Scientists Puzzled By Invasion Of Sea Squirts
If you look closely, you can see it — a thin layer of glistening, gelatinous goop forming on rocks and docks along the New England coast. It's the beginning of a crop of creatures known to scientists as a species of the genus Didemnum, but commonly called sea squirts, or, more descriptively, “alien vomit.”
By next month the invasion of the sea squirts is expected to be well under way. The cream-colored rubbery glaze will bubble out to form pale, warty blooms or squid-like tendrils, growing underwater on anything that sits still.
This particular brand of invasive sea squirt is new to New England researchers, and is still largely shrouded in mystery. No one knows for sure where it came from, or what its presence portends. But based on early observations, researchers worry it could endanger local shellfish and fish populations, crowd out other species, or signal larger environmental problems.
“This is so new, people are still trying to figure out what to do,” said Mary Carman, a research associate at Woods Hole Oceanographic Institution.
She said she began to study the creature seven years ago after she led kids into a Sandwich, Mass., tidal pool and they discovered it was teeming with what they said looked like peanut butter or macaroni.
Carman traces the growth explosion of the Didemnum squirts (there are other, less problematic varieties previously identified) to the 1980s, when “something happened to the environment, to the climate. Nitrogen levels grew to an all-time high. Algae blooms and algae growth are increasing. We're in a rapid warming trend,” she said. “Is it just coincidental?”
While scientists poke and prod at the organism and study its larvae under a microscope, New England shellfishermen are already fighting a war against the creatures.
These sea squirts have been found smothering mussels along Cape Cod, forming walls of choking jellyballs on oyster nets in Duxbury, Mass., and, in 2002, splotching a 40-square-mile patch of the ocean floor on Georges Bank, a vital offshore fishing ground. They have been found all along the New England coast, from Long Island Sound almost to the tip of Maine. They are multiplying in other parts of the world as well.
Seth Garfield has farmed oysters on Cuttyhunk Island for 24 years, and blames sea squirts — likely the Didemnum — for the loss of a third of his crop this year. Colonies of sea squirts grow on the outside of his nets, forming a rubbery wall that slowly suffocates and starves the oysters by blocking water flow and filtering out all the food and oxygen.
Garfield said his labor costs have increased over the past five years, as this single form of sea squirt has crowded off the other native sea squirts and seaweeds around Cuttyhunk, which sits between Buzzards Bay and Vineyard Sound. This season his staff will spend hours dragging the nets — each loaded with about 70 pounds of squirts, also known as tunicates — out of the water to clear the infestation every few weeks.
George Shamma, a Duxbury oyster farmer, kills them by regularly drying his cages in the sun, then clearing off the carcasses with a blast of pressurized water. “They simply attach to the outside of the cage, and they look sort of like ... a long version of a pumpkin stem. When they're alive, they're really tough,” he said.
To the chagrin of shellfish farmers, all signs point to a good year for the sea squirts, which got their playful nickname from the way they feed, sucking in nutrient-laden water through one siphon and then squirting out the remnants through a second tube.
The population growth is expected to peak in late summer.
“This stuff is growing like gangbusters!” said Page Valentine, a biologist with the U.S. Geological Survey in Woods Hole, who turned over rock after rock in the Sandwich tidal pool last week, finding evidence of the year's first growth when he expected to see nothing.
The bottoms of many rocks were laminated with new colonies of sea squirts, in a thin translucent layer that had thickened to form ugly, tough pimples in some places.
Carman and Valentine will visit the tidal pool every two weeks. They hope this summer to show conclusively whether sea squirts are killing shellfish, as anecdotes suggest, and to unravel the mystery of where the animals, which die off when the sea water gets too cold, are spending the winter.
Sea squirt researchers around the world had been working in relative isolation until recently. Last month, the first international conference on invasive sea squirts was held at Woods Hole. Researchers from the Netherlands to New Zealand swapped horror stories — vacuuming 26 tons of sea squirts off the bottom of a barge; seeing vast underwater stretches turn cream-colored each summer as squirts spread out; spending hundreds of thousands of dollars clearing the animals from a single location, only to find them growing back by the ton.
Researchers shared unpublished findings and failed eradication strategies. Didemnum can multiply 15-fold over a month; they seem to die when the temperature gets too cold or are left to dry out in the sun; and they have one known predator — a snail species that will eat dead sea squirts during the winter.
The conference was exciting for scientists who had been working for years on a mysterious species, but the take-home message was also sobering — there is no easy solution in sight.
“Under the best of circumstances, I might be able to tell you what you have, but I don't know if I could get rid of it,” said Gretchen Lambert, a Seattle taxonomist who was asked how much money she would need to eradicate a sea squirt invasion in a single location. “This is a major problem, so $35 million — $350 million — $35 billion isn't going to do it.”
One glimmer of hope lies in the possibility that sea squirts might eventually prove useful for human endeavors.
Japanese researchers injected live HIV and human cancer cells into several species of sea squirts, and found that the acidic creatures killed the diseases. A paper published this month described a bacteria living symbiotically with the sea squirt that may one day be used to fight cancer.
While microbiologists search for a way to turn sea squirts from bane to boon, other scientists around the world are launching the first coordinated efforts to share research about the organisms, hoping to solve the mysteries surrounding the animals and their sudden worldwide growth.
As part of that effort, researchers were scattered across the Sandwich tidal pool last week, and they sent out excited, urgent yelps when they found more evidence of new growth.
Dann Blackwood, a photographer with the U.S. Geological Survey who has been documenting the growth of Didemnum for 18 months, began snapping photos of the seemingly unearthly life forms.
“They're he-ere,” he sang, ominously. Article UID=4f56c48a-a773-41e5-93d6-d4b8d038ce26