HOBSON and CHESS: TROPHIC RELATIONSHIPS AMONG FISHES 



nificantly different from patterns followed by the 

 larger forms studied here. The zooplankters de- 

 scending into the depths by day tend to be the 

 larger individuals, so we wonder where the very 

 small ones are located. In sharp contrast to the 

 relatively few adult planktivores active in weak- 

 current areas of the nearshore shelf by day, large 

 numbers of juvenile and larval fishes (Figure 4) 

 clearly found planktonic food abundant. It may be 

 that very small zooplankters, unsampled by our 

 net and too small to be taken by most adult plank- 

 tivores, remain numerous in shallow weak- 

 current areas during the day. 



Mysids as Prey During the Day 



It is striking that when mysids swarm in dense 

 numbers near many reefs during the day they are 

 relatively unimportant as prey of the major 

 planktivorous fishes. They seem to escape the in- 

 terest not only of diurnal planktivores, but also of 

 the many nocturnal planktivores (e.g., Myripristis 

 spp. ) that hover within easy reach close among the 

 coral. 



To be sure, a number of the fishes we studied 

 took some of these mysids by day. Chromis caeru- 

 lea, C. agilis, Dascyllus reticulatus, and Poma- 

 centrus pavo included mysids as minor compo- 

 nents of their diet at the weak-current site. 

 Furthermore, Hiatt and Strasburg( 1960) reported 

 that C. atripectoralis preyed significantly on 

 mysids. But considering the preponderance of 

 mysids in the water column at so many places 

 during the day, these fishes took only token num- 

 bers. 



Probably the relatively large size of the mysids 

 is important in this context. The evolution of feed- 

 ing morphologies in diurnal planktivores appears 

 to have been determined by strong selective pres- 

 sures to take tiny prey (Davis and Birdsong 1973; 

 Hobson and Chess 1976). Significantly, most of the 

 zooplankters taken by these fishes (e.g., copepods, 

 larvaceans, and fish eggs) were <2 mm long, and 

 the size range of mysids that swarmed around 

 these reefs in daylight was 2 to 8 mm (Tables 2, 7). 

 In reporting a similar situation in the tropical 

 Atlantic Ocean, Emery (1968) speculated that 

 planktivorous pomacentrids fail to prey on swarm- 

 ing mysids because normally these fishes feed on 

 smaller prey. 



The failure of Myripristis spp. and other large- 

 mouthed nocturnal planktivores to exploit this 

 diurnal resource cannot be attributed to the size of 



the mysids, however, because these fishes find the 

 same mysids major prey at night. Apparently the 

 nocturnal fishes simply do not react to these read- 

 ily accessible mysids as prey during daylight. In 

 warm-temperature waters of southern California 

 the large juvenile olive rockfish, Sebastes serra- 

 noides, feeds primarily on zooplankters after dark, 

 but during the day sometimes preys on mysids 

 that are within reach of the rockfish where it hov- 

 ers in relatively inactive diurnal schools (Hobson 

 and Chess 1976). However, predominantly noc- 

 turnal habits seem to be characteristic of the olive 

 rockfish only during its large juvenile stage — both 

 before and after this stage it feeds mainly by day 

 (Hobson and Chess 1976). Therefore, even at that 

 time of its life when the olive rockfish feeds 

 primarily at night, we should not expect it to be as 

 strongly nocturnal as Myripristis spp. and the 

 other more specialized nocturnal forms that ig- 

 nore mysids by day at Enewetak. 



Possibly swarming mysids are protected from 

 predators by the nature of their aggregations. 

 Emery ( 1968) noted that mysid swarms respond to 

 predators just as fish schools do. The analogy can 

 be expanded. Like these nocturnal mysids, many 

 nocturnal fishes congregate in dense numbers 

 above the reef during the day, and at this time 

 they too are relatively undisturbed by the many 

 predators at large in the same area (Hobson 1965, 

 1968). It is widely believed that fishes are less 

 vulnerable to predators when they aggregate (e.g., 

 Bowen 1931; Springer 1957; Brock and Riffen- 

 burgh 1960; Manteifel and Radakov 1961; Wil- 

 liams 1964). Of the many theories that would ex- 

 plain this circumstance, we favor the existence of a 

 confusion effect, as advocated by Allen ( 1920) and 

 others. This theory suggests that visually orient- 

 ing predators which select individual prey have 

 trouble singling out a target among the many al- 

 ternatives they confront in an aggregation. That 

 mysids achieve some safety from predators by ag- 

 gregating is further supported by the experiments 

 of Welty (1934), who found that goldfish, Caras- 

 sius auratus, consumed fewer daphnia when these 

 prey were concentrated. (These comments apply 

 as well to the relative lack of diurnal predation on 

 larval fishes, which, in their dense schools close to 

 the reef, resembled swarming mysids.) 



Planktivore Morphology and 

 Their Distance From the Reef 



It was suggested earlier (Hobson 1974) that in 



151 



