adductor has been calculated as follows (Ko- 

 ba3^ishi, 1929): 



Translucent portion — 



October 1.12 percent. 



November 1.0 percent. 



White portion — 



October 1.43 percent. 



November 1.29 percent. 



The figures are not essentially different from those 

 for C. vir<iln.ica. The questions of how much of 

 the glycogen in the adductor muscle is part of the 

 muscular mechanism and how much of it is stored 

 have not been answered with certainty. 

 Proteins 



According to the data quoted from Tabulae 

 Biologicae (1926), the fresh adductor muscle of 

 0. imbricata contains 11.38 percent protein and 

 4.8 percent fat. No published data are available 

 for the protein content of the muscle of C. vir(jinica.. 

 It may be assumed that in this species the protein 

 content is not essentially different from that 

 usually found in plain muscles in which it forms 

 from 14 to 18 percent (Evans, 1926). 



The contractile mechanism of the adductor 

 nuiscle of bivalves has the same structural 

 elements as are found in vertebrate muscles: 

 myosin (Florkin and Duchateau, 1942), actin, 

 and adenosinetriphosphate (ATP). The actin and 

 myosin extracted from muscles of 0. edulis, Mij- 

 tilus edulis, and Pinna nohilis (Lajtha, 1948) have 

 solubility relationships similar to those of the 

 corresponding- substances of rabbit muscle (Szent- 

 Gyorgyi, 1951). The myosin is soluble in distilled 

 water, insoluble in dilute potassium chloride solu- 

 tion (0.002-0. OS m), and again soluble in 0.1 m 

 potassium clJoride and higher. It is also soluble 

 in the 0.1 m and stronger solutions of chloride and 

 magnesium chloride. Myosin and actin can be 

 precipitated at isoelectric points of 5.2 and 4.7. 

 They both show double refraction whicli dis- 

 appears in dilution or at higher concentration 

 (0.4 M potassium chloride for myosin). Actin has 

 a higher doulile refraction than myosin. It also 

 lias the peculiar property of undergoing reversible 

 change from the globular to the fibrous state and 

 vice versa, depending on the pH and ionic con- 

 centration of the medium. 



Besides actin and myosin the adductor muscle 

 contains another protein called paramyosin, which 

 differs in solubility and X-ray diffraction from 

 nwosin (DeRobertis, Nowinski, and Saez, 1954). 

 Paramyosin was first detected in the adductor 



164 



muscle of the clam {Merce.naria iyenus) mer- 

 cenaria) by usmg electron stains (Hall, Jakus, 

 and Schmitt, 1945). Preparations of muscle 

 fibrillae treated with phosphotungstic acid reveal 

 a periodic structure of alternate bands that show 

 affinity for the stain. The distance between the 

 bands averages 145 A. At the same time there 

 is a larger period of 720 A. which is repeated 

 every five spaces of the smaller period (145 X 5). 

 It was concluded by Hall, Jakus, and Schmitt 

 (1945) that the fibrillae of this type consist of 

 paramyosin. Its content in various bivalves 

 varies but is cjuite high in Mytilus edulis in which, 

 according to Lajtha (1948), it exceeds the content 

 of myosin. Paramyosin of the adductor muscle 

 of C. rirginica was separated from actomyosin by 

 precipitation with three volumes of ethanol at 

 room temperature (Philpott, Kahlbrock, and 

 Szent-Gyorgyi, 1960) and resuspension of the 

 precipitate in 0.6 M potassium chloride at pll 7.4, 

 which was then dialyzed against the same solution. 

 By such treatment the paramyosin passed into 

 solution and the actomyosin remained precipitated. 

 The yield of paramyosin extracted in percent of 

 total protein was 22 percent in the opaque part, 

 16 percent in the translucent part. On tlie basis 

 of biochemical studies the authors suggest that 

 paramy(isin is localized in the thick filaments, 

 while the thin filaments consist of actomyosin. 



Paramyosin is not found in vertebrate muscles 

 but is the principal protein in many invertebrates 

 ' (Engstrom and Finean, 1958). Although its par- 

 ticular role in muscular contraction has not been 

 determined with certainty, it appears probable 

 that this protein is responsible for the maintenance 

 of the tonus of the adductor muscles. 



PHYSIOLOGY OF THE ADDUCTOR 

 MUSCLE 



The zoologists of the middle 19th century were 

 aware of the difference in the function of the two 

 parts of the adductors of bivalves. They re- 

 garded the white part as a bunch of elastic bands 

 which counteracted the pulling force of the valve 

 ligament and the translucent part as an ordinary 

 nniscle wliich brought the valves together (Bronn, 

 1862, p. 359). Goutance (1878) and Jhering 

 (quoted from Marceau, 1904a) and later Jolyet 

 and Sellier (1899) maintained that the translucent 

 part of the adductor muscle of Pecten maximus 

 Consists of striated anastomosing fibers whose ex- 

 clusive function is to close the valves; they 



FISH AND WILDLIFE SERVICE 



