HOBSON: FEEDING RELATIONSHIPS OF FISHES 



Rank 



Table 48. — Food of Chromis ovalis. 



Items 



Also, unidentified fragments 



No fish 



with this 



item (n = 2) 



Mean percent 



of 

 diet volume 



Ranking 

 index 



for Abudefduf abdominalis and for other plank- 

 tivorous pomacentrids; its nocturnal behavior also 

 is like that described for these other species. 



Of six specimens ( 124: 121-138 mm) examined, 

 all four that were speared among corals shortly 

 before first morning light (two after dark nights, 

 two after moonlit nights) contained only a few 

 well-digested fragments, whereas two that were 

 speared from aggregations above the reef during 

 midafternoon were full of food (including fresh 

 items), as listed in Table 48. 



Swerdloff (1970b) reported the following 

 categories of prey in eight C ovalis from one col- 

 lection on the island of Oahu (ranked as percent 

 of the diet): copepods, 60.1%; tunica tes, 16.9%; 

 malacostracans, 9.5%; mollusks, 9.5%; poly- 

 chaetes, 2.3% ; fish eggs, 0.8%; and siphonophores, 

 0.8%. Gosline and Brock (1960) reported "a mass 

 of copepods" in the stomach of one individual of 

 this species. 



CONCLUSION. — Chromis ovalis is a diurnal 

 planktivore that takes primarily copepods. 



General Remarks on Damselfishes 



Pomacentrids are widely recognized as being 

 active by day and relatively inactive at night. For 

 example, they were so described in the Gulf of 

 California (Hobson, 1965, 1968a), and also in the 

 tropical Atlantic (Starck and Davis, 1966; Collette 

 and Talbot, 1972). Food-habit data from the vari- 

 ous members of this family in areas as widely 

 separated as the West Indies (Randall, 1967) and 

 the Marshall Islands (Hiatt and Strasburg, 1960) 

 show widely divergent habits: some are strictly 

 herbivorous, others are omnivorous, and still 

 others are strictly carnivorous. 



The habitat of each pomacentrid in Kona is 

 especially well defined. Two major categories 

 exist: those that forage on the bottom and those 

 that feed in the water column. 



BOTTOM FEEDERS.— Pomacentrids that for- 

 age on the sea floor have especially diverse diets. 

 Algae and organic detritus are the major foods of 

 many, especially among species of Pomacentrus 

 (Hiatt and Strasburg, 1960; Randall, 1967). In 

 Kona, P. jenkinsi is in this category, but P. pavo 

 in the Marshall Islands is primarily a predator on 

 small fishes and crustaceans (Hiatt and Stras- 

 burg, 1960). The diets of species of Abudefduf 

 appear even more diverse. Abudefduf sindonis in 

 Kona has food habits similar to those of P. jen- 

 kinsi, but the highly omnivorous A. sordidus 

 forages on a wide variety of benthic animals and 

 plants, whereas the predaceous A. imparipennis 

 takes mostly benthic crustaceans and poly- 

 chaetes. Abudefduf saxatilus in the West In- 

 dies is, according to Randall (1967), "one of the 

 most diversified of all fishes in its food habits," 

 feeding as it does on a wide assortment of plants 

 and animals from both sea floor and water column. 

 Similarly, A. troschelii in the Gulf of California 

 feeds on zooplankton and bits of algae from the 

 water column, as well as organisms from the sub- 

 stratum (Hobson, 1968a). 



WATER-COLUMN FEEDERS.— Planktivor- 

 ous pomacentrids are prominent on coral reefs 

 throughout tropical seas. Their characteristic 

 mid-water aggregations have been described in 

 the Indian Ocean (Eibl-Eibesfeldt, 1962), central 

 Pacific (Hiatt and Strasburg, 1960), Gulf of 

 California (Hobson, 1965, 1968a), and the tropical 

 Atlantic (Starck and Davis, 1966). In the Ba- 

 hamas, Stevenson (1972) showed that the height 

 in the water column at which Eupomacentrus 

 partitus feeds on plankton is determined largely 

 by light and current. The progressive ascent of 

 planktivorous pomacentrids into the water 

 column during morning twilight, as they rise to 

 their mid-water feeding grounds, and their 

 subsequent descent to the reef during evening 

 twilight, has been described in Kona (Hobson, 



985 



