Swaths of Cape Cod’s salt marshes are slowly disintegrating. For
the human observer, the most notable sign of their decline might
be the increase in night herons. They crouch like low, dark
smudges on the salt marshes at dawn after feeding on the surfeit
of Sesarma crabs through the night. Anyone looking will probably
see pieces of crabs on the ground around night heron roosts.
“You can find little balls of Sesarma legs and carapaces and
eyeballs,” said Mark Bertness, an ecologist at Brown University
who has been studying New England’s salt marshes for 20 years.
These crabs, which measure a little over an inch at most, have
many more predators than merely herons, especially when young.
“When they’re small, basically anything with a mouth will eat
them,” Dr. Bertness said. Blue crabs, striped bass and cod feast
on Sesarma crabs, for example. But these fish and shellfish are
being fished out by recreational fishermen, so over all, there
aren’t enough predators eating Sesarma crabs, according to a
study by Dr. B