PHYSICOCHEMICAL AND BIOLOGICAL STRESSORS 131 



TABLE 4 



MEAN SIZES OF ADULT MALE Callinectes, 



COMBATIVE EQUALS OF MEAN-SIZED 



MALES, AND ABUNDANCES WHERE EACH 



SALINITY GROUP IS DOMINANT 



the Pacific Colombian C. toxotes. It is much larger than C. arcuatus 

 (X carapace length for adult males is 87.3 vs. 49.0 mm, respec- 

 tively). If they were agonistically equivalent to their closest 

 Caribbean relatives (freshwater Callinectes vs. C. danae), C. arcuatus 

 would have to average 71.6 mm long to equal average C. toxotes 

 combatively. If they were agonistically equivalent to the most 

 mismatched Caribbean species (freshwater Callinectes vs. C margina- 

 tus), C. arcuatus would have to average 67.4 mm to have an equal 

 chance of winning against C. toxotes. Since the maximum size of C. 

 arcuatus (55.5 mm) is far less than either of these, interference 

 competition apparently cannot explain downstress limits of either 

 Jamaican brackish-water or Pacific Colombian freshwater crabs. 



Exploitation Competition and Predation 



Although I took quantitative data only on Pacific Colombian and 

 Jamaican brachyuran crabs, I repeatedly observed that numbers of 

 species in groups likely to be competitors and predators of portunids 

 (e.g., other reptant decapods, stomatopods, gastropods, cephalopods, 

 echinoderms, and fishes) increase sharply as the likelihood of severe 

 dilution decreases. In Jamaica the number of portunid species 

 quadruples from the freshest to the most equable marine waters. In 

 contrast, I found no obvious rise in the number of individuals along 

 the gradient, although population size and structure of many species 

 have undoubtedly been affected by human activities. As MacArthur 

 (1972) suggested, an increase in diffuse competition (i.e., number of 



