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Squirt Alert: Science News Online, Dec. 24, 2005

Squirt Alert: Science News Online, Dec. 24, 2005: "Designed to survive

Some 3,000 species of squirts inhabit waters from the equator to the poles. For most of Gretchen Lambert's 40 years spent studying these animals, 'I was kind of like the Maytag repairman,' she says. 'Nobody ever called.'

Squirts were a little-valued detail in the seas' vast diversity of inhabitants, explains the taxonomist, who is affiliated with the University of Washington's Friday Harbor Laboratories in Seattle. Demand for her expertise rose precipitously a dozen or so years ago, she says, when an unremitting tide of nonnative marine species began upsetting the balance of established ecosystems in near-coast waters around the world. Some of the more unusual pests were foreign squirts.

'Now, I spend virtually all of my time on invasive species,' she says—particularly on that 'very interesting scourge' known as Didemnum sp.

Like other sea squirts, this one begins life as a tadpolelike larva with eyes, a gut, a heart, and a primitive, backbonelike notochord. Once a sea squirt larva finds a suitable surface, it bonds to it. At that point, the larva metamorphoses into a barrel-shaped animal with a cover—its tunic—that's strengthened by hard clumps of cellulose. Squirts are the only animals to make cellulose, a primary structural component of plants.

The animals take their name from their two siphons at or near the top of the barrel. One draws in water laden with food—bacteria, algae, and other goodies—and squirts it into a filter basket lined with a sticky mucus. The critter then squirts out the water via the other siphon.

Most squirts are solitary animals, and some can grow several inches high and a few inches in diameter. Among the Didemnums, each individual, or zooid, is tiny, but it's usually part of a larger population. These colonies expand by budding new zooid clones from their edges.

The Didemnum in U.S. waters is like a squirt on steroids, notes Robert B. Whitlatch of the University of Connecticut in Groton. This 'beast,' as he refers to it, grows several times faster than any other sea squirt known. Within a few seasons, a single zooid can clone itself into a mat that's a meter in diameter. The zooids can also reproduce sexually, and each summer spew larvae into the water to establish distant colonies.

In colors ranging from white to peach, Didemnum mats have the slick feel and compressibility of cured silicone caulking, Whitlatch says. A colony's surface is acidic as is that of others of its genus—presumably, to make it unappetizing to potential predators. However, Whitlatch notes, 'with a pH of 2, equivalent to stomach acid,' the invasive species' coating is more acidic than that of its cousins.

How long can each animal live? 'We don't know,' Lambert says, 'but theoretically, it could be immortal' because it can continue to clone."

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