FISHERY BULLETIN: VOL. 74, NO. 3 



ly, nocturnal foraging may be the rule among 

 species like the ostracod Perasterope sp. A, the 

 cumacean Cumella sp. A, and the amphipod Batea 

 tmm^ret'i^a, where it seems the majority spend 

 most of the night in the water column. Working in 

 the tropical Atlantic Ocean, Emery (1968) noted 

 that polychaetes, cumaceans, and zoea rise into the 

 water column at night after spending the day 

 under reef shelter and speculated that they make 

 this ascent to forage. But most of the polychaetes 

 entering the water column after dark at Santa 

 Catalina are epitokus nereids, whose mid-water 

 activities probably relate to reproduction. 



Williams and Bynum (1972) doubted the nightly 

 ascents of amphipods in North Carolina estuaries 

 relate to feeding because they subsist on detritus. 

 But detritus can be available to zooplankters in the 

 water column, as reported by Gerber and Marshall 

 (1974) from a coral atoll in the central Pacific. 

 Significantly, however, many of the amphipods 

 that enter the mid-waters at night appear mor- 

 phologically maladapted for swimming. The 

 oedicerotids (including Monoculodes and Sijn- 

 chelidium), for example, are modified for burrow- 

 ing in unstable sand (Bousfield 1973), and the 

 caprellids (Figure 5V) seem especially unsuited for 

 existence in mid-water. It is unlikely that such 

 forms are in the water column to feed, especially 

 as relatively small proportions of their populations 

 are up there. Probably these and similar forms 

 make only brief, or infrequent excursions into the 

 water column for reasons yet undetermined. Wil- 

 liams and Bynum (1972) suggested that relative 

 numbers of caprellids entering the water column 

 may correlate with seasonal deterioration of their 

 benthic habitats. They also felt that among gam- 

 maridean amphipods the tubicolous forms (e.g., 

 Ampelisca and Ericthonias) may facilitate re- 

 production by entering the water column, where 

 mating pairs would have free access to each other. 

 We reject Williams and Bynum's additional 

 suggestion that the nightly ascent may be an 

 attempt to escape from predatory bottom-feeding 

 fishes. Most bottom-feeding fishes that prey 

 heavily on amphipods (and other similar organ- 

 isms) are diurnal. (Some of the relatively few 

 fishes that prey on amphipods at night, and the 

 circumstances surrounding this predation, will be 

 discussed later; Hobson and Chess in prep.) Most 

 predaceous fishes feed visually, and tiny, cryp- 

 tically hued forms on dark substrata seem to go 

 unseen in the low levels of illumination that 

 prevail at night (Hobson and Chess in prep.; see 



also Hobson 1968, 1974, for accounts of the same 

 situation on tropical reefs). For whatever the 

 reason, most amphipods that fall prey to predatory 

 fishes after dark are species that rise into the 

 water column. 



Compared to their diurnal counterparts, the 

 nocturnal planktivorous fishes are of relatively 

 large sizes and have large mouths— both charac- 

 teristics reflecting the relatively large size and 

 accessibility of organisms in the mid-waters after 

 dark. 



Evolutionary Implications 



Since early in the Mesozoic period, the evolution 

 of actinopterigian fishes has centered on a main- 

 stream of generalized predators (Schaeff'er and 

 Rosen 1961). Because such predators are adapted 

 for straightforward attacks at prey in exposed 

 positions, the water column, with its absence of 

 cover, has been a risky place for smaller organisms 

 throughout the evolution of modern nearshore 

 marine communities. In discussing the earliest 

 actinopterigian fishes, Schaeffer and Rosen stated 

 that food was probably first obtained by biting and 

 was swallowed whole. Although advances in mouth 

 structure have refined their means of seizing food 

 (Schaeffer and Rosen 1961; Gosline 1971), gener- 

 alized teleosts have continued to take their prey 

 intact. Consequently, these fishes have found 

 appropriate prey to be organisms large enough for 

 them to entrap in their relatively large mouths, 

 yet small enough for them to swallow whole. As 

 demonstrated at Santa Catalina, prey of appro- 

 priate size include animals that rise into the 

 nearshore water column after dark-mysids, am- 

 phipods, isopods, and others. 



The present situation at Santa Catalina Island 

 suggests that since early times predation pres- 

 sures from large-mouthed, generalized predators 

 have influenced major evolutionary trends among 

 shallow-water zooplankters. Most apparent are 

 trends toward nocturnal planktonic activity in 

 those zooplankters that would spend only part of 

 their time in the water column, and toward 

 reduced size among those zooplankters that would 

 spend all of their time in the water column. At the 

 same time it would appear that each of these 

 trends has elicited an evolutionary response 

 among planktivorous fishes, as discussed below. 

 We do not suggested that pressures exerted in 

 predator-prey interactions have been the only 

 force shaping these trends, but we believe their 



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