PHYSICOCHEMICAL AND BIOLOGICAL STRESSORS 129 



Upstress Distributional Determinants 



The simplest explanation for these patterns is that species 

 distributions respond to the likelihood of severe dilution. Some crabs 

 must either avoid or be killed in climates having constant (always 

 fresh or brackish) or occasional (e.g., back bays, where heavy rains 

 cause normally marine salinities to plummet) severe dilution. Such 

 patterns imply marked differences among species' tolerances of 

 dilution, differences my tests confirmed. Combined results from 

 Jamaica and the Florida keys give the following order of eury- 

 halinity: C. maracaiboensis = C. bocourti = C sapidus > C 

 exasperatus > C. danae > C. marginatus > C. ornatus > P. depressi- 

 frons^P. spinimanus ^ A. cribrarius ^ P. sebae. Using data from 

 experiments in which Callinectes spp. were transferred to 10% 

 seawater, I found that the (arcsin transformed) proportions surviving 

 are very strongly correlated with the species lowest salinity records 

 (r = —0.989, v^th 5 df ; P < 0.001). In contrast, their high-tempera- 

 ture tolerances are only weakly correlated with their highest 

 temperature records (r = 0.397; not significant), and I could discern 

 no marked relationship between species desiccation tolerances and 

 their potential danger from desiccation. Unfortunately, I was unable 

 to do planned experiments on anoxia tolerances, but current 

 evidence indicates that upstress distributional limits are largely 

 controlled directly (by mortality) or indirectly (e.g., by the 

 depressing effects of physiological stress on competitive abilities) by 

 the likelihood of severe dilution. 



There is a strong resemblance between taxonomic and ecophysio- 

 logical relationships in portunids from both oceans, undoubtedly 

 resulting from Pliocene interoceanic connection. Among Pacific 

 Colombian crabs, C. toxotes, whose closest Caribbean relatives are C. 

 maracaiboensis and C. bocourti, dominates the freshest estuarine 

 sites; C. arcuatus, whose closest relative is C. danae, dominates 

 intermediate salinities; whereas Portunus asper and Arenaeus mexi- 

 canus (Gerstaecker), whose congeners are among the less euryhaline 

 Caribbean portunids, occur only in equable shelf waters. 



Downstress Distributional Determinants 



Since all species can occur in marine salinities and all Caribbean 

 species can survive in undiluted seawater in the laboratory, climatic 

 conditions do not directly limit downstress distributions. What brings 

 about the observed patterns of serial replacement in both oceans? 

 Possibilities include interference and exploitation competition, pre- 

 dation, parasitism, and food quantity and quality. Some investigators 



