HOBSON: FEEDING RELATIONSHIPS OF FISHES 



were the major item, occurring in five individuals 

 (mean percent of diet volume: 50; ranking index: 

 31.25). Most of these xanthids were in the 

 megalops stage, except that their abdomens were 

 reflected under their carapaces. The only other 

 identifiable prey, occurring in three specimens, 

 were gammaridean amphipods (mean percent of 

 diet volume: 20; ranking index: 7.5). Four con- 

 tained unidentified crustacean fragments (mean 

 percent of diet volume: 30; ranking index: 15). 



CONCLUSION. — Apogon erythrinus is a noc- 

 turnal predator that takes mostly benthic crusta- 

 ceans. 



Apogon menesemus Jenkins — \ipapalu 



This species and the very similar A. snyderi 

 (below) are the largest and most abundant 

 apogonids in Hawaii (Gosline and Brock, 1960), 

 and they were the apogonids seen most often dur- 

 ing the present study. During the day, A. 

 menesemus hovers quietly in the deep shadows of 

 reef crevices, but during late evening twilight 

 emerges into the open. Throughout the night sol- 

 itary individuals hover about 1 m above the coral. 

 On several occasions after dark this cardinalfish 

 struck at the silver barb on my otherwise dark 

 spear: sometimes when this happened the spear 

 was faintly illuminated by my partner's diving 

 light, but other times moonlight provided the only 

 illumination. At first morning light A. 

 menesemus moves close to cover on the reef, and 

 during morning twilight returns to its daytime 

 shelter. When under cover during the day its col- 

 oration is relatively featureless, but when in the 

 open at night distinctive fin markings appear 

 (Figure 20a and b). 



Fifty-nine specimens (114: 90-134 mm) were 

 collected during day and night. Of the 14 that were 

 speared from reef caves during late afternoon, 

 only 2 had food in their stomachs — one contained 

 an extensively digested piece of meat that proba- 

 bly was the remains of prey captured the previous 

 night, whereas the other contained a relatively 

 fresh xanthid crab that appeared to have been 

 captured earlier that day. In comparison, 25 of 40 

 specimens collected at night, between 3 h after 

 sunset and first morning light, had food in their 

 stomachs — much of it fresh. Finally, of five speci- 

 mens collected from caves during early morning, 

 within 3 h after sunrise, four had food in their 

 stomachs. Items in the 31 individuals containing 

 identifiable material are listed in Table 20. 



Two individuals that each contained just a 

 single xanthid crab are the only ones that indi- 

 cated exclusively benthic feeding; significantly, 

 one of these was the lone individual, noted above, 

 that appeared to have fed while under cover dur- 

 ing the day. The other, collected in the open just 3 

 h after sunset, may also have taken its prey before 

 leaving shelter in the evening. A. menesemus 

 takes mostly free-swimming prey, presumably at 

 its regular nocturnal station above the reef. 

 Nevertheless, judging from the sand mixed with 

 food in one individual collected at midnight, some 

 prey are taken from the sea floor after dark. 



CONCLUSION. — Apogon menesemus is a noc- 

 turnal predator that feeds mostly on free- 

 swimming crustaceans. 



Apogon snyrferi Jordan and Evermann — upapalu 



This cardinalfish cooccurs with the very similar 

 A. menesemus, above, but the two species remain 

 at least partially segregated. During the day both 

 species occupy the same caves, but A. snyderi is 

 not so deep in the shadows and, in fact, frequently 

 hovers at the entrances. Like A. menesemus, A. 

 snyderi emerges into the open during evening 

 twilight, but during the night stays closer to the 

 sea floor; furthermore, whereas A. menesemus 

 mostly remains over coral, A. snyderi tends to 

 move over the sand patches within the reef and in 

 the fringes of the more extensive sand areas adja- 

 cent to the reef. On several occasions at night, A. 

 snyderi struck at the silver barb on my spear, just 

 as described above for A. menesemus. Apogon 

 snyderi does not have prominent nocturnal color 

 features, as does A. menesemus. When over sand 

 at night its body has a highly reflective bluish 

 cast, also shown to a lesser extent by A. 

 menesemus, but which is largely lost by both 

 species soon after they move over coral or rocks. 



Thirty specimens (96: 82-130 mm) were 

 speared during day and night. All 3 that were 

 taken from caves during the afternoon had empty 

 stomachs, whereas of 24 collected in the open at 

 night, between 2 h after sunset and first morning 

 light, 22 had food in their stomachs. The remain- 

 ing three were collected from caves during the 4 h 

 after sunrise, and while two of these had food in 

 their stomachs, the third was empty. Items in the 

 24 individuals containing identifiable material 

 are listed in Table 21. 



951 



