In a recent study, Kennish and Olsson (18) studied the effects of thermal 
discharges on the microstructural growth of Mercenaria 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 higher 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 
temperatures, resulting fro,m 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 calcitic prismatic structures within the innermost shell layers of 
Mytilus californianus . Lutz (23) found annual variation in the thickness of 
nacreous laminae within the inner shell layer of Mytilus edulis , and suggested 
that such variation might be growth rate and/or temperature dependent, with 
relatively fine 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 the shell of the intertidal gastropod, Nucella lapillus, 
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 lines of weakness in the shell. Kennish and Olsson (18) observed 
transgressing regions of crossed-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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