136 NORSE 



shelf sites. If the amount of smaller, edible detritus is proportional to 

 that of macrodetritus, the most climatically stressful environments 

 have superabundant food for their inhabitants, but species are 

 increasingly likely to be food-limited to seaward. Although upriver 

 species can afford to be inefficient at finding or assimilating food, 

 selection should favor increased efficiency among organisms in 

 climatically more equable but trophically poorer waters. Downstress 

 distributional limits may correspond to the points where an 

 individual's maintenance costs equal its income. 



Energy intake is affected by the availability, as well as the over- 

 all quantity of potential food. In lagoons and among coral reefs, 

 predation pressures have stimulated the evolution of highly devel- 

 oped antipredation mechanisms in benthic invertebrates (Bakus, 



1964). Thus potential prey are usually out of reach (e.g., they bore 

 into hard substrates), hard to find (cryptic), or in some way 

 unpleasant to would-be predators (e.g., they have startling or warning 

 coloration, sharp spines, envenomating apparatus, toxicity, or 

 noxiousness). In informal experiments (Norse, 1975), I found that 

 normally exposed reefal animals were immune from predation by 

 Callinectes, but estuarine organisms were not. Thus an estuarine crab 

 on a coral reef, hke the Ancient Mariner, could be surrounded by 

 food resources but unable to use them. 



We have seen correlative evidence supporting alternate hypothe- 

 ses that downstress portunid distributions are limited by interference 

 competition, exploitation competition, predation, parasitism, and 

 food-resource availability. Interactions of these factors could also be 

 limiting since, as climatic stress decreases, eurytopic portunids 

 increasingly become host to ectoparasites. This raises their main- 

 tenance costs while diminishing their abilities to compete for 

 increasingly scarce food resources, to win agonistic contests against 

 relatively stenotopic portunids, and to escape increasingly intense 

 predation. Unfortunately, if my observations are correct, we cannot 

 yet reject any of the hypotheses. Even if only one of the major 

 categories were correlated with changes in climatic stress and 

 portunid distributions, we still would not know the precise limiting 

 factors. Rather, the hypotheses generated by these observations 

 should be tested rigorously so that we can continue to approach an 

 understanding of the determinants of organismic distributions. 



The Callinectes spp. whose crab stages can live in the freshest 

 waters at the edges of both oceans are more than transients but less 

 than permanent residents; they are catadromous (Norse, 1975; 1977; 

 Norse and Estevez, 1977). Although this skews their distributions to 

 seaward, the species that reach peak abundances along rather than at 



