FISHERY BULLETIN: VOL. 72, NO. 4 



taken the previous day, but in the third individual 

 a wide variety of differentially digested items in- 

 dicated either nocturnal feeding or unusually slow 

 digestion. A fourth individual taken during morn- 

 ing twilight was empty. Items in the eight speci- 

 mens containing identifiable material, much of it 

 fragments torn from larger sessile animals, are 

 listed in Table 37. 



CONCLUSION. — Chaetodon fremblii preys on 

 a wide variety of benthic organisms during the 

 day, obtaining much of its food by tearing off 

 pieces of larger sessile animals. With some un- 

 certainty, it seems largely inactive after dark. 



Chaetodon lunula (Lacepede) — 

 masked butterflyfish 



This butterflyfish, one of the more numerous in 

 Kona, is most abundant where a coral-crested reef 

 face falls among basalt boulders, yet occurs in a 

 variety of habitats. Setting it apart from all other 

 chaetodontids reported here, I never saw this 

 species feed during the day. It generally hovers 

 close to the reef in daylight, sometimes solitary, or 

 in twos or threes, and often in large aggregations 

 (Figure 30). These aggregations form day after 

 day in the same locations, and several occurred in 

 the same places over the entire 15-mo period of the 

 study. The aggregations disperse at nightfall, and 

 after dark the species scatters over the reef, either 

 solitarily, or in twos or threes. 



Of the 26 specimens (134: 112-150 mm) ex- 

 amined, all 14 speared at night (more than 4 h 

 after sunset), or during morning twilight, had 

 stomachs full of food in varying stages of diges- 

 tion, much of it fresh; the other 12 were collected 



during afternoons (some from the daytime ag- 

 gregations), and although they too had full 

 stomachs, the contents generally were further di- 

 gested. There was no recognizable difference in 

 the composition of the diet between specimens col- 

 lected during each of these three periods. Items in 

 the stomachs are listed in Table 38. 



Clearly, C. lunula, like C. auriga and C. fremb- 

 lii, habitually tears pieces off the bodies of larger 

 sessile animals, but, more so than the others, also 

 takes whole organisms. In fact, its major prey, 

 based on these data, are opisthobranch gas- 

 tropods, which it takes whole. The opisthobranchs 

 are mostly one form of Anaspidea and one form of 

 Cephalaspidea. Significantly, all individuals of C 

 lunula that contained what seemed to be freshly 

 ingested opisthobranchs were speared at night. 

 Opisthobranchs in C. lunula speared during the 

 afternoon were consistently far digested. These 

 opisthobranchs are mostly about 4 to 10 mm long, 

 and are relatively solid pieces of meat that may 

 take longer to digest than many other kinds of 

 food. Similarly, the polychaete heads and proso- 

 branch gastropod heads taken by this fish are rela- 

 tively dense pieces of meat that probably resist 

 digestion (the shells of the prosobranch gastropods 

 were never present — only the heads, which this 

 butterflyfish apparently is adept at snipping off). 

 Smaller organisms that would be rapidly digested 

 like the amphipods and isopods, generally, but 

 with two exceptions, were absent in specimens 

 speared during the afternoon. Generally then, the 

 stomach contents appeared to have been taken 

 mostly at night. Finally, it may be significant that 

 the eyes of C lunula are relatively larger than the 

 eyes of all other species of this genus studied at 

 Kona. 



Table 37. — Food of Chaetodon fremblii. 



976 



