Sea Squirts: Ascidiacea

views updated

SEA SQUIRTS: Ascidiacea

NO COMMON NAME (Distaplia cylindrica): SPECIES ACCOUNTS
NO COMMON NAME (Botryllus schlosseri): SPECIES ACCOUNTS
NO COMMON NAME (Didemnum studeri): SPECIES ACCOUNTS

PHYSICAL CHARACTERISTICS

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.

GEOGRAPHIC RANGE

Sea squirts live in all oceans and seas.

HABITAT

Sea squirts live at all depths. Most live in shallow water. Some even survive on open shores under strong wave action. Some sea squirts live in ocean trenches as deep as 28,000 feet (8,400 meters). Most species of sea squirts live on rocks, shells, or algae (AL-jee), which are plantlike growths that live in water and have no true roots, stems, or leaves. Some sea squirts live on soft muddy or sandy bottoms. A few species of sea squirts live between gravel grains.

DIET

Sea squirts eat bacteria, plankton, and small swimming invertebrates. Plankton is microscopic plants and animals drifting in the water. Invertebrates (in-VER-teh-brehts) are animals without a backbone.

BEHAVIOR AND REPRODUCTION

Most sea squirts firmly attach themselves to the material on which they live. Species that are not firmly attached have threads on their body covering for anchoring themselves. The species that live between grains of gravel are not fixed and can move.

Sea squirts use muscle contractions to draw in seawater and to pump it out, opening and closing their intake and exit holes as they do so. For most sea squirts these movements are slow, but species that live in the deepest part of the ocean, where food is scarce, can quickly contract their muscles and close their large intake opening to catch small invertebrates.

CHORDATES

Chordates (KOOR-dayts) are the group of animals that have a notochord (NOH-tuh-koord), a flexible rod of cells that supports the body. In sea squirts the notochord disappears during the transformation from larvae to adults. The notochord is a simple form of the spinal cord of more complex animals.

Sea squirts make both eggs and sperm. In some colony-forming species fertilization (FUR-teh-lih-ZAY-shun), or the joining of egg and sperm to start development, takes place inside an individual squirt, but development takes place in the chamber the system shares, in special pockets in the body wall of the individual sea squirts, or in the sheet that holds the colony. Colony-forming sea squirts release freely swimming larvae. Larvae (LAR-vee) are animals in an early stage that change form before becoming adults. Some colony-forming sea squirts use asexual reproduction by budding to form colonies. Asexual (ay-SEK-shuh-wuhl) means without the uniting of egg and sperm for the transfer of DNA from two parents. In budding a bump develops on an animal, grows to full size, and then breaks off to live as a new individual.

Sea squirts that live alone either give birth to live young or release fertilized (FUR-teh-lyzed) eggs, which hatch into larvae. The larvae never feed but swim for a short time and then attach to the material on which they will live and transform into young sea squirts that have the same form as adults.

SEA SQUIRTS AND PEOPLE

Sea squirts may be a source for chemicals used to make medicines. In some countries, people eat sea squirts. Sea squirts that grow on the bottoms of ships interfere with mussel and oyster farming.

The Philosopher and the Sea Squirts

The Greek philosopher Aristotle studied sea squirts more than 2,300 years ago.

CONSERVATION STATUS

Sea squirts are not considered threatened or endangered.

NO COMMON NAME (Distaplia cylindrica): SPECIES ACCOUNTS

Physical characteristics: Distaplia cylindrica sea squirts form sausage-shaped colonies that reach a length of 23 feet (7 meters) and a width of 3 inches (8 centimeters). The colony is white or yellowish and has a soft texture. It is attached at one end to rocks and grows upward. The individual sea squirts in the colony are small and are located only in the surface layer of the colony, where they form many small, oval systems.

Geographic range: Distaplia cylindrica (abbreviated to D. cylindrica) sea squirts live in the Antarctic regions.

Habitat: D. cylindrica sea squirts live on rocky bottoms.

Diet: D. cylindrica sea squirts eat nutrient particles they strain from the water flowing through them.

Behavior and reproduction: D. cylindrica sea squirts attach themselves to rocks. They make both eggs and sperm, but individual sea squirts in different parts of a colony may have better developed male organs and those in another part of the colony may have better developed female organs. Larvae develop in pouches in the parent colony.

Distaplia cylindrica and people: D. cylindrica sea squirts have no known importance to people.

Conservation status: D. cylindrica sea squirts are not considered threatened or endangered. ∎

NO COMMON NAME (Botryllus schlosseri): SPECIES ACCOUNTS

Physical characteristics: Botryllus schlosseri sea squirts live in flat colonies on rocks or algae. The colonies reach a width of about 4 inches (10 centimeters). Systems of individual sea squirts are embedded in a soft, slimy dark brown sheet in circular patterns that look like daisies. The system's common opening looks like the center of the flower. The individual sea squirts have a white or yellow stripe on top.

Geographic range: Botryllus schlosseri (abbreviated to B. schlosseri) sea squirts live on both sides of the Atlantic Ocean.

Habitat: B. schlosseri sea squirts live in shallow water on rocks, shells, and algae.

Diet: B. schlosseri sea squirts eat nutrient particles they strain from the water flowing through them.

Behavior and reproduction: B. schlosseri sea squirts attach themselves to the surface on which they live. Fertilization and the development of larvae take place within the colony.

Botryllus schlosseri and people: B. schlosseri sea squirts have no known importance to people.

Conservation status: B. schlosseri sea squirts are not considered threatened or endangered. ∎

NO COMMON NAME (Didemnum studeri): SPECIES ACCOUNTS

Physical characteristics: Didemnum studeri sea squirts form flat colonies on rocks. The colonies are 3 feet (1 meter) or more in diameter and are less than 0.2 inches (5 millimeters) thick. The colony is white and covered with tiny pieces of a material that looks like bone.

Geographic range: Didemnum studeri (abbreviated to D. studeri) sea squirts live in the waters just north of the Antarctic regions. They are especially common in the Strait of Magellan.

Habitat: D. studeri sea squirts live in shallow water on rocks and on roots or stems of algae.

Diet: D. studeri sea squirts eat nutrient particles they strain from the water flowing through them.

Behavior and reproduction: D. studeri sea squirts attach themselves to the surface on which they live. The larvae develop inside the colony.

Didemnum studeri and people: D. studeri sea squirts have no known importance to people.

Conservation status: D. studeri sea squirts are not considered threatened or endangered. ∎