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Life of the Cla?ns 39
A certain amount of respiration may take place even when the valves
of certain bivalves are completely shut during exposure to dryness or to
heat from the sun. What little air may be trapped within the mantle cavity
of the animal is soon used up. Oxygen is then obtained anerobically (with-
out contact with air) by cleavage of reserve glycogen substances stored in
the clam's tissues. Carbon dioxide builds up and is dissolved in the fluid
in the mantle cavity, and the resulting increase in acidity may dissolve or
etch away portions of the shell. Shells of oysters and the Patella limpets
vv^hich are kept dry on the rocky coast for unusually long periods show
considerable etching on the inside. Shells of the Date Mussels {Lithophaga),
which live in a small volume of water in their rock burrows, are etched in
this manner, while those specimens which live in the same volume of well-
aerated water are not etched.
Bivalves can be forced to cease respiration for several days without
succumbing, but they are very susceptible to polluted waters and excess
amounts of silt. The "red tide" caused severe destruction to the marine fauna
on the west coast of Florida in 1946, and for several years afterward the
"shelling" on famous Sanibel Island was little better than it is on Coney
Island Beach, New York. "Red tides" have occurred from time to time in
California, Washington State, Japan, Austraha and elsewhere. They are caused
by an unusual increase in the numbers of single-celled dinoflagellates, Gony-
aiilax. It is believed that billions of these organisms not only deplete the
oxygen supply but also clog the gills of fish, moUusks and other animals which
die in vast numbers and further befoul the ocean. Fortunately, these "red
tides" spend themselves out, and the coastal waters return to normal in a
few years.
Another species of Gojiyaulax (G. catanella) may be ingested by mus-
sels and clams and, although it does no harm to the mollusk, it is highly toxic
to humans who may eat the infected shellfish. A number of deaths have
occurred on both of our coasts from this type of mussel poisoning. There
is no way of distinguishing poisonous from sound mussels by their appear-
ance, and heat does not destroy the poison. Mussel poisoning occurs along
the California coast from May 15 to October 15. There is another such
center in Nova Scotia.
Among the various schemes of classification of the bivalves, the type
of gill structure has been used by many students of phylogeny (the study
of molluscan ancestral trees), such as Lankester, Pelseneer, Ridewood and
others. Opponents to this system, such as Neumayr, Munier-Chalmas, Dall,
Cotton and others, have based their classification on the hinges of the shell
valves. Neither system is without its weaknesses, and in some modern schemes
the two systems are employed together.
There are four main types of gills: (i) Protobranch, in which the gills