HOBSON: FEEDING RELATIONSHIPS OF FISHES 



conservative predatory behaviors to be associated 

 with this morphology, and just such behaviors are 

 centered around nocturnal and crepuscular feed- 

 ing habits. The nocturnal habit involves mostly 

 predation on small, motile crustaceans, the cre- 

 puscular habit mostly predation on smaller fishes. 

 Together, crustaceans and fishes are the two 

 major types of prey taken by the generalized 

 predator. 



Nocturnal and crepuscular habits among 

 generalized carnivores are discussed separately in 

 the following sections. The separation is artificial, 

 as is the delimitation of a third category, that 

 dealing with generalized carnivores that feed reg- 

 ularly by day. In fact, as illustrated below, the 

 behavior patterns associated with these three 

 types of activity are closely interrelated. 



Generalized Carnivores as 

 Nocturnal Predators 



Early in the evolving relation between fishes 

 and their prey, the evolutionary lines of many 

 small, vulnerable organisms probably increas- 

 ingly shifted activity to periods of darkness. There 

 scarcely could be a more elementary solution for 

 animals threatened by active, visually orienting 

 predators. And because effective defense adjust- 

 ments in prey pressure predators to modify their 

 offense, it seems certain that various predators 

 early aquired means to follow their prey into the 

 night. Thus, in predatory fishes the nocturnal 

 habit itself would be a specialization, but a 

 specialization probably adopted in early pre- 

 teleostean times that has permitted much of the 

 continued widespread success of the generalized 

 predaceous feeding mechanism. 



The smaller generalized carnivores on reefs 

 today find their major prey among the abundant 

 crustaceans, which, as follows from the above, are 

 mostly nocturnal animals that expose themselves 

 at night (Longley, 1927; and others). Many 

 generalized predators that would feed on these 

 organisms have found nocturnal habits adaptive, 

 because only after dark does their straightforward 

 attack find suitable prey in the required exposed 

 position. In this feeding relation, the relatively 

 small size of the crustaceans undoubtedly has 

 influenced the size of the predatory fishes, most of 

 which are of small to medium size (less than about 

 300 mm long). 



Most nocturnal fishes in Kona prey on benthic 

 crustaceans, especially xanthid crabs; however, a 



number are adapted to take crustaceans and other 

 forms from the water column. The prey of these 

 fishes are mainly relatively large zooplankters (a 

 broad, perhaps loose concept of the term "zoo- 

 plankton" is used in this report), like crab 

 megalops, that are most abundant in the water 

 column at night. Adults of most nocturnal plank- 

 tivorous fishes in Kona do not feed significantly on 

 the many small plankters, like calanoid copepods, 

 that predominate in the water column during both 

 day and night. 



The extent to which the more primitive reef 

 fishes feed at night seems not properly ap- 

 preciated. Nocturnal habits are widespread 

 among basal percoids, whereas diurnal habits 

 tend to be characteristic of certain more 

 specialized offshoots. Even if one considers only 

 families that occur in Kona, all nearshore species 

 of the Kuhliidae, Priacanthidae, and Apogonidae 

 seem to be nocturnal, as are many species among 

 the Serranidae, Carangidae, Lutjanidae, 

 Sparidae, and Mullidae. 



Probably the nocturnal habits of these more 

 generalized percoids were inherited from ances- 

 tral beryciforms. The Holocentridae are the major 

 representatives of this once diverse order on near- 

 shore reefs today, yet as illustrated by their prom- 

 inence in Kona, they nonetheless are numerous, 

 widespread, and obviously successful. All of them 

 for which there are data are nocturnal, and there 

 is no reason to believe that this is not a primitive 

 characteristic. The anamalopids, which are the 

 only other beryciforms on nearshore reefs, also are 

 nocturnal (e.g. Harvey, 1922). Presumably these 

 modern beryciforms have competed successfully 

 with nocturnal forms among the more advanced 

 teleosts by having refined certain features that 

 are highly adaptive to feeding in the dark. Thus, 

 although much of their anatomy is essentially 

 that of their ancestors, they have acquired highly 

 specialized features — at least many of them 

 sensory — that have permitted more effective use 

 of this equipment. All other present-day be- 

 ryciforms live in the twilight zone of middepths or 

 in the deep sea, and their suitability to the di- 

 minished light of this habitat suggests that their 

 shallowwater ancestors perhaps were nocturnal 

 (Richard H. Rosenblatt, Scripps Institution of 

 Oceanography, pers. commun.). Perhaps during 

 the Cretaceous certain more specialized be- 

 ryciforms possessed diurnal habits, much as many 

 specialized perciforms do today. But if so, these 



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