Larval Paralichthys dentatus of only one size class (4 mm TL) were tested. 
The results stand in contrast to the findings with older Menidia larvae. A 
significant increase in escape ability (P < .01) occurred in P. dentatus larvae 
following thermal shock (Table 19-1). 
DISCUSSION 
Relationship of Results to Upper Thermal Limits 
Results of tests with four and six week old M. menidia indicate a possible 
adverse effect following an acute thermal increase to 30.0°C from an 
acclimation temperature of 20.5°C. The magnitude of this thermal elevation is 
close to the one hour TLM value of 31.4°C given by Hoff and Westman (14) 
for juvenile M. menidia acclimated to 20°C. The present study points to the 
increased sensitivity of behavioral stress indices to monitor effects of 
short-term or low level pollutant stress. Indeed, these findings strongly contrast 
the view of Austin et al ( 1). Based on mortality studies of a 13 minute shock of 
14°C above a 20°C acclimation temperature, he concluded that this treatment 
would not have any important effects on survival of larvae of this species. 
The absence of significant differences in escapes in newly hatched and two 
week old stressed M. menidia vs. the controls may be real, or could be due to 
the low number of tests run and the high variability observed within the shock 
groups. More data are necessary before final conclusions can be made on the 
sensitivity of these younger larvae to thermal shock. 
The increase in escape probability following thermal shock with larvae of P. 
dentatus may be due to an increase in altertness or in frequency of locomotory 
movements. Because the ambient water temperature was lower in tests with P. 
dentatus (15.1°C), the thermal shock did not approach reported lethal levels 
(32.0°C CTM at 15°C acclimation, Hoss et al (25)). Increased escape ability 
has been reported by Coutant (8) in juvenile Salmo gairdneri when thermal 
shock temperatures are well below lethal levels. 
Potential Mechanisms of Thermal Shock Effects on Predator 
Avoidance 
Although the phenomenon of differential predation in thermally shocked 
fish is now well documented (8, 25, 27), causal mechanisms for changes in 
vulnerability following thermal shock are not known. It has been demonstrated 
that the central nervous system is highly sensitive to temperature fluctuations 
(4, 23). The thermal sensory receptors are believed to consist of cutaneous free 
nerve endings (3), yet behavioral response to thermal shock is not necessarily 
limited to free nerve endings. Blood chemistry, membrane permeability, and 
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