showed the same cycle of increasing gonad activity 

 in the late spring and early summer when temper- 

 atures were increasing, in spite of the temperature 

 differential between the two areas. However, the 

 outfall population did not attain as high gonadal 

 index levels as the control, probably because 

 stressful temperatures above 20° C were reached 

 in June, leaving less energy available for gamete 

 production (Widdows 1976; 1978a. b). Food avail- 

 ability interacts with temperature to influence the 

 energy budget of mussels (Bayne 1973; Widdows 

 1978a, b), but food availability estimated by dry 

 weight of suspended matter is not significantly 

 different in the outfall and intake water at Morro 

 Bay (Hargreaves 1977). In July through October 

 outfall temperatures exceeded the energetically 

 extremely stressful! level of 25° C (Widdows 

 1978a, b), and the body weight/shell volume index 

 declined to levels well below the controls. Al- 

 though the reduced gonadal index of the outfall 

 population at Morro Bay strongly indicated a re- 

 duced reproductive output, M. ediilis under stress 

 apparently conserve the gonad up to a point at the 

 expense of other tissues, so that stressed mussels 

 continue to produce some gametes (Gabbott and 

 Bayne 1973; Bayne 1975). However, gametes from 

 stressed mussels result in embryos and larvae that 

 are less viable than those produced by adults not 

 under stress (Bayne 1972). 



In M. californianus the relationship of tempera- 

 ture to the energy budget for growth and reproduc- 

 tion is not well studied as it is in M. ediilis, nor 

 have the critically stressful temperatures been de- 

 termined for M. californianus. Mytilus california- 

 nus is reported to reproduce year-round with peak 

 periods of more intense spawning at various times, 

 particularly in spring and fall (Seed 1976). In the 

 present study the control population showed a 

 peak of gonadal activity in spring, corresponding 

 with the period of rising ambient temperatures. 

 The outfall population showed higher gonadal 

 index levels than the controls year-round, indicat- 

 ing that in M. californianus higher absolute tem- 

 peratures, rather than a temperature change as in 

 M. edulis, stimulate gametogenesis and increased 

 reproductive output. However, the body weight/ 

 shell volume index of M. californianus in the out- 

 fall was consistently lower than the control popu- 

 lation. If M. californianus conserves its gonad at 

 the expense of other tissues under the increased 

 energetic stress of elevated temperatures in the 

 same manner as M. edulis, this would explain the 

 lower body weights of the outfall mussels. 



It is not clear why all but the smallest outfall M. 

 californianus in my samples had relatively larger 

 shell volumes at the same shell length than the 

 controls. The difference may reflect greater shell 

 erosion of the control mussels, resulting from high 

 surf levels on the jetty, rather than reflecting a 

 temperature effect on the form of shell growth. 

 Seed ( 1968) showed that shell growth in M. edulis 

 is extremely variable, depending upon population 

 density and physical conditions. Mytilus califor- 

 nianus also shows great variation in shell form 

 from one locality to another (Coe and Fox 1944), 

 and intertidal height and latitude also affect shell 

 growth (Dehnel 1956). 



It is often difficult to apply results from control- 

 led laboratory conditions directly to field situa- 

 tions, where there are multiple and fluctuating 

 variables. I must acknowledge that the ability to 

 interpret the results of the present paper speaks 

 well for the realistic analysis of energetics and 

 stress in marine mussels in recent laboratory 

 work by others. However, very few marine inver- 

 tebrates have received this level of study critical 

 for the assessment of complex sublethal effects of 

 environmental disturbances such as thermal 

 effluents. 



Acknowledgments 



I am grateful to Cadet Hand, Virgil Schrock, 

 Ralph Smith, and George Trezek for their advice 

 and support. Bruce Hargreaves, Chris Harrold, 

 and John Pearse gave valuable comments on the 

 manuscript. Linda Hines, Brian Jennison, Marg 

 Race, Jim Rutherford, Jon Standing, Chris Tarp, 

 and John Warrick helped in many ways. The 

 Pacific Gas and Electric Company gave gener- 

 ously of their time and facilities. This study was 

 funded by National Science Foundation Grant 

 GI-34932 to George Trezek and Virgil Schrock of 

 the Department of Engineering, University of 

 California, Berkeley; Sea Grant NOAA 04-5- 

 158-20 to Ralph I. Smith and Cadet Hand of the 

 Department of Zoology; and a grant from the 

 Pacific Gas and Electric Company. 



Literature Cited 



Bayne. B L 



1972. Some effects of stress in the adult on the larval 

 development of Mytilus eduhs. Nature(Lond.i 237:459. 



1973. Physiological changes in Mytilus edulis L induced 

 by temperature and nutritive stress. J. Mar. Biol. Assoc. 

 U.K. 53:39-58. 



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