For many years people thought that sponges were a type of plant. This idea changed in the mid–18th century when scientists discovered that a sponge could circulate water through itself. They later found out that sponges eat bacteria and other types of tiny food found in the water. Based on these observations, scientists determined that sponges are actually animals. They are still not sure, however, where the thousands of kinds of sponges fit within the animal kingdom.

Where sponges live

Sponges live in water. They can be found throughout the world, from the tropical waters of the Bahamas to the cold Antarctic. Most live in the sea, but some live in freshwater. Sponges have been found at depths of more than 28,000 feet (8,500 meters) as well as in much shallower water. They live attached to the sea bottom or to hard objects. Some attach themselves by means of a single stalk, but others cover an object like moss covers a rock.

Physical features

Sponges come in a variety of sizes and shapes. Some are only as big as a bean, but others can grow to be 3–6 feet (0.9–1.8 meters) tall. Some look like cones, tubes, or globes; others are flat or bushy. Sponges can be rough, smooth, prickly, hard, or mushy. Although some sponges are drab in color, others are bright yellow, red, blue, green, or violet. A few animals eat sponges, but most leave them alone because of their unpleasant taste and smell.

The body of a sponge is made up of a soft mass of living cells covered by a thin layer of cells called pinacocytes. It is supported by a skeleton. Sponges do not have specialized organs and tissues for breathing, eating, and getting rid of wastes. Instead, individual cells take care of these functions themselves. Pores on a sponge's skin let water flow inside so that cells can take in needed things such as oxygen and food.

Reproduction and growth

Most sponges can produce both male and female sex cells. A male sperm cell released by one sponge is carried by the water to a second sponge, where it is captured and transported to a female egg. The sperm cell fertilizes the egg, which develops into a larva, or developing sponge. The larva is released into the water. Within a day or two the swimming larva attaches itself to a firm surface and begins to grow into an adult sponge. Some sponges release fertilized eggs instead of larvae, and development takes place in the surrounding water.

Sponges also can reproduce without the help of another sponge. In a process called budding, a sponge forms branches with independent circulatory systems. The branches then break off to create new individuals. Another process, called gemmulation, involves a sponge releasing food-filled cells from its surface. These cells can settle elsewhere and grow. A third process, called regeneration, enables pieces of torn sponges to grow into complete animals.

Uses

The skeletons of sponges have been useful to humans for many centuries. In ancient times sponges were used to pad war helmets and suits of armor. Doctors have used soft sponges during surgery. Sponges also have been used for bathing, mopping, and painting. Most of the sponges people use today are artificial, but natural sponges are still used for some arts and crafts projects.