Figure 19. — Oysters affected by boring sponge ( Cliona ). The sponge colonies in the shell of theoyster at left have died, 

 but the holes made by them are clearly visible. The oyster at right is still affected by vigorous, healthy colonies of 

 sponges, portions of which are seen protruding from the shell. 



The attack on oyster shells usually begins 

 at the oldest part of the valve near the hinge. 

 From here the sponge excavations begin to 

 tunnel toward the shell edges. Sometimes as 

 much as 50 percent of the shell substance 

 may be destroyed. Eventually the sponge may 

 attack the part of the shell where the muscle 

 is attached and, as a result, the oyster cannot 

 open or close its shell. At this stage the oyster 

 usually is killed by predators, such as crabs. 



Physical exhaustion of an oyster often re- 

 sults from its efforts to repair damages 

 caused by a sponge. The repairs are neces- 

 sary to prevent the sponge from penetrating 

 the shell because this will expose the interior 

 to pathogenic organisms. 



Sponge damage can be reducea by main- 

 taining oyster beds in clean condition, chiefly 

 by removing old shells badly infested with 

 sponges. Recently, an extremely effective 

 treatment of sponge-infested oysters was 

 developed by Bureau of Commercial Fisheries 

 biologists. It is a simple procedure consisting 

 of dipping oysters and old shells in a strong 

 brine solution during transplanting. The brine 



should be a saturated solution of common salt 

 stirred continuously. Normally, 1 minute of 

 immersion is sufficient to kill all sponges. 

 An additional value of the treatment is that, 

 as already mentioned, it also destroys many 

 other enemies and competitors of oysters. 



A bivalve, a boring clam of the genus 

 Martesia ( Diplothyra ), often enters the oyster 

 shell by boring a small, round hole, and ex- 

 cavating a hemispherical cavity in which it 

 spends its life, often attaining a length of 

 three-eighths of an inch. However, this clam 

 usually does not penetrate the shell or feed 

 on the oyster meat and, in general, does 

 comparatively little damage to the oyster. 



Polychaete worms of the genus Polydora , 

 usually called mud or blister worms, the 

 majority belonging to the species P. websteri , 

 often live on inside surfaces of oysters and 

 cause considerable damage. When the worms 

 are in the free-swimming stage, they enter 

 the oysters and attach themselves to the inner 

 surfaces of the shells close to their edges. 

 After attachment a young worm gathers mud, 

 placing it in a mat around itself. To prote-t 



26 



