itself from irritation caused by the mud, the 

 oyster lays a film of new shell substance 

 covering the mud and creating a blister. These 

 blisters vary considerably in size from about 

 one-half square inch to almost 1 square inch. 

 The worm continues to live in the blister, 

 forming a U-shaped tunnel with openings to 

 sea water at the shell edges. 



Cases are known where large numbers of 

 Polydora infest external shell surfaces of 

 living oysters. It has not been proven, how- 

 ever, that in such cases oyster mortality 

 results directly from the Polydora attack 

 rather than from secondary effects on weak 

 individuals by masses of material covering 

 and eventually smothering them. 



When Polydora infect oysters in the usual 

 way, they are able later to enlarge their bur- 

 rows by edging out the calcareous shell mate- 

 rial. The formation of blisters reduces the 

 volume of the shell cavity and forces the 

 oyster to spend much energy producing the 

 secretion to cover the blisters. Heavily in- 

 fested oysters are poor and watery, and 

 mortality may be great. Nevertheless, in some 

 instances, even heavily infested oysters 

 possess good, healthy meats. 



Polydora can be controlled successfully by 

 dipping infected oysters in a saturated salt 

 solution, as described for controlling sponges. 



The common black mussels, Mytilus edulis , 

 are dangerous enemies of oysters because 

 they connpete with them for space and food. 

 Upon reaching setting size, mussel larvae, 

 like those of oysters, descend to the bottom 

 in search of a surface to attach to. Unlike 

 oysters, however, they attach by a slender 

 thread called a byssus. Sometimes the mussels 

 set so heavily that within a short time they 

 smother large numbers of young oysters. 



Since nnussels and oysters feed on the same 

 kinds of organisms, they compete directly for 

 food. In addition to this competitive activity, 

 mussels deposit on oyster beds large quan- 

 tities of feces and pseudofeces produced as a 

 result of their normal feeding activities, and 

 this often forms large masses of black, de- 

 composing material that sometimes smother 

 the oysters. Furthermore, if young oysters 

 are crowded by mussels, their shells become 

 deformed. Thus, it is virtually impossible to 

 grow good oysters if the beds are heavily 

 populated with mussels. 



Mechanical methods for controlling mus- 

 sels, such as picking them by hand or squash- 

 ing them with various devices, have always 

 been ineffective. Recently, several simple 

 chemical methods for killing them have been 

 developed by Bureau of Commercial Fish- 

 eries biologists and are now being further 

 evaluated before release for general use. One 

 method is using a material known as Victoria 

 Blue, which kills most mussels if they are 

 immersed in a 0.5 percent solution for 5 



seconds. A better and less expensive method, 

 however, consists of dipping mussel-infested 

 oysters in a solution of copper sulfate. Ex- 

 periments showed that this solution should be 

 weaker than 2.5 percent. Solutions containing 

 between 0.5 and 1.0 percent are recommended 

 if mussels can be kept out of water for 24 

 hours or longer after dipping. This method is 

 not recommended if the oysters are less than 

 1 inch long because considerable mortality 

 may result. 



Another serious competitor of the oyster is 

 the slipper limpet, a snaillike animal belong- 

 ing to the genus Crepidula . Most common of 

 these is C^. fornicata , sometimes found in 

 large numbers on oyster beds of the east 

 coast. It was introduced with shipments of 

 American oysters to British and Dutch oyster 

 beds which, at one time, suffered greatly from 

 this competitor. Crepidula was also introduced 

 to our Pacific coast with shipments of oysters 

 from New England. 



Generally, Crepidula does not interfere with 

 fattening of oysters. However, it should be 

 considered a dangerous oyster pest because 

 it occupies space needed for setting and growth 

 of oysters. As a rule, oysters and Crepidula 

 set at about the same time. Crepidula grows 

 much faster than the oyster and soon out- 

 grows it. In growing, the shells of Crepidula 

 spread over nearby oyster spat, covering them 

 so that they suffocate and die. In one instance, 

 examination of oyster shells collected from a 

 bed in Long Island Sound showed seven small 

 oysters smothered under a single Crepidula . 



In Europe, Crepidula -infested oysters were 

 dipped in a solution of corrosive sublimate. 

 However, since corrosive sublimate is an 

 extremely poisonous compound containing 

 mercury, its use in contact with edible prod- 

 ucts, such as oysters, may be dangerous. 

 Fortunately, it was found that the use of a 

 saturated salt solution, to which reference has 

 already been made, is extremely effective in 

 freeing oysters of Crepidula . Affected Crepid- 

 ula are usually unable to attach to a new sub- 

 stratum and are quickly destroyed by their 

 enemies, such as crabs, starfish, and fish. 



Anomia, often called the jingle shell, is 

 another bivalve which competes with oysters 

 by setting in large numbers on surfaces, such 

 as old oyster shells, to which young oyster 

 spat also attach. Like Crepidula , Anomia 

 grows imuch more rapidly than oyster set, 

 soon outgrowing it. In one case, 22 dead oyster 

 spat were found under a single Anomia nneas- 

 uring only five-eighths of an inch in diameter. 



Barnacles compete with young oysters for 

 space and, also, to a large extent, for food and 

 perhaps oxygen. Sometimes, when they set 

 late in the season, barnacles cover the re- 

 cently set oysters so heavily that the oysters 

 are smothered or their shells become ab- 

 normally shaped. 



27 



