Niels-Viggo Hobbs has spent a great
deal of time in recent years exploring tide pools and the rocky shoreline of
Rhode Island, and he said that the ecology of the shore has changed
dramatically in the last two decades due to one relatively recent invader: the
Asian shore crab.
“Twenty years ago if you went to the
rocky shore and turned over rocks, you would have found mostly one or two
species of native crabs,” said Hobbs, a doctoral candidate at the University of Rhode Island. “But within a year of the Asian shore crab showing up in Rhode
Island in 1998, if you turned over those same rocks, you’d have something like
20 or 30 Asian shore crabs.”
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| Asian shore crabs by Niels-Viggo Hobbs |
Asian shore crabs aren’t large –
their shells are typically an inch or less across – but they reproduce quickly
and are more tolerant of cold and being out of the water than the region’s
native crabs. Hobbs said that a one-meter
area that may have harbored a dozen native crabs two decades ago now harbors hundreds
of Asian shore crabs.
“They eat whatever they can get
their claws on, and they reproduce like crazy, so they have a lot of mouths to
feed,” Hobbs said. “But they also become food for other marine life as well.”
Lobsters are one such species for
which the Asian shore crab is a double-edged sword. While the shore crabs may consume
large quantities of larval lobsters, the crabs are also probably eaten in large
numbers by adult lobsters. Commercially important fish, such as black sea bass,
also benefit from the availability of large quantities of Asian shore crabs.
Hobbs said the green crab, which is
the second most abundant crab species on Narragansett Bay shores, is also an
invasive species. But it arrived in the region in the ballast of ships about
200 years ago – long before scientists paid much attention to crabs -- so it is
difficult to know how its arrival may have affected the marine environment.
“The one thing we know happened
after the arrival of green crabs is that snail shells got thicker and tougher,”
Hobbs said. “They were apparently really good at cracking open snail shells, so
the snails evolved thicker shells.”
The focus of Hobbs’ research is on
the cost of aggression among crabs of the same species – for instance, how does
fighting among themselves affect how quickly they grow and mature, how likely
they are to be injured or die, and other factors. So he put pairs of crabs in
glass jars for what he called “crab fight club” to observe how aggressively
they behaved.
He found a great deal of variation.
Spider crabs, which have the smallest claws, exhibit almost no aggression at
all, whereas the rare lady crab is just the opposite.
“Those results likely correlate with
how abundant they are in the wild,” explained Hobbs. “Hundreds of spider crabs
can be piled up in one small area, and if they were as aggressive as lady crabs
then they would just kill each other.”
What is most interesting to Hobbs is
that Asian shore crabs aren’t all that aggressive. Since they don’t fight among
themselves, they can rapidly build up large populations, making it difficult
for native crabs to share the same habitat.
“One of the major mechanisms helping
the Asian shore crab succeed is that they’re not all that aggressive to their
own species,” Hobbs said. “They’re incredibly tolerant of other Asian shore
crabs, allowing them to be in such abundance that they can act like a wall
keeping other species from moving into an area.”
Has the invasion of Asian shore
crabs and green crabs made the Narragansett Bay ecosystem unhealthy?
“I can’t say it’s a less healthy
ecosystem; it’s just a different ecosystem than it would have been without them,”
Hobbs said. “There could be negative ecosystem effects – in addition to
outcompeting native crabs, Asian shore crabs eat a lot of mussels, so they
could be having an effect on important filter feeders in the bay. But there is
no evidence that they’ve had such an impact that the ecosystem changes are
significantly negative. So far it’s just lower level ecological communities
that have been negatively affected at this point.”
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