Biology Reference
In-Depth Information
feet of water. The deep beds of mud coating the bay floor are riddled with
cities of burrowing, filtering, siphoning, and sliding animals. These inhab-
itants of the benthos include the shrimps, worms, clams, and other crea-
tures that filter the water or sift the mud for plankton and other micro-
scopic meals. Today, 95 percent of these bottom-dwellers are species
native to other estuaries. Only two native species—the Bay Mussel and a
Polychaete Worm—are among the most commonly found species in the
benthos. For this reason, scientists now consider the bay the most invaded
estuary in the country (see p. 199, “Preventing Invasions,” and p. 202, “A
Few Bad Actors”).
Clams
The bay's benthic communities were never as rich as those of East Coast
estuaries, but local bivalves were more than plentiful enough to sustain the
estuary's Native American residents. Scientists think the closest thing to a
native clam from San Francisco Bay may be
Macoma balthica
. Reaching
about an inch in diameter, this clam is a versatile feeder, able to dine both
on bottom deposits and food suspended in the water. Its unusually long
siphon sweeps across the mud like the hose of a vacuum cleaner. These
long straws also enable the clam to survive extreme changes in salinity by
burrowing deep into the sediments and finding pockets of salty water se-
questered among rock, sand, and silt.
One not-so-native clam seems to surpass most of the bay's other ben-
thic residents in sheer chutzpah and toughness: the Overbite Clam (
Cor-
bula amurensis
). Though no bigger than a quarter, this bivalve has it all:
speed, strength, and the ability to withstand extremes. It can siphon plank-
ton from the water faster than any ordinary clam, spin anchoring byssal
threads strong enough to secure its position in turbulent waters, and han-
dle waters that are fresh one minute and salty the next. Other characteris-
tics bolster its supercritter credentials: unusual stomach enzymes enabling
it to digest bacteria; the ability to reproduce when just a few months old;
and shells of unequal sizes (hence the overbite), buffered by a tough rub-
bery lip, that resist the prising of hungry crabs.
“The design of this clam is amazing,” says the U.S. Geological Survey's
Jan Thompson, an aquatic ecologist and engineer whose life work has
been the study of bottom-dwellers and the physical processes that allow
them to feed on the water column. “It's ecologically and biologically pretty
plastic.”
The Overbite Clam is a relative newcomer to San Francisco Bay. It most
likely arrived in the mid-1980s by hitchhiking on an Asian container ship.
The species occurs naturally in Korea, Japan, and China, but derives its
scientific name from China's Amur River. The clam must have felt right at
