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BackgroundThe familiar, greenish-brown "sea grapes" that attach to pilings, oyster shells and stones in the littoral zone of the Chesapeake Bay are not plants, but one of the few advanced invertebrate species in the phylum Chordata. Also called the sea squirt, or common sea grape, Molgula manhattensis of the class Ascidacea, is a tunicate-animals named for their flexible, tough outer covering or "tunic". (Sea squirts are also named for their capacity to squirt a stream as far as two feet, when removed from the water.)
Tunicates are sessile animals, spending their mature lives attached to a solid object, but, surprisingly, derive from free-swimming, tadpole-like larvae with primitive notochords-a fibrous sheath surrounding nerve cells. Notochords permit a chordate organism to bend from side to side, rather than just telescoping forward when its muscles contract. The mature sea squirt superficially resembles a knobby potato with leathery skin, and contains two openings-protuberant excurrent and incurrent siphons-through which water enters and leaves the body. Water flows through the incurrent siphon and into a slitted, mucous-coated pharynx, surrounded by a large cavity or atrium. Food particles filter into this opening and through the digestive tract, which is continuous with the pharynx. Waste products pass through the anus and out the excurrent siphon. Most tunicate species are hermaphroditic and contain both male and female reproductive organs capable of producing eggs and sperm. Some reproduce asexually, by budding, which can yield many individuals from a single animal. Other species generate eggs that remain inside the atrium of the adult until the tadpole stage has been achieved. The common sea grape spawns by releasing both eggs and sperm into the water.
After about three days, the sea squirt's eggs develop into tadpole-like larvae, which have sucker mechanisms along the side which allow the animal to adhere to objects such as pilings and stones and begin their metamorphosis. After three or four days, the larvae will appear to have reabsorbed their notochords, gill slits will become visible, and the organism's digestive, reproductive, and circulatory organs will develop. The sea squirt grows to a mature length of two inches, 1 5/8 inches wide, and its "tunic" appears to be gray-green, sometimes sheathed in mud. This species can tolerate a wide range of salinities and pollution. True to its scientific name, Molgula manhattensis occurs along the length of the Atlantic coast and in New York harbor, and is the most common species in the Chesapeake Bay.
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