Sea Squirts: Ascidiacea

Some sea squirts look like large, upright tubes and live alone. Others live in colonies and form a crust on rocks and other hard surfaces. Both types of sea squirts have a protective body covering. This layer is clear or brightly colored, usually red, brown, or yellow but sometimes blue. In some species the body covering is coated with spines. Most sea squirts are 0.04 to 6 inches (1 millimeter to 15 centimeters) long, but some are much larger. Some species that live alone can be as tall as 20 inches (50 centimeters), and some colonies grow to an area of about 10 square feet (1 square meter).

Sea squirts have two body openings: one for taking water in and one for pumping, or squirting, water out. The water-intake opening leads to a large chamber that takes up most of the inside of the sea squirt and is lined with slits. Seawater, which contains food particles and oxygen, is drawn into the large chamber and then is pumped into a second chamber. The slits are small enough to keep the food particles inside the first chamber, which leads to the digestive system. After entering the second chamber, the water is pumped out of the animal through the exit hole.

In colony-forming sea squirts, the body covering forms a sheet that holds the individual sea squirts. Each sea squirt in the colony has its own water-intake opening and main chamber, but several individuals in a system share a second chamber and water-exit hole. The shape of the systems varies. In some species the individual sea squirts are arranged in a circular system around the shared opening. In other species the individual sea squirts form long double rows along a canal of water-exit openings.