PHYSICOCHEMICAL AND BIOLOGICAL STRESSORS 135 



females, which tend to occur in higher salinities; in some cases 

 gill chambers were clogged with hundreds. Stenohaline marine 

 portunids, however, usually have few or no Octolasmis on their gills 

 (De Turk, 1940; Humes, 1941; Walker, 1974). In low salinities 

 euryhaline crabs have few external or gill foulers, but, when they 

 enter higher salinities, they become susceptible to fouling. Marine 

 species, however, are less vulnerable, presumably because of morpho- 

 logical, physiological, behavioral, or life-historic adaptations for 

 preventing larval settling. Crabs with fouled carapaces and limbs 

 suffer higher energetic maintenance costs and diminished walking 

 and swimming abilities. Those with gill foulers must compete with 

 the parasites for oxygen, are prevented from cleaning foreign matter 

 (sand grains, ciliates, nemerteans, bryozoans) from their gills, further 

 diminishing their oxygen uptake, and probably have weakened gill 

 discs, which provide an avenue for entry of pathogenic bacteria and 

 protozoa into their circulatory systems. Not surprisingly, crabs 

 having heavily fouled gills are noticeably less active than less-fouled 

 conspecifics. 



Food Availability 



Both quantitative and qualitative aspects of food change along 

 the gradient in terrestrial influence on aquatic climate. Available data 

 show that Callinectes have varied diets (Darnell, 1958; Tagatz, 1968; 

 Estevez, 1972), and V. Fox-Norse and I observed that they have 

 a surprisingly varied repertoire of feeding behavior. They hunt prey 

 as diverse as polychaetes and mullet, scavenge for carrion, scrape 

 aufwuchs from sea-grass blades and rocks, eat detritus-rich sediments, 

 and even filter feed when they encounter dense concentrations of 

 zooplankton (Norse, 1975; Fox-Norse, in preparation). In the 

 Caribbean study areas, water clarity permits autochthonous plank- 

 tonic and benthic primary production, but much benthic macro- 

 phyte productivity and allochthonous material from terrestrial plants 

 overhanging streams enter detritus-based food webs (Fenchel, 1970; 

 Odum and Heald, 1972). On the Pacific Colombian coast, since 

 estuaries and inshore shelf waters appear too turbid to allow major 

 planktonic and benthic plant production, most energy input is in the 

 form of wood, bark, and leaves from mangroves and other trees. 

 Since Callinectes feed largely on detritus or detritus-feeding organ- 

 isms, their productivity should be a function of the amount of 

 available detritus. The trawls collected animals, whose total biomass 

 did not vary greatly along transects, and macroscopic terrigenous 

 detritus, which far outweighed animal biomass in fresher sites but 

 decreased rapidly, constituting perhaps only 1% of the hauls in deeper 



