FISHERY BULLETIN: VOL. 72, NO. 4 



probably did not survive the widespread extinc- 

 tions that decimated reef communities at the close 

 of the Mesozoic. 



If, as suggested, many reef fishes close to the 

 main line of actinopterygian evolution long ago 

 assumed nocturnal habits in answ^er to the noctur- 

 nal habits of their prey, then one is not surprised 

 to find that widespread predator-prey relations 

 are centered around the nocturnal habit and that 

 the participants are mostly among the more 

 generalized members of the reef community. One 

 especially widespread activity pattern is dis- 

 played by the many fishes that assemble in schools 

 on or close to nearshore reefs during the day, then 

 disperse at nightfall and feed on small organisms 

 that become exposed after dark. This is the basic 

 activity pattern of many carangids, lutjanids, 

 pomadasyids, and sciaenids — all among the more 

 generalized perciforms (Hobson, 1965, 1968a, 

 1972, 1973). 



In addition to these basal percoids, it is 

 significant that of the relatively few fishes of 

 preacanthopterygian groups associated with 

 modern reefs, many either follow this pattern 

 themselves, or closely relate as predators to other 

 fishes that do (see next section). A diurnally 

 schooling-nocturnally active pattern is especially 

 widespread, if not universal, among the inshore 

 clupeids, order Clupeiformes — as described ear- 

 lier for Harengula thrissina, an exceedingly 

 numerous fish close to shore in the Gulf of Califor- 

 nia (Hobson, 1965, 1968a). Starck and Davis 

 (1966) found this same pattern in all five clupeids 

 that they studied on reefs in Florida, and I ob- 

 served it in Herklotsichthys punctatus in the Mar- 

 shall Islands (unpubl. data). Pertinent informa- 

 tion on nearshore clupeids is limited because so 

 few investigators have distinguished between 

 diurnal and nocturnal activity; nevertheless, 

 there are at present no data refuting the general- 

 ization that these fishes feed at night. 



There are fewer of these diurnally schooling, 

 nocturnally active fishes on Kona reefs than on 

 most other tropical reefs, perhaps for reasons dis- 

 cussed earlier (Hobson, 1972). Still, the pattern is 

 well defined there in certain of the mullids, genus 

 Mulloidichthys, and in the lutjanid Lutjanus vai- 

 giensis, and is especially apparent in the atherinid 

 Pranesus insularum, just as in its congener P. 

 pinguis of the Marshall Islands (Hobson and 

 Chess, 1973) — both of the preacanthopterygian 

 order Atheriniformes. 



Generalized Carnivores as 

 Crepuscular Predators 



In the same way that many generalized pred- 

 ators are nocturnal because suitable prey are 

 most available to them after dark, other 

 generalized predators — those that prey mostly on 

 smaller fishes — are primarily crepuscular be- 

 cause that is when these prey become most vul- 

 nerable to their mode of attack (Hobson, 1968a). 

 Moreover, just as is true of the nocturnal forms, 

 the crepuscular piscivores, which also are among 

 the more generalized of the reef fishes, experience 

 certain long-established predator-prey relations. 

 Significantly, many of these crepuscular pisciv- 

 ores are members of the same basal percoid 

 families, the Serranidae, Carangidae, and Lut- 

 janidae, that have produced some of the nocturnal 

 predators discussed above. Many of the crepuscu- 

 lar piscivores, however, tend to be larger than the 

 nocturnal species, which might be expected, in- 

 asmuch as the nocturnal fishes are among their 

 major prey (Hobson, 1968a). Schools of nocturnal 

 carangids, pomadasyids, mullids and, especially, 

 clupeids, are well-known targets of such pisciv- 

 ores. 



During the twilight periods of greatest piscivo- 

 rous activity (Hobson, 1968a, 1972), these noctur- 

 nal fishes are still in their diurnal schools. And 

 although the schools effectively protect them from 

 predators during most of the day (Manteifel and 

 Radakov, 1961; Eibl-Eibesfeldt, 1962; Hobson, 

 1968a), this protection is reduced during twilight 

 (Hobson, 1968a). At this time of maximum danger 

 from predators, most other smaller reef fishes, 

 both diurnal and nocturnal, are under cover; thus, 

 the schooling fishes, which are still in the water 

 column, become the most numerous prey of proper 

 size exposed to the space-demanding attacks of the 

 generalized piscivores (Hobson, 1968a). After 

 dark, the smaller fishes seem relatively safe from 

 at least most such predators (Hobson, 1973), but 

 during the changeover between day and night, 

 they are vulnerable (Hobson, 1968a; Munz and 

 McFarland, 1973). 



The large piscivores are exceptionally abundant 

 in certain parts of the Gulf of California where the 

 diurnally schooling, nocturnallj active fishes are 

 numerous (Hobson, 1968a). As suggested earlier 

 (Hobson, 1972), the relatively few such large pred- 

 ators on Hawaiian reefs, compared with most 

 other tropical areas, may relate to the relative 

 dearth in Hawaii of schooling prey. 



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