BIOLOGICAL POLLUTION - Marine Pollution: What Everyone Needs to Know - page 197

intertidal habitats along much of the northeast coast of the
United States. There is little evidence that they have major
predators or parasites. They also like the moist, shady envi-
ronment created by cord grass and mussels in salt marshes.
The cord grass attracts ribbed mussels by giving them some-
thing to attach to; the mussels, in turn, give the crabs crevices
in which to avoid predators, a process referred to as a facilita-
tion cascade. The cord grass provides valuable shade for both
mussels and crabs. In this case, the crabs' use of the habitat
does not seem to crowd out native species. It is encouraging
that the salt marsh habitat can apparently accommodate this
new resident without severe problems. However, its broad
ecosystem effects and economic impacts are as yet unclear,
and there are indications that their populations are declining
in favor of native species.
What are some invasive sedentary attached organisms and
what harm do they do?
Tunicates or sea squirts are fouling organisms that attach to
hard substrates. Invasive tunicates are found mostly associ-
ated with artificial structures like floating docks, pilings, and
aquaculture installations, but they also settle on natural habi-
tats. Some invasive tunicates, (golden star tunicate, Botryllus
schlosseri ; violet tunicate, Botrylloides violaceus ) settle on eel
grass blades and reduce light penetration, thereby reducing
the growth and survival of the grasses, which are important
habitats for numerous animals. The sea squirt, Didemnum vex-
illum , has a history of invading and overgrowing marine com-
munities in temperate waters, including New England and
mid-Atlantic coasts, as summarized by Gretchen Lambert. It
reproduces rapidly, spreads easily, and can colonize and domi-
nate large areas of benthic habitat. They can overgrow native
organisms such as mussel beds. Areas with large amounts of
open space, regardless of species richness, are vulnerable to
Didemnum . Processes that fragment its colonies may accelerate