HOBSON: FEEDING RELATIONSHIPS OF FISHES 



Thus, a major activity pattern of these large 

 piscivores closely interrelates with a major activ- 

 ity pattern of the smaller nocturnal predators. For 

 this reason, and because so many members of the 

 two groups are closely related taxonomically, it is 

 apparent that the crepuscular pattern probably 

 has had a longevity comparable to that of the 

 nocturnal pattern. A good indication of this long 

 history exists in the Gulf of California, where the 

 day-night activity pattern of the nocturnal clupeid 

 Harengula thrissina closely interrelates with the 

 crepuscular activity not only of certain basal per- 

 coids, but also o{ Elops affinis, order Elopiformes, 

 a member of the most primitive of all extant tele- 

 ostean genera (Hobson, 1968a). 



Generalized Carnivores as Diurnal Predators 



Thus, nocturnal or crepuscular habits are adap- 

 tive for many generalized carnivores. Others with 

 basically the same feeding mechanism, however, 

 have acquired morphological and behavioral 

 characteristics suited to capture small, motile 

 crustaceans and, especially, fishes in daylight. 

 Despite the fact that crustaceans are most exposed 

 to direct attacks at night, and smaller fishes gen- 

 erally are most vulnerable to such attacks during 

 twilight, various predators are equipped to exploit 

 the exceptions to these generalizations. 



True, selective pressures applied by generations 

 of visually orienting predators have refined the 

 defense mechanisms that protect so many prey 

 organisms during daylight. But there are occa- 

 sional lapses in all these defenses when the prey 

 are briefly vulnerable. For example, nocturnal or- 

 ganisms resting under a thin layer of sand occa- 

 sionally betray their presence by moving. And 

 small fishes that usually are within retreating 

 distance from cover sometimes stray too far into 

 the open; or others, enjoying the security of a 

 school, occasionally drift too far from their fellows. 

 Still others, normally alert to surrounding 

 danger, are momentarily distracted. At such 

 times, these organisms are open to attack. But 

 normally such events fail to occur in the presence 

 of large, free-swimming predators that are ac- 

 tively hunting. Potential prey are sensitive to cues 

 that mark the hunting predator, and take defen- 

 sive action when a hunter appears — cryptic forms 

 stop moving, others move closer to cover, and 

 schooling forms draw themselves closer together 

 (Hobson, 1965, 1968a). Above all, in this alerted 

 state the prey are less likely to make a defensive 



mistake. This does happen occasionally, of course, 

 as when large carangids swim slowly among 

 schooling prey for hours during the day without an 

 aggressive move, and then suddenly attack 

 — presumably having sensed a vulnerable target 

 (Hobson, 1968a). Probably this offensive tactic de- 

 pends on the prey eventually becoming con- 

 ditioned to the predator's presence, and finally 

 making a mistake. But it seems unlikely that such 

 predators could depend on these relatively infre- 

 quent successes. They remain best suited for cre- 

 puscular attacks. 



The problem of being within striking range 

 when prey are momentarily available during the 

 day because of a defensive lapse is probably best 

 solved by those predators that lie in wait under 

 concealment — the ambushers — or by those that 

 stalk. Both tactics have produced some highly 

 specialized forms that are more appropriately con- 

 sidered in the next section. However, many of 

 those that use concealment to ambush their prey 

 look much like the nocturnal or crepuscular pred- 

 ators discussed above, and so are considered 

 here. 



This is especially true among certain basal per- 

 coids, like the serranids. For example, many 

 species of Epinephelus ambush prey from a con- 

 cealed position, and much of this activity occurs in 

 daylight (Hiatt and Strasburg, 1960; and others). 

 Most of these predators are cryptically hued for a 

 sedentary existence among rocks or coral — 

 usually they are brown or grey, with the hues 

 often arranged in blotches or spots. Such predators 

 rest unseen until a small organism within strik- 

 ing distance makes a defensive error. 



Generalized predators adapted for this tactic 

 are well known to feed regularly during both day 

 and night, as exemplified by certain species of 

 Epinephelus (Longley and Hildebrand, 1941; 

 Starck and Davis, 1966; Hobson, 1968a). There is 

 evidence, however, that feeding habits of these 

 predators differ between day and night. In the 

 Gulf of California, E. labriformis preys almost 

 entirely on crustaceans at night, but heavily on 

 fishes during the day (Hobson, 1968a). I have al- 

 ready commented on the increased vulnerability 

 of small crustaceans at night; apparently fishes 

 are more vulnerable to the predatory tactics of this 

 fish in daylight. The diurnal piscivorous habit of 

 Cephalopholis argus {Epinephelus argus of some 

 authors, e.g. Smith, 1971) in Kona is consistent 

 with this probability. 



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