MULTIPLE-FACTOR AND SYNERGISTIC STRESSES 733 



reported that the tolerance level aiid differences between populations 

 greatly increased with salinity. The seasonal distribution of this 

 copepod, however, is contrary to what would be predicted on the 

 basis of his data. Bradley suggested that other copepod species have a 

 competitive advantage during the summer and fall and thereby 

 restrict populations of E. affinis. In contrast, after determining the 

 salinity and temperature tolerances and the salinity preferences of 

 the meiofaunal species Derocheilocaris typica, Kraus and Found 

 (1975) found that this species was distributed in nature by active 

 selection of tolerable and avoidance of intolerable salinity- 

 temperature regimes. Distribution patterns were in general agreement 

 v^th laboratory findings, unlike the work of Bradley (1975). 



The levels of various environmental factors may fluctuate with 

 time, and Thorp and Hoss (1975) studied the effects of salinity and 

 cyclic temperatures on the survival of two sympatric species of grass 

 shrimp. Cyclic temperatures appeared to be detrimental in combina- 

 tion with other stresses. Initially, the physiological tolerance to 

 wdnter conditions of temperature and salinity was determined so as 

 to provide a basis for examining the ecological interaction between 

 these two species. However, neither salinity nor temperature toler- 

 ances appear to be of primary importance in habitat partitioning of 

 these sympatric species. 



The role of other environmental factors also has been studied in 

 recent years. Roland and Ring (1977) reported on the interrelations 

 of cold, freezing, and desiccation tolerances of the limpet Acmaea 

 digitalis. Skoog (1976), working with snails, found that adult and 

 larval Lymnea peregra had a greater thermal tolerance than adult and 

 larval Theodoxus fluuiatilis, a result that could be correlated with the 

 field distribution of the two species. Also, differences in desiccation 

 limits could be correlated with field observations. The freezing 

 tolerance of the intertidal mollusc Modiolus demissus increased after 

 acclimation to either low temperature or high salinity (Murphy and 

 Pierce, 1975). It was proposed that low-temperature acclimation 

 does not act to increase freezing tolerance by reducing the amount of 

 tissue water frozen but rather that it increases cellular tolerance to 

 greater levels of dehydration by an unspecified mechanism. 



Vargo and Sastry (1977) analyzed the combined effects of two 

 other environmental factors, dissolved oxygen and temperature. 

 They determined the tolerance limits of five zoeal stages and 

 megalops of the crab Cancer irroratus. Interstage variation was 

 observed in that the first, second, and fourth zoeal stages showed 

 similar responses, but the responses were different from those of the 

 third and fifth stages, which were similar to each other. Although the 



