The Sinister Sea Squirt


Didemnum sp. / Photo courtesy UConn/Avery Point

Didemnum sp. / Photo courtesy UConn/Avery Point




Hartford Courant, July 1, 2006 (draft)

By David Funkhouser

GROTON — The R/V Connecticut bobs in a light chop, floating midway between Mystic and Fisher’s Island. From the bridge, the dark blue waters of Long Island Sound sparkle on a sunny May morning. A fresh breeze blows, the air is clear, and everything looks just fine.

About 70 feet down, the state of the Sound is a lot murkier.

Capt. Dan Nelson shifts his glance from the view outside to the monitor of a computer that controls the 76-foot research vessel’s position and taps the keys to make an adjustment. With a throbbing hum, thrusters in the bow and stern fight a stiff current to hold us at a spot a mile west of Latimer Reef.

This is where we will hunt for Didemnum sp. A dozen students, teachers and crew have joined marine researchers Robert B. Whitlatch and Ivar G. Babb on this voyage to track down one of the most recent foreign invaders in Long Island Sound. The “sp.” stands for the generic “species,” because we are not sure exactly what beast this is, only that it is a sea squirt, of the genus Didemnum, and that its presence might be really bad news.

Didemnum sp. may have been brought here by ships that ply international waters, possibly from Asia. Whatever its origin, the creature is just the latest in a long list of problems confronting Long Island Sound, from mercury pollution to oxygen depletion to global warming. If it thrives here as well as it has elsewhere, it could pose a significant threat to shellfish and other important species. 

Just over an hour after embarking from the dock at the University of Connecticut’s Avery Point campus, the Connecticut settles at the first target zone. Babb, director of the National Undersea Research Center at Avery Point, supervises the crew’s use of a large winch that lowers an ROV — remotely operated vehicle — off the stern and into the water.

Babb, a tall man with graying brown hair and a moustache, is enthusiastic about the project. This is a shakedown cruise for the ROV, which will travel to Florida, North Carolina and Maine this summer to conduct research, then return to Groton to resume the search for didemnum in September.

Within minutes, the ROV reaches the bottom. In a cramped room under the bridge, Whitlatch, Babb and a half dozen others lean in over the shoulder of Craig Bussell, who sits at table full of computer and video monitors. Bussell holds in his lap a box the size of a large dictionary with two joysticks, which he manipulates to direct the ROV. A ball cap that covers his shaggy long brown hair reads: “Stingray ROV Stunt Pilot.”

The monitors and computers are wired into a framework of electronics the size of a small refrigerator. Out of this tangle of circuitry, a two2-inch-thick orange cable filled with fiber optic lines snakes up and across the ceiling, out across the stern and down into the water to the ROV.

The ROV, a propeller-driven, 600-pound metal sled mounted with lights, instruments, cameras, scoots along the bottom, looking for Didemnum sp. 

“They’re popping up everywhere,” says Whitlatch, a professor of marine science at UConn whose work on didemnum is funded by the National Sea Grant Program, which sponsors research at universities nationwide. Whitlatch attended the first international conference on invasive sea squirts in April in Woods Hole, Mass. To his great surprise, 70 people from 14 countries showed up. 

Dressed in a gray UConn sweatshirt and jeans, his wavy, graying hair tucked under a green ball cap, Whitlatch stares into a video monitor that displays the sea floor 68 feet down. The mostly sandy bottom is populated by scattered boulders, many surmounted by bright yellow sponges and orange corals. Cunners and other small fish amble into the frame and zip away.


At 11:15, Bussell maneuvers the ROV close to a boulder colored with a variety of sponges. Just above a tiny cave in the sand carved out by a lobster, we finally see it: a 20 centimeter patch of milky white didemnum. 

Didemnum sp. is one of five “invasive,” or non-native, species of sea squirts that have appeared in the Sound over the past two decades. Sea squirts, known as tunicates for the tunic-like sheath that covers them, are small, rubbery animals that live clinging to whatever they can grab onto underwater — rocks, pilings, ropes and boats.

Most species live as individuals; they may grow several inches wide and long. Some, like didemnum, whose individual members are just 2 mm tall (61/147th of an inch), form colonies. Didemnum grows in large mats that blanket the sea floor, and in long, dreadlock-like tendrils hanging from trap lines and piers. 

Sea squirts eat by pulling in sea water through a tube, sifting it through a mucous filter for food, and ejecting the water out another tube — hence their nickname. They eat whatever organic matter, from tiny phytoplankton to bacteria to detritus, they can suck in. They thrive in nutrient rich waters such as the Sound, and many other highly populated areas along the U.S. coasts.

They do not travel far on their own; but have become world travelers on boats, perhaps in ballast water or anchor compartments. Some believe this species arrived here attached to oysters imported from Japan as seed stock.

Once established, they tend to spread quickly. Taxonomist Gretchen Lambert of the University of Washington says didemnum reproduces both sexually, ejecting larvae into the water, and by budding tiny clones at the edge of the colony. Waves can cause clumps of it to break off and spread elsewhere.

