MERCENARIA MERCENARIA (MOLLUSCA: BIVALVIA): 

 TEMPERATURE-TIME RELATIONSHIPS FOR SURVIVAL 



OF EMBRYOS AND LARVAE^ 



V. S. Kennedy,^ W. H. Roosenburg,^ M. Castagna,'* and J. A. Mihursky^' 



ABSTRACT 



To estimate the effects of entrainment of Mercenaria mercenaria embryos and larvae in the cooling- 

 water systems of steam-electric power plants, we used a thermal gradient apparatus. Cleavage 

 stages, trochophore larvae and straight-hinge veliger larvae were subjected to 11 different 

 temperatures for 8 different time periods. There was a direct relationship of mortality with tempera- 

 ture increase and, at higher temperatures, with increase in time exposure. As the clams aged, 

 temperature tolerance increased, with cleavage stages most sensitive to higher temperature and 

 straight-hinge larvae least sensitive. Multiple regression analyses of percentage mortality on 

 temperature and time produced estimating equations that allow prediction of percentage 

 mortality under different conditions of temperature and time exposure. Entrainment of M . mercenaria 

 embryos and larvae in cooling systems of power plants should be as short as possible if mortality 

 is to be held to a minimum. 



Passage of plankton through the coohng system 

 of steam-electric power plants is a matter of 

 concern (Coutant, 1970). Mortality caused by such 

 entrainment (e.g. Marcy, 1971) might lead to loss 

 of species from the vicinity of a power plant, with 

 various ecological and economic consequences. It 

 is estimated that increased demand for cooling 

 water may necessitate the location of power plants 

 in estuarine and marine environments (Tarzwell, 

 1972). Thermal tolerances of planktonic organ- 

 isms in these environments must be determined to 

 allow estimation of lengths of entrainment and 

 increases in temperature that are least harmful 

 to entrained organisms. 



The hard clam, Mercenaria mercenaria (L.), 

 is an abundant and commercially important 

 bivalve found in shallow inshore waters of the east 

 coast of North America. It is easily spawned in 

 the laboratory and has been the subject of nu- 

 merous investigations on the influence of various 

 factors on its larval biology (see Loosanoff and 

 Davis, 1963; Calabrese and Davis, 1970 for appro- 



'Contribution No. 565 of the Natural Resources Institute, 

 University of Maryland and Contribution No. 552 of the Vir- 

 ginia Institute of Marine Science. 



^Natural Resources Institute, University of Maryland, Ches- 

 apeake Biological Laboratory, Solomons, Md.; present address: 

 Fisheries Research Board of Canada, Biological Station, St. 

 John's, Newf , Can. 



^Natural Resources Institute, University of Maryland, Chesa- 

 peake Biological Laboratory, Solomons, MD 20688. 



''Virginia Institute of Marine Science, Eastern Shore Labora- 

 tory, Wachapreague, VA 23480. 



priate references). We used an aluminum thermal 

 gradient apparatus (Thomas, Scotten, and Brad- 

 shaw, 1963) to determine thermal tolerances of 

 hard clam embryos and larvae at different com- 

 binations of temperature and time exposure. This 

 simulated exposure of these organisms to heat 

 for varying time periods in power plant cooling 

 systems. The research was undertaken in the 

 summer of 1972 and spring of 1973 at the Eastern 

 Shore Laboratory, Virginia Institute of Marine 

 Science, Wachapreague, Va. Similar experiments 

 have been made on embryos and larvae of the 

 coot clam, Mulinia lateralis (Say) (Kennedy et 

 al., 1974). Reference should be made to that paper 

 for fuller details of experimental apparatus and 

 techniques. 



MATERIALS AND METHODS 



Mercenaria mercenaria were stimulated to 

 spawn by fluctuating water temperatures (Loo- 

 sanoff and Davis, 1963) over the range of 22° to 

 30°C. Gametes from 3 to 32 females and 2 to 30 

 males were pooled in each experiment to provide 

 genetic diversity (Calabrese and Davis, 1970). We 

 used three developmental stages: early cleavage 

 stages (2 h old); trochophore larvae (10-11 h); 

 straight-hinge veliger larvae (32-50 h). 



Wild stock collected as needed in the summer 

 near Wachapreague provided the gametes. After 

 the experiments ended in 1972, the preservative 



Manuscript accepted February 1974. 

 FISHERY BULLETIN: VoL 72, No. 4, 1974. 



1160 



