In a recent study, Kennish and Olsson (18) studied the effects of thermal 

 discharges on the microstructural growth of Merc en aria mercenaria in Barnegat 

 Bay, New Jersey. They found that clams from within a mile and a half of the 

 mouth of Oyster Creek, which carries the heated effluent from the Oyster 

 Creek Nuclear Power Plant, had a much higlier number of breaks in their shells, 

 thinner shells, and slower summer growth than did clams farther from the river. 

 Counting the growth increments back from the shell margin, they determined 

 that many of the breaks occurred concurrently with rapidly decreasing water 

 tem.peratures, resulting frojn abrupt shut-downs of the power plant, or rapidly 

 increasing temperatures associated with abrupt renewal of plant operations. 

 The growth rate of M. mercenaria generally increases with increasing 

 temperatures and peaks between 20-24°C; Haskin (communicated to Kennish 

 and Olsson) found decreased growth above 26°C. The thermal effluent raised 

 the water temperature in areas around the mouth of Oyster Creek 3-5°C above 

 ambient. Kennish and Olsson (18) also suggested that the thermal effluent may 

 be adversely affecting physiological functions other than growth. At the station 

 nearest the effluent, no spawning breaks were observed within the shells, while 

 they were seen in specimens from all the control sites. 



Shell Structural Changes 



In addition to changes in patterns of microstructural growth increment 

 sequences, changes in the type of crystalline structure deposited under various 

 environmental conditions have been observed within the shells (particularly 

 within the inner shell layers) of numerous species of bivalves. Dodd (10) 

 described environmentally-controlled variation in the relative proportions of 

 nacreous and calcific prismatic structures within the innermost shell layers of 

 Mytilus californianus . Lutz (23) found annual variation in the thickness of 

 nacreous laminae within the iimer shell layer of Mytilus edulis, and suggested 

 that such variation might be growth rate and/or temperature dependent, with 

 relatively fme laminae being formed with increased growth rate and rising 

 temperatures in the late spring. Bryan (5) examined the effects of oil spill 

 remover (detergents) on tlie shell of the intertidal gastropod, Nucella bpillus, 

 following the Torrey Canyon spill in March of 1967. The addition of toxic 

 detergent BP 1002 applied to the Kuwait crude oil spill was effective in 

 temporarily sealing the shell edge by continuing the inner nacreous layer to the 

 outer surface. Subsequent shell growth on thin nacre produced a growth mark 

 and Unes of weakness in the shell. Kennish and Olsson (18) observed 

 transgressing regions of aossed-lamellar structure within the outer shell layer 

 of Mercenaria mercenaria associated with shell deposition occurring during 

 periods of extreme ecological stress (winter freezes, high summer temperatures, 

 and thermal shocks from abrupt changes in operations of a nuclear power 

 plant). Farrow (14) noted similar transgressing regions of crossed-lamellar 

 structure within the outer layer of Cerastoderma edule associated with winter 



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