Sea Squirts: Ascidiacea

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.

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.