chambers lined with choanocytes — collar cells that beat a whip-like flagellum to drive the current and trap food particles — and leaves through one or more larger openings called oscula. Between the inner and outer cell layers lies a jelly-like layer with wandering cells that digest food, carry nutrients and build the skeleton.

That skeleton is made of tiny mineral needles called spicules, of either silica (glass) or calcium carbonate, and/or a network of protein fibres called spongin. The soft, springy bath sponges once harvested from the Mediterranean and Caribbean are spongin skeletons. Sponge cells are remarkably flexible: many can change job as needed, and in classic experiments, sponges pushed through a fine mesh have reassembled into new sponges.

Sponges are among the earliest-branching animals on the tree of life, and fossil and chemical evidence suggests they were present in ancient oceans hundreds of millions of years ago. Their simplicity is not a weakness: the same basic design has persisted almost unchanged while countless more complex groups have come and gone.

Types of sponges on coral reefs

Four classes, with demosponges dominating the reef.

Most sponges a reef diver sees are demosponges. The giant barrel sponge (Xestospongia) of the Caribbean and Indo-Pacific can grow large enough for a diver to fit inside and is thought to live for centuries, earning the nickname “redwood of the reef.” Tube and vase sponges in purple, yellow, pink and azure line Caribbean walls. Encrusting sponges form thin coloured sheets over rock and dead coral. Boring sponges such as Cliona etch tunnels into coral skeletons and shells, leaving only their small yellow or orange openings visible. Some species, like the “do-not-touch-me” and fire sponges, can irritate the skin. See blue sea sponges.

How sponges feed and filter the reef

Living pumps that can process their own volume of water many times over.

Sponges are filter feeders on a heroic scale. By beating millions of flagella, a large sponge can pump a volume of water many times its own body size every day (see how much water sponges filter), capturing bacteria, tiny plankton and organic particles, many of them too small for other filter feeders to catch. That makes sponges important water cleaners on reefs, especially in caves and crevices where currents are weak.

Sponges also take in dissolved organic matter — sugars and other compounds leaked by corals and algae that most animals cannot use. Research in the 2010s showed that some reef sponges turn this dissolved material into cells that they rapidly shed as detritus, which is then eaten by snails, worms, brittle stars and other detritus feeders. This “sponge loop” helps explain how coral reefs stay so productive in nutrient-poor tropical water: sponges recapture energy and nutrients that would otherwise be lost. Many sponges also host dense communities of symbiotic bacteria and other microbes, which can make up a large share of the sponge’s mass and help it process nutrients.

Sponges in the reef ecosystem

Habitat, food, bioeroders and chemical warriors.

A single large sponge can be an apartment block. Shrimp, brittle stars, small crabs, worms, gobies and other animals live inside its canals or on its surface, and some species are found nowhere else. Sponges also compete with corals for space; when corals die after bleaching or disease, fast-growing sponges often spread over the bare surfaces, and on many Caribbean reefs sponges have become as abundant as corals.

Few animals eat sponges, because spicules and chemical defences make them unpalatable. The exceptions are specialists: the critically endangered hawksbill turtle feeds mainly on sponges, as do several angelfish and some nudibranchs. By eating fast-growing sponges, hawksbills and angelfish may help keep sponges from overgrowing corals. Boring sponges, meanwhile, are among the most important bioeroders on reefs, dissolving and chipping away limestone and producing fine sediment. On a healthy reef this balances coral growth; on a stressed reef, especially under ocean acidification, bioerosion can outpace it.

How sponges reproduce and grow

Sexually through larvae, and asexually by budding and fragmentation.

Most sponges reproduce sexually. Many are hermaphrodites that produce both eggs and sperm, though usually at different times. Sperm are released into the water in a cloudy plume — a spawning sponge can look as if it is smoking — and are drawn into neighbouring sponges, where fertilisation takes place. Many reef sponges brood their embryos and release free-swimming larvae, which drift for a short time before settling on a hard surface and growing into a new sponge.

Sponges also reproduce asexually. Broken fragments torn off by storms can reattach and grow, which helps some species spread quickly after disturbances, and some sponges produce buds or resistant cell packages that can survive harsh conditions. Growth rates vary enormously: some encrusting sponges spread fast enough to overgrow corals within months, while giant barrel sponges grow only slowly over decades. Because sponges have no fixed shape, the same species can look very different depending on depth, light and current.

Sponges, medicine and people

Chemical defences that became cancer drugs.

Because sponges cannot move or flee, many defend themselves chemically, producing a rich array of unusual compounds to deter predators, fouling organisms and competitors. Chemists have studied these for decades. Compounds first found in a Caribbean sponge in the 1950s inspired cytarabine, a drug still used to treat leukaemia, and eribulin, used for some breast cancers, is a synthetic drug based on a compound originally isolated from a sponge. Many other sponge compounds are being tested for antiviral, anti-inflammatory and antibiotic activity; see invertebrates as a source of medicine.

People have also used sponges directly for thousands of years as bath and cleaning sponges. Commercial sponge diving once supported whole communities in the Mediterranean and in places such as Tarpon Springs, Florida, and some natural sponges are still harvested or farmed today. A few are even eaten in parts of the world, though most are inedible; see can you eat a sea sponge.

Threats to reef sponges

Tough, but not invulnerable.

Sponges are generally more tolerant of warming and acidification than corals, which is one reason they are spreading on degraded reefs. But they are not immune. Marine heatwaves have caused sponge bleaching and die-offs in some regions, disease has killed many sponges in the Caribbean, and sediment from runoff and dredging can clog their pores. Physical damage from anchors, careless divers and fishing gear can kill large, slow-growing sponges that took decades or centuries to grow. Deep-sea glass sponge reefs are especially vulnerable to bottom trawling. Some deep-water sponges are thought to live for hundreds of years or more (see long-lived deep-water sponges), so losses there are effectively permanent on human timescales. Protecting sponges is part of protecting the whole reef, as covered in coral reef conservation.

Divers can help by keeping good buoyancy near walls and caves where large sponges grow, never sitting in or pulling on barrel sponges, and avoiding souvenirs made from wild-harvested sponges.

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