132 NORSE 



species) on a resource gradient increases the chance that one species 

 sandwiched between others will be eliminated. 



By analogy, what can be called "diffuse predation" also increases 

 to seaward. Activities of some crab predators (wading birds, crab- 

 eating raccoons in Colombia, and crocodilians in Florida, Jamaica, 

 and Colombia) are not greatly affected by the salinity of the water 

 containing their prey. Distributions of many other predators are 

 affected by salinity, however. Man, American eels, and Callinectes 

 are the only other known Callinectes predators in Jamaican fresh 

 waters. More predator species occur in brackish waters, however, 

 including ariid catfishes in both oceans in Colombia; snook in both 

 oceans in Colombia, in Florida, and in Curacao; tarpon and gray and 

 schoolmaster snappers in the Caribbean; and some sciaenids in Pacific 

 Colombia. Still more occur in marine waters; these include many 

 sciaenid species along mainland coasts and moray eels, balistids, 

 snappers, grunts, goatfishes, serranids, and sea turtles in all areas 

 where they have not been fished out. In fact, Randall (1967) found 

 that the majority of 212 Caribbean inshore and reef fish species eat 

 crabs. From these observations, it appears that predation (and 

 exploitation competition since many of these predators consume 

 foods that would otherwise be taken by portunids) steadily increases 

 to a maximum in lagoons and coral reefs, probably because many 

 predators are orthostenohaline. This trend may not be so simple in 

 some areas, however. Freshwaters on Caribbean islands, which are 

 not easily reached by oligostenohaline fishes and decapod crusta- 

 ceans, are depauperate in comparison with analogous mainland 

 communities. On the continents both downstress and upstress 

 distributions of freshwater Callinectes may be limited by competi- 

 tion and predation from diverse marine and freshwater stenohaline 

 communities. 



In addition to studying extrinsic evidence of predation pressure 

 from observed instances of predation and numbers of predator 

 species and individuals, we can assess its impact on a species by 

 examining the intrinsic responses that have evolved to thwart 

 predation. Several of these responses indicate that predation pressure 

 is more intense in higher salinities. 



All Callinectes are more or less countershaded, although ventral 

 surfaces of crabs in some environments may be covered with reddish 

 or black fUms. Countershading is almost ubiquitous among demersal 

 animals but has a unique denouement in Callinectes. In advanced 

 brachyurans the abdomen is usually folded beneath the body. In 

 male and immature female Callinectes, the actual dorsal abdominal 

 surface and surrounding ventral sternites are indistinguishably light 



