HOBSON and CHESS: TROPHIC RELATIONSHIPS AMONG FISHES 



Table 5. — Food habits of nocturnal planktivorous fishes from Walt Island, Enewetak Atoll, site of 

 weak currents. See Table 4 legend for explanation of listed values. 



'Numbers-of plankters (from Table 2) provided only for rough measure of relative abundance. 



^Most polychaetes in guts of fishes were nereid epitokes. 



■'Predominant calanoids in the three larger fish species were Pleurommama xiphias and Euchaeta marina, which were 

 relatively large (3 to 5 mm). 



^Tanaids and insects were not present in plankton collections but were in several fish guts. Both are known from plankton 

 collections elsewhere (e.g.. Hobson and Chess 1973, 1976). 



vaiuli was numerous, but perhaps no more so than 

 where currents were weak (see above), and here 

 too it confined itself to the immediate proximity of 

 the reef 



The nature of the substrate can be important. 

 Chromis caerulea and Dascyllus reticulatus, for 

 example, swam in tight well-defined aggregations 

 above specific growths of branching coral — 

 particularly large heads oi Pocillopora spp. (Fig- 

 ure 5A). Pomacentrus coelestus generally sta- 

 tioned itself low in the water column above out- 

 croppings of coral rock and rubble, its relation to 

 the substrate much like that of the similarly hued, 

 but deeper-bodied, P. pavo. Chromis agilis, C. 

 lepidolepis, and C. margaritifer generally swam in 

 small widespread groups over patch reefs. Com- 

 pared with their congener C caerulea, they 

 showed less affinity to specific substrata or loca- 

 tions on the reef. Thus C. caerulea invariably re- 



sponded to a human intruder by sheltering among 

 the branches of a large coral head directly below 

 its feeding station (Figure 5B), whereas C. agilis, 

 C. lepidolepis, and C margaritifer frequently re- 

 sponded to the same stimulus by moving away, 

 and taking shelter in a variety of places only when 

 the stimulus was intensified. 



In places where many of these diurnal plankti- 

 vores were concentrated, a relation was evident 

 between their morphologies and the distances 

 they swam from the reef: those with feeding sta- 

 tions farther from the reef tended more toward 

 cylindrical bodies and deeply incised caudal fins 

 (Figure 6). This generalization proved valid de- 

 spite exceptions among such deep-bodied forms as 

 Dascyllus reticulatus (Figure 7) and Amblygly- 

 phidodon curacao, in which the effect of their 

 deeper bodies is even further enhanced by longer 

 fin spines. Thus, for example, 7 D. reticulatus, 47 



141 



