HOBSON: FEEDING RELATIONSHIPS OF FISHES 



Myripristis arnaenus, like its congeners, emerges 

 from its daytime retreats about 30 min after sun- 

 set, and at least many individuals move offshore, 

 especially when there is no moonlight. Myripristis 

 arnaenus shows essentially the same day-night 

 distinction in coloration that is described above 

 for its congeners. 



Of 19 individuals (176: 116-210 mm) speared at 

 different times of day and night, all 14 collected 

 above the reef at night (later than 4 h after sun- 

 set), or from shelter sites within 2 h after the 

 species had returned to cover in the morning, con- 

 tained food, whereas all 5 collected from shelter 

 sites during late afternoon were empty. Items in 

 the 14 individuals containing identifiable prey are 

 listed in Table 18. 



CONCLUSION. — Myripristis arnaenus is a 

 nocturnal planktivore that takes mostly crab 

 megalops and other crustaceans. 



General Remarks on Squirrelfishes 



Squirrelfishes are known throughout tropical 

 seas to hunt prey after dark. For example, they 

 have been thus described in the Marshall Islands 

 (Hiatt and Strasburg, 1960), the Gulf of California 

 (Hobson, 1965, 1968a), and the West Indies (Ran- 

 dall, 1967). Similar behavior has been noted in the 

 Florida Keys by Starck and Davis (1966), who 

 noted there were no distinctive nocturnal color 

 features in tropical Atlantic holocentrids, such as 

 are reported for all but two of the Hawaiian 

 species above. 



The two major categories noted above in the 

 introduction to the squirrelfishes each represents 

 a generally different mode of predation. All feed 

 primarily on crustaceans, but whereas species of 



Holocentrus and Holotrachys lima capture their 

 prey close to the sea floor, species of Myripristis 

 hunt prey up in the water column. 



BOTTOM FEEDERS.— The seven holocentrids 

 in this category feed mostly on benthic forms, but 

 also take some prey that are free-swimming at the 

 base of the water column. Xanthid crabs com- 

 prised the major prey item for all species except 

 the largest, Holocentrus spinifera, which con- 

 tained a slightly larger volume of caridean 

 shrimps. Xanthid crabs are ubiquitous benthic 

 animals in all Kona inshore habitats, and are 

 widely active in exposed positions after dark. 



Of the seven bottom-feeding squirrelfishes, only 

 three similar species, Holocentrus diadema, H. 

 lacteoguttatum, and H. xantherythrus, hunt 

 significantly over sand in addition to feeding on 

 hard reef substrata; however, even these three do 

 not range away from cover during this activity, 

 which is limited to sand pockets on the reef and 

 only the fringes of more extensive sandy areas. 

 Nevertheless, it is probably because of this habit 

 that these three have more varied diets than do 

 the others. Hiatt and Strasburg (1960) reported 

 that some of the holocentrids in the Marshall Is- 

 lands forage on sandy bottoms, citing sand- 

 dwelling gastropods as being prominent prey of//. 

 diadema in that area. 



The other four bottom feeders, Holocentrus 

 sammara, H. spinifera, H. tiere, and Holotrachys 

 lima, restrict their activity largely to hard sub- 

 strata on the reef, and prey more heavily on carid- 

 ean shrimps — especially on snapping shrimps. 

 Some of the larger individuals of Holocentrus 

 sammara and H. spinifera capture the caridean 

 Saron marmoratus; although individuals of this 

 shrimp exceeding a length of 30 mm are numerous 



941 