Also, she says, the creature can go into a state similar to hibernation to survive adverse conditions.

“What this means is that they’re potentially immortal, like many other clonal animals,” says Lambert, who has tried for six years now to identify exactly what species of didemnum Didemnum this is.

 Didemnum sp. — or some variation of it — has turned up in a lot of places, from Puget Sound to the Gulf of Maine to the Netherlands. Holland. It threatens mussel aquaculture in Prince Edward Island and New Zealand. On the fishing and scalloping grounds of George’s Bank, at a depth of about 100 meters, didemnum mats now cover 86 square miles (by comparison, the city of Hartford covers 18 square miles).

Didemnum first appeared in the eastern end of Long Island Sound in 2000, Whitlatch says. In two years, it had spread “massively.”

“Their growth rate is 300 times faster than with other sea squirts we have been studying,” he says.

Last summer, the creature took over a portion of the bottom between 2 and 4 square kilometers, not far from where we are looking on this day. It’s been seen as far east as Clinton.

Sea squirts as a group are known to harbor various toxic compounds, thought to be a defense against predators, Whitlatch says. The didemnum invading Long Island Sound has the consistency of cured silicone caulk, the pH of stomach acid and no known predators — though a tiny snail has been seen on it, possibly grazing.


After a couple hours of surveying and numerous didemnum sightings, the crew winds the ROV back to the surface, and Capt. Nelson steers the R/V Connecticut to a second site east of the Latimer Light, with Stonington Harbor just north of us. This is where the large mat was discovered last summer.

Here the bottom is gravelly, with many boulders: prime turf for didemnum. The water is 100 feet deep, and we see lots of marine life — sea stars, rock crabs, sting rays, flounder. A school of sea robins, pectoral fins spread like wings, scatters along the bottom. We interrupt a pair of mating squid, who pull back from the ROV but hover, staring at it. 

Didemnum is everywhere — on boulders and old lobster traps and along the gravelly sea floor. Most patches are small, but appear a healthy orange-yellow. Jeff Mercer, a 24-year-old master’s student, was diving in this area last summer when he discovered the didemnum mat — a mosaic of colonies that covered much of the sea floor. 

The mat, he says, shrinks in winter, when water temperatures cool. He notes that on George’s Bank, where the temperature at the sea floor remains around 5 degrees centigrade (41 degrees F), didemnum has not died back.

This could be important for the Sound, because water temperatures here have been rising. This winter was the warmest on record in the east end, and from January to March the temperature did not go below 5 degrees C, Whitlatch says.

He predicts there will be plenty of the stuff around come September.

Didemnum is clearly changing the habitat, but it is unclear exactly how. The species prefers pebbly bottom over sand and mud. It overgrows most things, including shellfish and small sea animals that serve as food for other species. It competes for microscopic food and generally makes it harder for other species to survive.

Sea squirts also foul docks, boats, lines and traps. Whitlatch said shell-fishermen he has spoken to estimated between 20 percent and 80 percent of their costs involve biological fouling. He said ongoing studies will look at how sea squirts are affecting commercial shellfish such as scallops, oysters and blue mussels.

Whitlatch said vinegar and quicklime have been used to try to kill off didemnum. The beast does not survive long out of water. But dealing with something that thrives 100 feet underwater is a complicated matter.

Sea squirts are not all bad news for humans: Scientists in Japan said this monthJune that they had discovered a compound in one sea squirt that may help in the treatment of Alzheimer’s. Another compound derived from a sea squirt is in clinical trials as a treatment for soft tissue sarcomas. 

 Some of the critters are even edible: They are on menus in Chile and some Asian countries — though not didemnum. Babb tried them on a visit to Chile, where he saw them dried and hanging in markets. He said they were rubbery and tough, and “tasted like iodine, bleachy.”

“It’s an acquired taste,” he says.

For more on didemnum, and a sea squirt recipe, visit “Wrack Lines,” a magazine published by the Connecticut Sea Grant College Program at UConn:


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3 responses to “The Sinister Sea Squirt

  1. Linda Auker

    I am a Ph.D. candidate at the University of New Hampshire, studying the effects of this very same species on mussel biology and ecology. Your article came up in my Google alert for “Didemnum.” I just wanted to thank you for writing this piece and you did a great job describing this “sinister” species.

    • David Funkhouser

      Thanks for your note. This story ran some time ago; I recently posted it here as part of my portfolio, glad you found it. I can’t take credit for the headline, I took that from what ran over the story in the newspaper.

  2. Very well-written. I got here doing a search on, “The Sinister Squirting Effect”, the name of a college radio show I DJ’ed way back when (heck, it beats raking leaves). It turns out my older daughter is interested in majoring marine biology (and we’re quite familiar with Fisher Island and LI Sounds from boating around Noank). I’ve shared the article with her and will subscribe. Hooray for randomness!!!

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