The fast-spreading species blankets sea beds, perplexes scientists
Sea squirt threatens native marine life
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By Lori Valigra
The sea squirt, also called an ascidian or tunicate because its body is covered by a
tough “tunic,” is thought to have come to New England about 15 years ago. Scientists still disagree
on which of several species it is, so they are calling it Didemnum sp., or “mystery” tunicate.
The name is appropriate considering that no one knows exactly where it came from, when it became
invasive, whether it will cause irreversible damage to shellfish beds, and what to do about it.
The creature appears to have no nutritional value and no predators. Fish do not like to eat it
because it is so acidic, and it already shows signs of overtaking mussel and oyster beds, choking
anything trying to live off the sea floor. One upside may be in its chemical makeup: it is being
studied for its potential medicinal value. But overall, it is viewed as unsightly and unmanageable;
even a fragment of a colony moved to another area by currents, fishing nets, boat hulls or other
means can multiply rapidly through both sexual and asexual production.
“It's a nuisance,” said Page Valentine, a research geologist at the U.S. Geographical Survey
(USGS) in Woods Hole, Massachusetts. “But it has the potential to cause a big impact to aquaculture
and fisheries habitats. The sea squirt forms a mat on the bottom of the sea, and there are fish and
other species that live off the bottom.”
Creature invades Georges Bank
The USGS caused a ripple among researchers when it discovered a six mile square area of the sea
squirt on Georges Bank at a depth of about 48 meters (157 feet) in November 2003. A year later it
found colonies covering 40 square miles in an area of the seabed that was highly productive for
both fish and sea scallops. In many parts of the colonized area, the critters covered 50 percent
or more of the seabed. The USGS plans another trip to look at sea squirt colonies this fall.
Because it's difficult to study the sea squirt in offshore waters, Valentine and his colleagues
have been using information from studies in a tide pool in Sandwich, Massachusetts on the eastern
end of the Cape Cod Canal. That research shows an alarming rate of growth for the sea squirt under
the right conditions. For example, a 7 square centimeter fragment from a colony can grow to 45
square centimeters [seven square inches] in only 15 days, and to 103 square centimeters [16 square
inches] in 30 days.
Mary Carman, research associate at Woods Hole Oceanographic Institution in Massachusetts, and
organizer of the first International Invasive Sea Squirt Conference held in Woods Hole in April,
has been collecting samples of the creatures in Sandwich for years. She said they can change their
shape. While flat like a large chicken breast in deep sea beds, they can extend long tendrils when
they attach to pilings and floating docks.
Various types of sea squirts have been found worldwide. The Georges Bank species was thought by
some scientists to have originated in northern Europe, where a similar species has spread in waters
in The Netherlands for years. But others, including Gretchen Lambert, a taxonomist at the University
of Washington Friday Harbor Laboratories, said recent research indicates it might have come from
Japan. This invasive sea squirt has also been found in Washington state and New Zealand.
Lambert suspects the sea squirt in Georges Bank may have been there for years before they were
discovered. The creatures are hardy: a large population can decrease greatly and enter a stage of
hibernation, and then come back in full force. James Carlton, director of the Williams-Mystic Marine
Studies Program at Williams College, Mystic, Connecticut, said there typically is a lag time that can
be three years or so from the arrival of a species until it is observed. And most sea squirt
relocations go unrecognized. One reason is that while alien sea squirt and other marine animals
move via commercial shipping and recreational activities, most never become invasive. Carlton,
Lambert and other researchers say it is critical to discover what in the environment or genetics
makes alien species turn into invasive species, because the arrival of non-native species will
continue.
“The issue hasn't received much research attention,” said Claire Carver, a consultant at Mallett
Research Services in Dartmouth, Nova Scotia. “We need to adjust our management strategies and
techniques, because the problem isn't going to go away.”
Difficult to control
Thomas Noji, division chief of the National Marine Fisheries Service in Highlands, New Jersey,
and a member of the steering committee of the Gulf of Maine Mapping Initiative (GOMMI), said it is
important to understand the life history, distribution and natural traits of Didemnum better. “What
is the current potential threat on Georges Bank? What can be done to contain it? Can we contain it?”
he said. “I'm not so sure.”
Efforts to contain or eliminate invasive sea squirt species in New Zealand and elsewhere by
killing them with chlorine have had limited success. The major reason is that if just one ship
housing the creatures enters a harbor, or even one colony fragment survives, a whole new infestation
can begin. Valentine said the Sandwich tidal pool has shown the sea squirt will degenerate if it
is exposed to air or shallow water. “We may be able to slow the spread by avoiding fragmentation
of colonies and controlling them in aquaculture by exposing them to air,” he said.
Despite control efforts, the spread of the sea squirts will loom as a threat. Emily Darbyson,
a researcher in the biology department of Dalhousie University in Halifax, Nova Scotia, gives an
example of another sea squirt species, Styela clava or clubbed tunicate. Native to the Sea of Japan,
it first showed up in the southern Gulf of St. Lawrence in 1998, and then spread to Prince Edward
Island. In PEI, it is causing major problems for the mussel aquaculture industry by overgrowing
the mussels. The clubbed tunicate can reach a density of up to 600 individual sea squirts per square
meter. The colonies hang from lines and fall to the sea bottom, where sea squirts spread by rafting,
hull fouling, fishing and aquaculture gear. “Tunicates can spread anywhere in Atlantic Canada in two
days' travel time,” Darbyson said.
Lambert's advice on sea squirt invasions: “We need to get rid of it now while we can, because we
don't know what triggers it to go from alien to invasive.”
To learn more about the mystery sea squirt in Georges Bank please visit
http://woodshole.er.usgs.gov/project-pages/stellwagen/didemnum/index.htm.
WITH NICKNAMES like macaroni, pancake batter and the blob, one would hardly expect a warm welcome for
an invasive sea squirt that is spreading on the rich fishing grounds of Georges Bank and areas along
the New England coast. Colored pale yellow to pink and textured like cartilage, the sea squirt
latches onto just about anything hard, including oyster beds and docks, and filters minute food
particles out of sea water. It has no known predators, and is spreading quickly in the cold waters
from Long Island Sound to northern Maine.
Lori Valigra is the assistant editor of the Gulf of Maine Times. She can be reached at
timeseditor@gulfofmaine.org.