It is easy to see why people ask this question. A sponge does not swim, crawl or visibly react when you swim past it. It looks more like a plant, a rock or a piece of coral than an animal. And the only sponges most people handle are dry, lifeless objects bought in a shop. But a sponge on a reef is as alive as the fish swimming around it. It is busy every second of the day, pumping water, catching food and building its body, and some sponges live for centuries.

Sponges are one of the groups I find most underrated on a dive. On the Caribbean reefs I have dived, from Cozumel to the Florida Keys, they are everywhere: giant barrels, purple tubes, orange elephant ears and encrusting sponges in every colour. They are often overlooked because they do not move, but they are among the most important animals on the reef.

What makes a sponge an animal?

Sponges belong to the animal kingdom, in their own phylum, Porifera, and they share the key features of animals.

Their cells lack the rigid cell walls of plants, and they cannot make their own food by photosynthesis; instead they eat, filtering bacteria and tiny organic particles from the water. They develop from embryos and produce swimming larvae. Their cells are specialised for different jobs and cooperate as one organism, and their genes include many of the same building blocks found in other animals. Genetic studies place sponges near the base of the animal family tree, among the earliest groups to branch off. Our page is a sea sponge an animal or a plant? goes into this in more detail, and are sponges multicellular? explains how their cells work together.

What a living sponge actually does all day

A living sponge is constantly at work, even though you cannot see it moving.

Thousands or millions of collar cells, called choanocytes, beat tiny whip-like flagella to pull water in through pores in the sponge’s surface and push it out through larger openings called oscula. As the water passes, the sponge filters out bacteria and fine particles to eat and absorbs oxygen. Large sponges can pump an astonishing volume of water every day; see how much water sponges filter. Wandering cells inside the sponge carry food around the body and build new skeleton, and the sponge grows, repairs damage and reproduces, both sexually by releasing sperm and eggs or larvae, and asexually by budding or fragmentation. Many sponges also host huge communities of bacteria inside their bodies, sometimes making up a large part of the sponge’s mass.

Sponges can also respond to their surroundings. Some close their oscula or contract their whole body when disturbed or when the water becomes cloudy with sediment. Researchers have filmed freshwater sponges making slow, coordinated contractions that work like a sneeze, flushing sediment out of their canals. All this happens without a single nerve cell.

How to tell if a sponge is alive or dead

On a reef or a beach, a few signs tell you whether a sponge is still living.

Colour and surface. Living sponges usually have rich, even colours and a smooth or velvety skin. Dead sponges fade to grey, brown or white and their outer surface breaks down, exposing the fibrous or spiky skeleton beneath. Feel. A living sponge in the water feels firm, rubbery or slimy, depending on the species; a dead one feels rough, crumbly or stringy. Smell. A sponge that has been out of the water and is dying or dead often gives off a strong, unpleasant smell as its tissue decays. Water flow. In calm water, you can sometimes see particles being drawn towards a living sponge or pushed out of its oscula. Fouling. A dead sponge skeleton is quickly covered by algae and other organisms, while a healthy sponge keeps its surface clean.

Please look rather than touch, though. Some sponges, such as the Caribbean fire sponge, cause painful skin irritation, and handling any sponge can damage its delicate surface.

So what is the sponge in your bathroom?

A natural bath sponge is the skeleton of a dead sponge, specifically of a few species whose skeleton is made of soft, springy protein fibres.

Most sponges have skeletons of hard mineral spicules, which would be useless for washing. A few species of demosponge, mainly in the genera Spongia and Hippospongia, have a skeleton made almost entirely of spongin, a flexible, collagen-like protein. Harvesters collect these sponges from the seabed, then let the living tissue rot away, beat and wash the sponges to remove the remaining cells and debris, and dry them. What remains is a soft, absorbent network of fibres. Mediterranean and Caribbean waters, including the Gulf of Mexico off Florida, have long been the main sources; our pages on natural bath sponges, sponge diving and Mediterranean sea sponges cover the trade. Most kitchen and bath sponges sold today are synthetic, made from plastics or cellulose, and have never been alive at all.

Sponges can come back from pieces

One of the most remarkable signs that sponges are alive is their power of regeneration.

Break off a piece of many sponges and, under the right conditions, it will reattach and grow into a new sponge. Sponge farmers have used this for over a century, cutting living sponges into pieces and attaching them to ropes or frames in the sea to grow into new, harvestable sponges. In a famous experiment published in 1907, the American biologist H. V. Wilson pressed living sponges through fine cloth to separate them into individual cells and small clumps. The cells crawled back together and reorganised into new, functioning sponges. Few animals can do anything like it.

As a coral farmer, I find this especially interesting, because coral aquaculture relies on a similar principle: fragmenting a living colony and growing the pieces on. Corals and sponges are very different animals, but both show how much life can survive in a small fragment when conditions are right. You can see how I grew corals from fragments in my own systems in my coral farm gallery.

Why living sponges matter to the reef

Living sponges do important work, which is one reason scientists worry when sponge populations decline.

By filtering huge volumes of water, sponges remove bacteria and fine particles and help keep reef water clear. Research on Caribbean reefs has shown that sponges also take up dissolved organic matter released by corals and algae and turn it into cells and detritus that other animals can eat, a process sometimes called the sponge loop, which helps recycle nutrients on reefs that sit in nutrient-poor water. Sponges provide shelter for shrimp, crabs, brittle stars and small fish, and they are food for hawksbill turtles, angelfish and some nudibranchs; see what eats a sea sponge. Some sponges compete with corals for space, and a few even bore into coral skeletons, so their role is complicated, but a reef without living sponges would be a very different and probably poorer place.

In my own reef systems, I have always thought of filter feeders as part of the cleanup crew, quietly removing what the skimmer and filters miss. Sponges do that on an enormous scale in the wild.

How long do sponges live, and how do they die?

Some sponges are among the longest-lived animals on Earth; others live only a season.

Many small sponges live for a year or a few years, and some freshwater sponges survive the winter only as tough resting bodies called gemmules. Large reef sponges such as the giant barrel sponge are thought to live for centuries, and some deep-sea glass sponges have been estimated to be thousands of years old; see deep sea sponge lifespan. Sponges die from predators such as hawksbill turtles and angelfish, disease, smothering by sediment, storms, warming water and exposure to air. Air is a particular danger: if a sponge is lifted out of the water, air enters its canals and can become trapped, killing the tissue. That is why reef keepers are told to move sponges underwater, as our page on sponges in reef tanks explains.