HOBSON and CHESS: TROPHIC RELATIONSHIPS AMONG FISHES 



crease again until just before dawn. In contrast, 

 large numbers of M. praliniis and M. violaceus 

 remained concentrated in the waters above the 

 nearshore patch reefs throughout the night. 



Also prominent in daylight were Apogon spp., 

 which concentrated close to reef cover. These in- 

 cluded adults of A. gracilis, which schooled quietly 

 at the bases of the same coral heads above which 

 juveniles of the species (see above) actively fed; 

 nevertheless, the true numbers of apogonids were 

 fully appreciated only after nightfall, when many 

 large species unseen during the day emerged from 

 reef shelters. The most prominent of the larger 

 apogonids entering the water column was A. 

 savayensis, although some of the smaller species, 

 notably A. ^ract/ts and A. nouaeguinae, were more 

 numerous. Larger apogonids were solitary at 

 night, but smaller ones often were loosely aggre- 

 gated, including A. gracilis, of which the adults 



Table i. 



-Composition of plankton at Walt Island, Enewetak 

 Atoll, site of weak currents. 



joined the juveniles in the water column after 

 dark. 



Samples From Walt Island 



PLANKTON. — Major materials (zooplankters, 

 algae fragements, and crustacean molts) taken in 

 the plankton net during day and night at the Walt 

 Island site of weak currents are listed in Table 1. 

 Zooplankters, grouped by major taxonomic 

 categories and with data pooled from the three 

 sampled depths (surface, middepth, and near bot- 

 tom), are listed in Table 2. Additional data for 

 calanoid copepods are presented in Table 3 to sup- 

 port certain points developed in the Discussion. 



Table 3. — Size distribution of calanoid copepods, day and night, 

 at Walt Island, Enewetak Atoll, site of weak currents. 



' Including Euchaeta manna. Pleurommama xiphias. and Undinula vulgaris. 



^Including Candacia sp.. E. marina. Neocalanus sp., Pleurommama xiphias. 

 and U. vulgaris. 



^Including Acartia sp.. Metndia sp., Pleurommama sp., and Scolothricella 

 sp. 



"Including Acartia sp., Candacia sp., E. marina, and U. vulgaris. 



^including Acartia sp , Candacia sp., and Euchaeta sp 



Table 2.- 



-Occurrence, number, and size of zooplankters collected day and night at Walt Island, 

 Enewetak Atoll, site of weak currents. 



Day (n = 9) 



Night (n = 9) 



'Most of them planktonic stage of Tretomphalus. 



^All appeared to be Mysinae sp. Mysids constituted 52.8% of the volume of daytime collections. 

 ^Included Mysinae sp. and Sinella sp., the latter unseen in daylight. Mysids constituted 44.8% of the volume of nighttime 

 collections. 



139 



