MULTIPLE-FACTOR AND SYNERGISTIC STRESSES 



731 



^' c 



Fig. 3 Boundaries of lethal conditions for the American lobster, 

 Hoinanis americanus. Regions are shown where temperature (T), 

 salinity (S), and dissolved oxygen (O), respectively, act alone as 

 lethal factors. (After McLeese, 1956.) 



on the survival of an adult aquatic animal, the American lobster, is 

 that of McLeese (1956) (Fig. 3). A sublethal but stressful exposure 

 to one factor may become lethal when an animal is exposed 

 concurrently to a second sublethal but stressful factor. The net result 

 is a reduction in the size of the zone of compatibility. For example, 

 w^hen salinity is optimal, the lobster can survive at a higher 

 temperature than it can w^hen exposed to lov^^ salinity and elevated 

 temperature. Larval stages are also sensitive to multiple factor 

 exposure, as seen in the work of Vernberg, DeCoursey, and O'Hara 

 (1974) (Fig. 4). In this case the multiple effects of temperature, 

 salinity, and cadmium also reduced the size of the compatibility zone 

 for larval fiddler crabs. 



Laboratory determination of the response to temperature and 

 salinity does not always correlate with the occurrence of that species 

 in the field. For example, Bradley (1975) reported that the thermal 

 tolerance of the copepod Eurytemora af finis determined at various 

 salinities with or without acclimation varied with season. He also 



