Figure 379. — Left valve of adult Crassostrea virginica 

 heavily infested by boring sponge, Cliona celata. Woods 

 Hole. 



continues, and in most cases the oyster's protective 

 measures prevent direct contact between the 

 sponge and the mantle. However, should the 

 deposition of shell material be delayed by adverse 

 conditions, the sponge makes direct contact with 

 the mantle and produces lysis of the epithelium 

 and underlying connective tissue. Dark pig- 

 mented pustules form exactly opposite the holes 

 in the shell. This extreme case observed in 

 oysters kept for several months in the laboratory 

 is shown in fig. .380. The tissue of these oysters 

 is flabby, and the mantle is easily detached from 

 the shell sm'face. 



All oyster bottoms are, to a certain degree, 

 infested by boring sponges which are found in 

 both live oysters and empty shells. There are 

 certain areas, however, where the infestation is 

 particularly heavy and the growth of the sponge 

 is very rapid. After the death of an oyster the 

 sponge continues to grow on the shell, form hi g 

 large, irregular masses 2 or more feet wide and 

 several inches thick. About 30 years ago such 

 large specimens were common in the bays and 

 harbors of southern Cape Cod, but now they are 



foimd only in deep offshore waters. The effect of 

 the bormg sponge can be estimated by determining 

 the percentage of oysters with heavily infested 

 and brittle shells and by comparing then- solid 

 and glycogen contents with those of uninfested 

 oysters. 



Boring clatn 



Oyster shells m the south Atlantic are often 

 infested with a bormg clam, Diplothyra smithii 

 Tryon of the famOy Pholadidae. Many papers 

 on oyster biology refer to this clam as Martesia 

 sp., but the taxonomy of the family revised by 

 Turner (1955) corrects the nomenclature and re- 

 stricts the name Martesia to wood-boring clams. 



The boring clam D. smithii is about one-half 

 inch long. It is usually found inside the shell 

 material m a cavity which increases in size with 

 the growth of the clam. The range of distribution 

 extends from northern Cape Cod (Provincetown , 

 Mass.) south to the east and west coasts of 

 Florida, Louisiana, and Texas. I have found no 

 live clams in oyster shells during my long-con- 

 tinued studies in New England waters, and only 

 a few live specimens have been recovered from 

 dead oyster shells aromid Tangier Sound in the 

 Chesapeake Bay. In southern waters the boring 

 clam is very common, particularly on some reefs 

 on the Texas coast. In 1926 oysters from Mata- 

 gorda Bay, Tex., were found to be so heavily 

 infested by Diplothyra that over 200 clams of 

 various sizes were found in a single adult (fig. 

 381). In order to make this count the shell was 

 dissolved in hydrochloric acid and the bodies of 

 the clams were collected. 



As the cavity bored by the clam increases and 

 approaches the inner shell surface, the oyster pro- 

 tects itself by depositmg layers of conchiolin over 

 the nearly perforated areas. Very rarely does one 

 find an oyster in which there is a direct contact 

 between the clam and oyster mantle. On the 

 outer surface of the shell the presence of clams is 

 indicated by small holes. The weakening of the 

 shell structm-e is the main effect of the boring 

 clam on the oyster. 



Mud worms 



Of the several species of Polydora found in the 

 intertidal zone of the Atlantic and Pacific coasts 

 of the United States, only two, P. websteri Hart- 

 man and P. ligni Webster, are important to 

 oyster ecology. P. websteri is found in oyster 

 shells and on the inner surfaces near the valve 



FACTORS AFFECTING OYSTER POPULATIONS 



421 



