PHYSICOCHEMICAL AND BIOLOGICAL 

 STRESSORS AS DISTRIBUTIONAL 

 DETERMINANTS OF CARIBBEAN 

 AND TROPICAL EASTERN PACIFIC 

 SWIMMING CRABS 



ELLIOTT A. NORSE 



Department of Zoology, University of Iowa, Iowa City, Iowa 



ABSTRACT 



Zones where influences of terrestrial— limnetic and marine ecosystems overlap 

 provide natural physicochemical stress gradients for groups that evolved at either 

 end. Portunid crabs, originally a marine gi'oup, now occur from equable offshore 

 marine biotopes to stressful rivers and streams. Field studies on population and 

 community structure in coastal areas of the eastern Pacific (Colombia, Panama, 

 and Mexico) and the Caribbean (Jamaica, Florida, Curagao, and Colombia) and 

 experiments on physiological tolerances and biological interactions of Caribbean 

 portunids showed that Euphylax robustus in the Pacific and Portunus, Arenaeus, 

 and Cronius spp. in both oceans occur predominantly farther offshore, deeper, 

 or in higher salinities than CalUnectes spp., which are more euryhaline than 

 members of the other genera. Among CalUnectes, high-temperature and 

 desiccation tolerances are much less important than hyposalinity tolerances in 

 permitting species to penetrate estuaries and shallow areas, and likelihood of 

 severe dilution directly or indirectly limits distributions as stress increases. 

 Seaward distributional limits result from increased interference and exploitation 

 competition, predation, parasitism, decreasing food quantity and quality, and 

 the interactions of these factors. Thus biological stress increases as physico- 

 chemical stress decreases. Distributions of populations along gradients are 

 envisioned as responses to risk of mortality from physicochemical and biological 

 stressors, with abundance peaks corresponding to zones of minimum combined 

 risk. 



When a species belonging to a group that evolved in marine 

 environments leaves the sea for land or freshwater, it has an 

 opportunity to exploit new resources and leave behind old problems. 



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