124 UNDERWATER GUIDE TO MARINE LIFE 



Bivalves are adapted mainly for protection. Their thick shells are so effective 

 in this respect that this ancient group has evolved very little during the many 

 million years of its existence. It is true that about all they do is feed and 

 breed and secrete a shell. In general, they move only a little, their major 

 response to disturbing stimuli being a closing of the shells. 



Almost all bivalves are filter-feeders. They have incurrent and excurrent 

 siphons through which sea water enters and leaves the bodv. This water brings 

 oxygen and microscopic, planktonic nutrients to the bivalve. The bivalves form 

 a very compact group. Some, such as the mussel, Mytihis, are sessile and 

 attach themselves to hard substrate by means of tough threads called a "byssus." 

 The jingle shells, Anoniia, have fragile shells and are common in all seas at- 

 tached to all sorts of solid objects in shallow water. The ark shells. Area, have 

 heavy shells, and one species, the bloody clam. Area fexata, is the only mollusc 

 with red blood. Oysters, Ostrea, are also attached to solid substrate. They have 

 the capability of changing from one sex to the other alternately. A few species 

 of oysters form commercially valuable pearls. Pen shells, Pinna, are common, 

 thin-shelled, tropical species which have a sharply triangular shape. They 

 frequently are seen partly buried, pointed end down, in sand. The upper end is 

 very sharp and can inflict deep cuts. 



Among the nonattached and motile pelecypods are the quahog or cherrystone 

 clams, Venus, which are typical of the many burrowing clams of sandy or 

 gravelly beaches. The soft-shelled clams, Mya, have a thinner shell than the 

 quahog and prefer softer bottoms like soft sand or mud. The same is true of 

 the angel-wing clam, Barnea, and the razor clam, Ensis, which also have 

 fairly delicate shells. Clams with thin shells have, in general, longer svphons 

 and bury themselves deeper than do the harder-shelled species. Thev are also 

 more active and faster diggers than the hard-shells. 



One species of pelecypods has left the general protectively shelled pattern of 

 adaptation described in previous species. The shipworm, Teredo, is actually 

 an almost naked clam that uses its reduced shell to bore through wood. Like 

 termites, shipworms utilize wood for food. 



The scallops, Peeten, are actively swimming pelecypods. Thev clap their 

 shells vigorously together to force water out of the body cavity and thus move 

 by a sort of jerky jet propulsion. Scallops have photoreceptors around the edge 

 of the mantle and also have long, sensitive tactile and chemoreceptive tentacles 

 attached to the mantle's edges. 



All clams and oysters are edible, and some are delicious, but because of their 

 filter-feeding habits, they may at certain times of the year and in some waters 

 become poisonous. Obviously, pelecypods from foul or polluted waters should 

 be avoided. In the summer, toxic bacteria and protozoans mav become plentiful 

 enough in some waters to cause pelecypods, with these microorganisms in their 

 digestive tracts, to become poisonous. It is also said that during the breeding 

 season some clams and oysters are toxic due to the accumulation of sex products 

 in them. At any time of year, these animals spoil quicklv. They should be eaten 

 only when absolutely fresh. It is generally a good rule to eat pelecypods only 

 during months which have an "r" in them; the cooler months, September 

 through April. 



