CHAPTER VIII 

 THE ADDUCTOR MUSCLE 



Page 



Anatomy — '■'- 



M icroscopic structure ^^^^ 



White muscle fibers '■''■' 



Dark muscle fibers IM 



Attachment to shell 'tiO 



Chemical composition of the adductor muscle 161 



Inorganic salts 162 



Organic components 162 



Glycogen. 162 



Proteins 164 



Physiology of the adductor muscle 164 



Chemical changes during muscular activity 167 



Normal shell movements... 168 



Method of recording 16S 



Five major types of shell movement. 169 



Type A --- 176 



Type B... .--. -- 171 



Type C 171 



TypeD 172 



Type E 172 



Duration of periods of opening and closing 172 



Effect of temperature.. 174 



Effect oflight and darkness 17.i 



Effect of tide 17S 



Power of adductor muscle -- 17S 



Tests made in air and in water 177 



Cycles of shell movements 1^" 



Bibliography l^^l 



ANATOMY 



The adductor muscle of tlie oyster is a massive 

 organ that controls the opening and closing of the 

 valves. It occupies a slightly asymmetrical posi- 

 tion at the ventroposterior part of the body and 

 is surrounded by the following internal organs: 

 tlie visceral mass, pericardium, epibranchial cham- 

 ber of the gills, and cloaca (fig. 72). Tlie rectum 

 adheres to the posterior side of tlie muscle. Tiie 

 protrusion of the visceral mass, containing the 

 crystalline style sac and the lowermost part of the 

 gonad, covers the anterior side of the muscle. A 

 wedge-shaped visceral ganglion located inside the 

 epibrancliial chamber rests in a slight depression 

 on the side of the muscle under the visceral pro- 

 trusion. The ganglion can be exposed by cutting 

 through the wall of tlie epibranchial chamber and 

 lifting the tip of the visceral mass. 



The adductor muscle of the monomyarian mol- 

 lusks, i.e., those which have only one muscle (sucJi 

 as edible oysters, pearl oysters, scallops, and 



Spondylus), corresponds to the posterior adductor 

 of other bivalves. The anterior adductor, present 

 in larvae, disappears during metamorphosis shortly 

 after the attachment of the larva. 



Shortly after the metamorphosis of the larva 

 the posterior adductor muscle develops into the 

 most conspicuous and the heaviest organ of the 

 oyster. In valves of C. vinjinica and in some 

 other species of edible oysters the muscle scar 

 where the adductor is attached to the shell is 

 darkly pigmented. The shape and dimensions of 

 tliis area are variable (see p. 30 ch. II). 



The weight of tlie nniscle of C. virginica ac- 

 counts for 20 to 40 percent of the total weight of 

 the tissues. After spawning, when other parts of 

 the body are watery and poor in solids, the rela- 

 tive weight of the adductor increases. Exam])les 

 of this condition, usually encountered after the 

 discharge of a large number of sex cells and be- 

 fore tlie accumidation of the reserve materials 

 (glycogen) in the connective tissue, are given in 

 table 18. It may be deduced from these data 

 that the weight of the adductor muscle is not 

 affected by the changes in the chemical composi- 

 tion which take place in otlier organs. For fur- 

 ther discussion of this problem the reader is 

 referred to chapter XVII of this book. 



The adductor is cttmprised of two distinct parts. 

 Al)out two-tliirds of the total bulk of the muscle is 

 translucent, oval-sliaped, and slightly concave at 



T\BLE \H.— Relative weiqht of the adilurtor mtmrle of six 

 adult C. virginica {/, to ~) inches in height) during the 

 spawning season {August) in Woods Hole, Ma.'is. (fresh 

 basis), 1951 



Oyster 



Ripe male 



Ripe male 



Ripe female 



Kipe female 



Partially spawned female 



Spawned out, sex undetermined 



Weight 



Meat 



Grams 

 17.8 

 15.0 

 18.7 

 6,5 

 6.5 

 5.2 



Adductor 

 muscle 



GraTus 

 3.5 

 3.7 

 4.4 

 2.1 

 2.3 

 2.2 



.\dductor 



muscle 



(total 



weight) 



Percent 

 19.7 

 21. K 

 23.5 

 32.3 

 35.3 

 42.3 



152 



FISHERY bulletin: VOLUME 64, CHAPTER VIII 



