HOBSON; FEEDING RELATIONSHIPS OF FISHES 



Table 42. — Food of Abudefduf imparipennis. 



reef, where it picks organisms from the plankton. 

 Although members of an aggregation are close to 

 one another, each feeds independently. The plank- 

 ters are taken with what seems to be a visually 

 directed action in which the fish suddenly thrusts 

 both jaws forward, then immediately retracts 

 them. Presumably the sudden expansion of the 

 oral cavity sucks the prey in. 



A given aggregation maintains station over a 

 particular location although its position in the 

 water column is influenced by several factors. Fish 

 size is important, because the larger individuals 

 tend to be farther above the reef than the smaller 

 ones. Prevailing light is another factor; thus, 

 when clouds pass in front of the sun, and light 

 diminishes, individuals of all sizes descend closer 

 to the reef. In addition, the appearance of a large 

 predator, or some other disturbance, intermit- 

 tently sends this fish diving to cover on the reef. 



However, after such an event it quickly returns to 

 its feeding stations in the water column. 



As light progressively fades late in the day, this 

 species gradually descends to the reef so that by 

 evening twilight it is sheltered among the coral 

 (Hobson, 1972). On dark nights it remains under 

 cover, relatively inactive but alert; however, 

 under bright moonlight it swims in exposed posi- 

 tions close to the reef. Then, during morning 

 twilight, it begins to ascend to its daytime feeding 

 stations in the water column (Hobson, 1972). 



Of 14 specimens ( 142: 105- 162 mm) examined, 

 the 4 that were speared as they hovered among the 

 rocks on dark nights (between 4 and 6 h after 

 sunset) contained only well-digested fragments, 

 whereas all 10 that were speared from mid-water 

 aggregations during afternoons had their stomach 

 full of food (including much fresh material), as 

 listed in Table 43. 



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