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 (without 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 which 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, Australia and elsewhere. They are caused by an unusual increase in the numbers of single-celled dinoflagellates, Gonyaulax. It is believed that billions of these organisms not only deplete the oxygen supply but also clog the gills of fish, mollusks 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 Gonyaulax (G. catanella) may be ingested by mussels 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 appearance, 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: (1) Protobranch, in which the gills
