GROWING FROM THE BOTTOM UP—WHOI Scientist Mary Carman examines the invasive, filter-feeding sea squirt species of the genus Didemnum living in the tidepools at Sandwich Town Beach on Cape Cod. This species spreads up from the bottom of rocks as it grows, covering everything in its path, including vegetation and shellfish. "Anyone who likes to eat seafood should worry about this," Carman said. (Photo by Tom Kleindinst, WHOI Graphic Services)
MANY SQUIRTS, PACKED TOGETHER—Some species of sea squirts appear to be one large organism, but when viewed under a microscope, it is evident that they are actually a colony made of hundreds and thousands of tiny, linked "squirts," called zooids. Here, two species of sea squirts grow next to each other. The small, orange sea squirts are of the genus Botrylloides. The whitish, larger squirts are of the genus Didemnum. (Photo by Dann Blackwood, USGS)
LIVING WATER PISTOLS—As sea squirts feed on algae and bacteria, or when they are gently squeezed, they shoot water out of tiny pipes. These bursts of water give it the name "sea squirt." (Photo by Dann Blackwood, USGS)
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ANATOMY OF A SQUIRT Sea squirts are tunicates, a type of sea life with a firm, rubbery outer covering called a "tunic," from which the name derives. As each organism feeds on algae and bacteria, they suck water in through one pipe and push it out a second. These bursts of water gave it the name "sea squirt."
Sandwich Town Beach was empty at low tide on a winter afternoon
when scientist Mary Carman yanked on hip boots and waded among the eel
grass and barnacles, her brown eyes scanning the clear water. Spotting
a butter-colored mass on a rock, she rolled up her jacket sleeve and
plunged in her bare hand. Out came something resembling soggy scrambled
eggs.
“Alien vomit, that’s what kids call it,” Carman said. In
fact, the cold, rubbery animal was a troublesome species of sea squirt
that likely invaded the New England coast from Asia or Europe in the
early 1990s.
Lacking natural predators, it is now found along
the coast from Connecticut to Maine, as well as offshore at valued
fisheries on Georges Bank, where dense mats of sea squirts are found in
a 40-square-mile area of seafloor. At an international conference April
21-22, 2005, sponsored in part by the Ocean Life Institute at Woods
Hole Oceanographic Institution (WHOI), scientists and natural resource
managers will address how—and if—they can keep the invasive sea squirt
population in check.
Worry for shellfish enthusiasts “Nothing
really wants to eat it. Nothing grows on it. And nothing
seems to prevent it from spreading,” said
Dann Blackwood of the U.S. Geological Survey (USGS) in Woods
Hole, who works with scientists photographing, videotaping,
and mapping the spread of the sea squirts on Cape Cod and
New England
While invasive sea squirts won’t harm people, scientists
worry about effects on marine life. Evidence shows that sea
squirts smother scallops and mussels, push out native species
of sea squirts, and coat the seafloor, possibly making areas
uninhabitable to fish eggs and shellfish larvae.
“Anyone who likes to eat seafood should worry about
this,” Carman said.
Scientists suspect the sea squirt hitched a ride in water
used as ballast on cargo ships, or on imported shellfish
used in the aquaculture industry. Like green crabs, kudzu,
gypsy moths, purple loosestrife, and tens of thousands of
other non-native animals, plants, insects, and microbes that
have settled in the United States over the last several centuries,
the sea squirt invasion could have potential ecological and
financial impacts, scientists fear.
The damages and control costs related to all invading species
in the U.S. are estimated at $137 billion per year, according
to a study by Cornell University ecology and agricultural
sciences professor David Pimentel. Costs associated with
sea squirts are not yet known, but among the more notorious
examples of damages caused by an invasive species are zebra
mussels, which have spread to nearly two dozen states since
the 1980s. They often outcompete and overwhelm native species,
and block water intake pipes that cost tens of millions per
year to repair.
A "horror-movie" creature Sea
squirts are tunicates, a type of sea life with a firm, rubbery
outer covering called a “tunic,” from
which the name derives. Of the nine types of sea squirts
found on Cape Cod, six are invasive species. Carman focuses
her research on a species of the genus Didemnum,
which forms dense mats made from many small, linked individuals.
As each organism feeds on algae and bacteria, they push
water in and out of tiny pipes. These bursts of water gave
it the name “sea squirt.”
At the Sandwich tide pool, Carman showed how it kills.
Wiggling her finger into a sea squirt’s wrinkly folds,
she pointed out shellfish trapped within. Like a creature
from a horror movie, the sea squirt had spread up and around
a rock, smothering everything in its path, including shellfish.
Carman initially encountered the animal seven summers ago
as a naturalist teaching youth education programs on Cape
Cod.
“It was difficult to avoid them,” she said. “Tons
of them were on the docks, on pilings, in tide pools, looking
like dead brain tissue.” In tide pools and rocky sea
beds, they often form wide, lumpy mats. On docks, lines,
and boat hulls, they grow pale, stringy stands, like multi-armed
octopi.
Kids peppered her with questions about this odd creature,
and she quickly learned that scientists had no ready answers.
She began work in the Geology & Geophysics Department
at WHOI and acquired funding from USGS, WHOI, and WHOI Sea
Grant.
“What started as a hobby has turned into a full-fledged
research project,” said Carman, who has a degree in
paleontology and previously studied microscopic invertebrate
fossils at The Field Museum in Chicago.
Into the field In December 2003, she began
working on field studies with research geologist Page Valentine
of the USGS in Woods Hole. At least once each month, they
venture to the Sandwich Town Beach for research.
Their experiments include snipping sea squirts and moving
portions to isolated, contained areas of the tide pool to
see how they reproduce and grow (they do—and quickly—the
scientists have learned). The scientists also observe if
predators feed on sea squirts (only periwinkle snails, and
only if the sea squirts are dead).
“Invasive sea squirts do present a problem, but we’re
figuring out some things we could do,” Valentine said.
At Georges Bank, for example, strategies could involve
keeping fishing gear away from the sea squirts, to avoid
dragging them to unaffected areas.
On shore, a different approach may be needed. Sea squirts
do not survive long when exposed to air, so Valentine and
Carman are studying how mussel and oyster farmers may be
able to stymie sea squirt growth by drying out equipment
that could host the organism.
Preventing sea squirts from entering new areas may be the
best solution, said Judy Pederson, manager of the MIT Sea
Grant Center for Coastal Resources.
“Consider trying to remove every invasive marine
larvae,” said Pederson. “It’s really, really
tough. The best remedy is to not allow it there in the first
place.”
For April’s international sea squirt conference at
WHOI, Carman said she’s expecting at least 40 speakers
and 100 attendees, including scientists from the western
United States, Canada, Brazil, New Zealand, India, Denmark,
Spain, and the Netherlands. Coastal communities in these
regions are also dealing with invasive sea squirts, she said.
“I’m glad to have these folks involved,” she
said. “We need to unite a lot of smart people to figure
out the next steps for dealing with this creature.”
About 3,000 different species of sea squirts exist worldwide. Some are yellow in color and stretch many feet in length. Others are the size and color of plums. “They come in all shapes,” said WHOI scientist Mary Carman “and in all colors of the rainbow.”
For all the headaches that invasive sea squirts can cause, native species are valuable. Some people use them for fishing baits. Scientists use them for genetics research and are exploring them for chemicals that could treat cancer. In nature, sea squirts filter water, keeping it clear. Sea squirt waste is a food source for other marine organisms.
