HOBSON: FEEDING RELATIONSHIPS OF FISHES 



waters of the surge zone are frequented by 

 four species of Acanthurus (achilles, glau- 

 copareius, guttatus, and leucopareius). 2) The 

 sand patches on deeper, more tranquil reefs are 

 home to four species of Acanthurus {dussumieri, 

 mata, olivaceus, and xanthopterus). 3) Finally, 

 basalt and coral substrata on reefs below the surge 

 zone (to a depth of about 90 m) are inhabited by 

 three species of Acanthurus (nigrofuscus, ni- 

 groris, and sanduicensis); two species of 

 Ctenochaetus {hawaiiensis and strigosus); two 

 species of Zebrasoma iflauescens and veliferum; 

 the adults of the latter often occur in the surge 

 zone); and three species of Naso {brevirostris, 

 lituratus, and unicornis). 



In erecting categories according to foraging 

 types, Jones (1968) classified the bottom feeders 

 either as browsers or grazers. The browsers are 

 described as "strictly herbivores that bite and tear 

 off bits of multicellular benthic algae, generally 

 without ingesting any of the inorganic sub- 

 stratum." Browsing surgeonfishes include those 

 characteristic of the surge zone and those char- 

 acteristic of subsurge reefs, except for the 

 two Ctenochaetus. The browsing species of 

 Acanthurus and Zebrasoma feed chiefly on fine 

 filamentous algae, whereas the browsing species 

 of Naso tend to feed on the leafy and fleshy forms. 



Surgeonfishes classified by Jones (1968) as 

 grazers are described as "Fishes that purposely 

 pick up large quantities of the substratum while 

 feeding. . . irrespective of whether the material is 

 rasped away from rocks, or picked up as loose 

 sand." This category includes the surgeonfishes 

 characteristic of the sand patches, all of which are 

 species of Acanthurus, and the two reef-dwelling 

 species of Ctenochaetus. The sand-patch Acan- 

 thurus species pick up mouthfuls of sand, 

 whereas the reef-dwelling Ctenochaetus species 

 ingest sediment that has accumulated over rocks 

 and dead coral. In examining these sediment- 

 packed guts, Jones found material from the two 

 groups distinguishable by particle size — being 

 coarse and grainy in the sand-patch Acanthurus, 

 fine and silty in the reef Ctenochaetus. He con- 

 cluded that the major food of both groups are 

 diatoms and detritus that have accumulated 

 around the particles in the surface layers of the 

 sediment. 



Surgeonfishes are widely recognized to be active 

 by day and relatively inactive at night (e.g. in the 

 Gulf of CaUfornia by Hobson, 1965; and in the 



Florida Keys by Starck and Davis, 1966). Al- 

 though quiescent, these nocturnally resting acan- 

 thurids are most often described as alert; how- 

 ever, Collette and Talbot (1972) reported that 

 Acanthurus coeruleus sleeps while sheltered 

 among coral at night in the Virgin Islands. In 

 the Gulf of California, Prionurus punctatus 

 aggregates above the reef on bright moonlit nights 

 (Hobson, 1965), as does Naso hexacanthus in 

 Kona. 



Family Zanclidae: moorish idol 



Zanclus canescens (Linnaeus 

 moorish idol, kihikihi 



The moorish idol (Figure 39a) is closely related 

 to the surgeonfishes, and some ichthyologists (e.g. 

 Greenwood et al. , 1966) consider it to be a member 

 of that family. It lacks the caudal spine common to 

 all surgeonfishes, however, and most classi- 

 fications assign it to the monotypic family 

 Zanclidae. 



This fish is numerous in all Kona inshore 

 habitats, where it swims over the reef during the 

 day, usually in groups of four to six individuals. 

 When feeding, it regularly probes the narrow 

 cracks and crevices of the reef with its elongated 

 snout. At night it is relatively inactive, but alert, 

 close among rocks or coral, and at this time its 

 coloration differs strikingly from that displayed in 

 daylight (compare Figure 39a and b). 



Of 21 specimens (108: 74-137 mm) speared at 

 various times of day and night, all 9 that were 

 collected at night (later than 4 h after sunset and 

 before sunrise) had empty stomachs, whereas all 

 12 that were taken during the day (between mid- 

 morning and late afternoon) had full stomachs 

 that included fresh material. Items in the speci- 

 mens containing identifiable material are listed in 

 Table 62. 



The sponges, which greatly predominate in the 

 diet, were all small species that presumably live in 

 narrow reef crevices. This fish appears to be 

 specialized in this diet, although Randall (1955) 

 reported only algae in two specimens from the 

 Gilbert Islands. 



CONCLUSION.— ZancZus canescens is a diur- 

 nal species that feeds mostly on small sponges. 



1003 



