HOBSON and CHESS: TROPHIC INTERACTIONS 



impact has been significant. (A trend toward 

 transparency, most developed in the larger zoo- 

 plankters present in the water column during the 

 day, is obviously adaptive for organisms threat- 

 ened by visually feeding predators. Although this 

 trend is only briefly mentioned here, its impor- 

 tance in defense against predators is emphasized 

 by Hamner et al. 1975.) 



Because most generalized predaceous fishes 

 probably have been visual feeders since early 

 times, their prey would have long since found 

 water-column activities most safely performed 

 under cover of darkness. Not surprisingly, the 

 zooplankters that are vulnerable to large-mouthed 

 fishes are mostly nocturnal forms that spend the 

 daytime amid benthic cover. But only organisms 

 that have the capacity to leave the water column 

 can enjoy the advantage of cover during vulnera- 

 ble periods. 



The organisms that spend all their time in the 

 water column have had to adapt to being fully 

 exposed during daylight, when the visual sense of 

 their predators is most eff'ective. Under this cir- 

 cumstance one would expect long-established 

 selective pressures favoring sizes smaller than 

 those that can be entrapped by the relatively large 

 mouths of generalized predators. That such selec- 

 tive pressures operate today among zooplankters 

 in daylight is well documented. Brooks and Dotson 

 (1965), for example, described the larger zoo- 

 plankters in a lake being eliminated by the plank- 

 tivorous clupeoid fish Alosa pseudoharengus, 

 reported by Emery (1973) to feed by day. 



Because successful defenses in prey create 

 pressures that modify the offenses of predators, 

 early tendencies in prey toward nocturnal habits 

 or reduced size would have generated appropriate 

 evolutionary responses among predators. Cer- 

 tainly a long-standing selection for nocturnal 

 capabilities is evidenced by the many large- 

 mouthed predaceous fishes that forage in the 

 mid-waters at Santa Catalina after dark, includ- 

 ing the walleye, the salema, and the queenfish. 

 Large eyes, an obvious advantage in predators 

 that hunt at night, have been widely acquired by 

 these fishes (see Figures 6-10). Similarly, the small 

 mouth and other specialized features of diurnal 

 planktivorous fishes, like the blacksmith, clearly 

 are adaptive for feeding on the very small organ- 

 isms that constitute the diurnal zooplankton (see 

 Figures 11-13). In their feeding morphologies and 

 body forms, the nocturnal planktivores have 

 diverged less than have their diurnal counterparts 



from the generalized predators that gave rise to 

 them all. This does not necessarily mean that the 

 diurnal planktivores are more recently evolved. 

 Each is the product of an equally long evolution, 

 and while the diurnal planktivores have been 

 molded by selective pressures favoring the 

 capacity to take tiny organisms, the nocturnal 

 species have been influenced during the same 

 period by selective pressures favoring the capacity 

 to detect and capture prey in the dark. 



ACKNOWLEDGMENTS 



We thank Russell Zimmer, and his staff at the 

 Catalina Marine Science Center, University of 

 Southern California, for making facilities availa- 

 ble. We also acknowledge with gratitude the 

 following individuals for identifying specimens of 

 various species cited in this study: Robert Given 

 and Kristian Fauchald, University of Southern 

 California; Abraham Fleminger and David This- 

 tle, Scripps Institution of Oceanography; Diana 

 Laubitz, National Museum of Canada; Bruce Ben- 

 edict and Bradley Meyers, Marine Biological 

 Consultants; and Jack Word, Southern California 

 Coastal Water Research Project. For constructive 

 criticism of the manuscript we thank Richard 

 Rosenblatt, Scripps Institution of Oceanography; 

 Richard Vance, University of California, Los 

 Angeles; John E. G. Raymont, University of South 

 Hampton, South Hampton, England; and E. Bous- 

 field, National Museum of Natural Sciences, Ot- 

 tawa, Canada. Johanna Alban, Southwest Fisher- 

 ies Center Tiburon Laboratory, National Marine 

 Fisheries Service (NMFS), NOAA, typed the 

 manuscript, and Kenneth Raymond, Southwest 

 Fisheries Center La Jolla Laboratory, NMFS, 

 NOAA, drew Figure 2. 



LITERATURE CITED 



BOUSFIELD, E. L. 



1973. Shallow-water gammaridean Amphipoda of New 

 England. Cornell Univ. Press, Ithaca, 312 p. 

 Bray, R. N., and A. W. Ebeling. 



197.5. Food, activity, and habitat of three "picker-type" 

 micro-carnivorous fishes in the kelp forests off Santa 

 Barbara, California. Fish. Bull., U.S. 73:815-829. 

 Brooks, J. L., and S. I. Dodson. 



1965. Predation, body size, and composition of plankton. 

 Science (Wash., D.C.) 150:28-35. 

 Davis, W. P., and R. S. Birdsong. 



1973. Coral reef fishes which forage in the water column. 

 Helgolander wiss. Meeresunters. 24:292-306. 



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