Biology Reference
In-Depth Information
home upon arrival, because the Amur, like San Francisco Bay, is very tur-
bid. Within two years, the clam dominated the bay's bottom community.
Biologists like Thompson soon discovered that
Corbula
amurensis
popula-
tions in Suisun Bay, for example, could filter the entire volume of water in
the bay twice in one day. Like most clams, it feeds using two siphons: one
to suck in water and mud to filter for food, and one to jettison the filtered
water back out again.
The Overbite Clam's pumping rate is phenomenal. To study this,
Thompson set up a bed of clams in a lab trough, then added water and
phytoplankton and continued recirculating the water to simulate the mov-
ing waters of the bay. Then she measured how quickly the clams removed
the phytoplankton from the water. In this case, a bed of 500 clams filtered
all measurable phytoplankton out of 400 gallons of water within a day. In
addition, scientists noted that clams on the leading edge of the bed re-
mained burrowed in the sediment, whereas clams on the downstream end
sat upright, leaving about half of their shell exposed to the flowing water.
“By doing this the downstream clams are getting less of a shadow effect
from what their brothers are eating upstream,” says Thompson.
The Overbite Clam is now an established part of San Francisco Bay. At
times, depending on estuarine conditions and predators, billions of them
form a solid carpet across Suisun Bay. This intruder's sheer numbers have
fundamentally changed the aquatic food web, and some even blame it for
driving several species of delta fish to the brink of extinction.
The Overbite Clam is one of four clam species from Asia now found in
San Francisco Bay and delta waters. One of the others, more commonly
referred to as the “Asian Clam,” arrived here in the 1930s and quickly colo-
nized a different ecological niche: the fresher environs of the upper estu-
ary and delta. But
Corbicula fluminea
can't match its overbite cousin,
whose pumping rate per gram of tissue is considerably higher.
Shrimp
To glide over the bay floor is to observe the eyes and feelers of a thousand
shrimp poking upward like blades of grass from the muddy surface. Most
shrimp in the bay are tiny bundles no longer than a paper clip, bristling
with delicate appendages and saturated in vivid hues. The Coon-striped or
Dock Shrimp (
Pandalus danae
), common around Central Bay piers, may
have translucent stripes and delicate red and yellow spots. Another, known
as the Broken-back Shrimp (
Heptacarpus flexus),
), has what appears to be a
two-part body, whereas the Opossum Shrimp (
Neo mysis mercedis
) is
named for the females' habit of carrying eggs and young in a pouch by
their last pairs of legs. Bay fishers trawl for the California Bay Shrimp
(
Crangon franciscorum
).
