Sponges: Ancient Animals That Help Coral Reefs

Sponges come in many colors, shapes, and sizes, and people often think they are plants. In fact, sponges are animals, and they are very old. Fossils from some species are about 600 million years old, from the earliest part of Earth's history. Today, around 8,550 sponge species are alive, and they belong to a group called Porifera.

Sponges and corals have some things in common, because both live in water and stay in one place. However, they are very different living things, with their own body structures, ways of feeding, and ways of reproducing. Even so, sponges play an important part in coral reef ecosystems. They filter the water, remove bacteria from it, and process carbon, nitrogen, and phosphorus, so a varied sponge community can affect water quality on a reef.

On reefs where nutrients are scarce, some sponges are believed to make carbon available to other living things. They release waste that other organisms eat, and this may boost productivity across the whole ecosystem. Sponges also help protect the reef from sharp changes in nutrients, temperature, and light, which supports the survival of other reef species.

A sponge's skeleton suits the place where it lives, so sponges can grow on hard rock, on soft sand and mud, or even on floating debris. Water passes through the porous outer surface of a sponge and brings it food and oxygen. Inside, tiny hairlike structures called flagella make currents that strain bacteria out of the water and catch food. The sponge's strong skeleton lets it handle the large amount of water that moves through it every day.

Why are sponges important to a coral reef, and how do they help other reef species survive?