Connaughton and Taylor: Seasonal cycles in the sonic muscles of Cynoscion regalis 



701 



ever, a seasonal increase was noted in the sonic 

 muscle mass of the male haddock, Melanogrammus 

 aeglefinus (Templeman and Hodder, 1958). The sonic 

 muscles of haddock are present in both sexes, but 

 the seasonal increase in volume of the muscles was 

 noted only in the males. 



Maximal levels of total plasma testosterone ob- 

 served in this study ranged between 3.5 and 5.5 

 ng-mL -1 . Peak testosterone levels of 2.4 ng-mL -1 were 

 noted in the closely related spotted seatrout, 

 Cynoscion nebulosus. 2 Similarly, maximal levels of 

 11-ketotestosterone in the spotted seatrout fell be- 

 tween 8 ng-mLr 1 and 10 ng-mL -1 . Unbound 11- 

 ketotestosterone levels in the weakfish, presumably 

 expressing only a fraction of the entire plasma pool 

 of this steroid, were roughly one order of magnitude 

 less than maximal levels in spotted seatrout. 



The similarity of the shapes of the androgen and 

 SMSI curves suggests that plasma androgen levels 

 may play a role in the seasonal cycling of the sonic 

 muscle. Seasonal hypertrophy of the sonic muscles 

 appears to be triggered by increasing plasma andro- 

 gen levels in the spring. Similarly, the increased sonic 

 muscle mass noted during the summer appears to 

 be maintained by high plasma androgen titers. As 

 androgen levels peaked and began to fall, sonic 

 muscle mass continued to increase for a period of 

 one to three weeks, then began to drop off as atro- 

 phy directly followed peak mass. There was no pla- 

 teau in plasma androgen levels, nor was one noted 

 in the plot of changing SMSI. Fine and Pennypacker 

 ( 1986) noted an increase in the mass and a darken- 

 ing in the coloration of the sonic muscles of male and 

 female toadfish after gonadectomy and administra- 

 tion of either testosterone or 11-ketotestosterone. In- 

 jection of testosterone in male anurans can initiate 

 calling behaviors and has been shown to accentuate 

 the sexual dimorphism of the calling apparatus 

 (Obert, 1977; Sassoon and Kelley, 1986). 



In mammals, increased androgen levels can induce 

 increased muscle protein synthesis and muscle gly- 

 cogen storage, resulting in muscle hypertrophy 

 (Lamb, 1975). Increasing the workload of a muscle 

 can also result in hypertrophy of the muscle. Work- 

 induced hypertrophy can occur in the absence of pi- 

 tuitary growth hormones, insulin, or androgens. In- 

 creased muscle mass in work-induced hypertrophy 

 is the result of increased protein concentrations in 



2 P. Thomas, N. J. Brown, and C. R. Arnold. 1982. Seasonal varia- 

 tions of plasma androgens and gonad histology in male spotted 

 seatrout, Cynoscion regalis (Family: Sciaenidae). In C. J. J Rich- 

 ter and H. J. T. Goos (eds.), Proceedings of the international 

 symposium on reproductive physiology of fish, p. 111. Centre 

 for Agricultural Publication and Documentation, Wageningen, 

 Netherlands. 



the tissue. Much of this new protein is myofibrillar 

 and is believed to result in increased cross-sectional 

 area of the muscle fiber (Goldberg et al., 1975). In- 

 creases in muscle aerobic enzyme activities, mito- 

 chondrial protein concentrations, myoglobin concen- 

 trations, and muscle glycogen storage have been 

 noted in exercise-induced hypertrophy in mammals 

 (Holloszy, 1967; Edgerton et al., 1969; Barnard et al., 

 1970). It is possible that the hypertrophy experienced 

 by the sonic muscles of weakfish in the spring may 

 involve both of these pathways. Data presented here 

 indicate that elevated plasma androgen levels may 

 have played a role in the seasonal increase in mass 

 noted in these muscles. Increasing androgen levels 

 may play a direct anabolic role in muscle hypertro- 

 phy, or they may cue work-induced hypertrophy by 

 initiating drumming behaviors, or both. Field hydro- 

 phone data from this population collected in 1992 

 (Connaughton and Taylor, in press) indicate that 

 drumming activity begins approximately 4-6 weeks 

 before maximal sonic muscle mass is reached. 



The decreasing mass of the sonic muscles of weak- 

 fish in mid- to late-summer may be the result of de- 

 creasing androgen levels and decreased workload. 

 Field recordings of voluntary drumming indicated 

 that this behavior ceased abruptly after the spawn- 

 ing season (Connaughton and Taylor, in press). At- 

 rophy caused by disuse in mammalian systems re- 

 sults in a decrease in fiber cross-sectional area and 

 muscle mass (Desplanches et al., 1987; Musacchia 

 et al., 1988). The decreased use of the sonic muscles 

 after the spawning season might result in atrophy 

 and subsequent weight loss in the sonic muscles. 



Observations of specimens collected in 1991 sug- 

 gested that while the sonic muscle condition declined 

 throughout the summer and fall, the specimens were 

 still capable of producing sound when handled. If the 

 sonic muscles were capable of producing sound re- 

 gardless of their condition, then the seasonal hyper- 

 trophy of these muscles must play a role other than 

 activation of the muscles. Muscle hypertrophy in 

 mammals can result in more powerful muscle con- 

 tractions by that muscle (Goldberg et al., 1975). An 

 increase in the strength of the sonic muscle contrac- 

 tion might increase the amplitude of the drumming 

 call, allowing the male to be heard at greater dis- 

 tances or at increased intensities at a given distance, 

 or both. Also, potential increases in aerobic capacity 

 and in concentration of mitochondria may increase 

 the stamina of the sonic muscles, permitting calling 

 bouts of longer duration. 



If male drumming plays a role in weakfish repro- 

 ductive behavior, the condition of the sonic muscles 

 may affect an individual's reproductive success. How- 

 ever, maintenance of peak condition of this other- 



