NOTES 



ZOOPLANKTERS THAT EMERGE FROM 



THE LAGOON FLOOR AT NIGHT AT 

 KUR£ AND MIDWAY ATOLLS, HAWAII 



Many zooplankters in nearshore marine habitats 

 are in the water column at night, but spend the 

 daytime sheltered on or near the sea floor (Emery 

 1968; Glynn 1973; Porter 1974). The diel move- 

 ments these organisms make between the water 

 column and the sea floor are major features of 

 nearshore ecosystems, and strongly influence 

 many of the fishes in these habitats (Hobson 1968, 

 1973, 1974, 1975; Hobson and Chess 1976, 1978). 

 Some of these zooplankters are holoplanktonic 

 forms that swarm close to bottom structures by 

 day and disperse above the reef at night. Included 

 are various calanoid copepods (e.g., Acartia spp.), 

 cyclopoid copepods (e.g., Oithona spp.), mysids 

 (e.g., Mysidium spp.), and larval fishes (Emery 

 1968; Hobson and Chess 1978). Although such 

 forms often occur in caves and other reef openings 

 large enough to accommodate their free- 

 swimming habit, they should be distinguished 

 from the many meroplanktonic forms that by day 

 live in or on the substrate (although this distinc- 

 tion between meroplankton and holoplankton is 

 not always clear-cut).' At least some of these neri- 

 tic holoplankters seem just loosely associated with 

 specific substrata. For example, by day the 

 calanoid A. tonsa swarmed close to coral reefs in 

 the tropical Atlantic (Emery 1968) and to kelp 

 forests in the warm temperate eastern Pacific 

 (Hobson and Chess 1976), and also occurred in 

 open waters offshore (Fleminger 1964). The 

 meroplanktonic forms which by day characteristi- 

 cally assume what is essentially a benthonic mode 

 have a much stronger affinity to specific nearshore 

 substrata, and these are the major topic of this 

 paper. Included are various polychaetes, os- 

 tracods, copepods, mysids, cumaceans, tanaids, 

 isopods, gammarid amphipods, and various larval 

 forms (Hobson and Chess 1976, 1978, in prep.). 



Two recent studies, one on the Barrier Reef 

 (AUdredge and King 1977) and the other in the 



'We define meroplankton as those zooplankters that are in or 

 on the substrate during part of the diel cycle, and holoplankton 

 as those that are in the water column at all hours. As pointed out 

 earlier (Hobson and Chess 19761, these terms have carried dif- 

 ferent meanings for different authors, 



FISHERY BULLETIN VOL 77, NO 1. 1979 



Philippine Islands (Porter et al 1977; Porter and 

 Porter 1977), have attempted to quantify the 

 emergence of zooplankters from various coral-reef 

 substrata. These are important papers because 

 they draw attention to what unquestionably is a 

 highly significant and long-neglected aspect of 

 nearshore ecosystems. We suspect, however, that 

 there are problems with these studies. If so, the 

 problems should be promptly recognized because 

 undoubtedly they will spawn similar investiga- 

 tions by other workers elsewhere (e.g., see Randall 

 et al. 1978). AUdredge and King collected their 

 samples in Plexiglas^ traps that rested on the bot- 

 tom and retained organisms that rose into the 

 water column; however, zooplankters from the 

 surrounding water had access to these traps 

 through gaps between the traps' rigid lower edges 

 and irregularities on the sea floor. Earlier (Hobson 

 and Chess 1978), we stated that these collections 

 need to be repeated with this possibility of error 

 eliminated. Obviously, if many zooplankters en- 

 tered the traps from the surrounding water col- 

 umn, the samples cannot be considered measures 

 of the organisms that emerged from the underly- 

 ing substrata. The Porter group used traps that 

 were tethered above the sea floor, and so would 

 seem to have offered even greater access to zoo- 

 plankters from the surrounding water. In fact, the 

 probability that such forms entered the traps 

 seems to us so great that we would have expected 

 that their intent was simply to sample zooplank- 

 ters near the reef And yet, in prefacing their 

 findings with statements like (p. 107)". . .volumes 

 of plankton produced per m^ per hour by different 

 reef substrates during the day and during the 

 night are given in Table 1." they clearly implied 

 that each trap sampled only those organisms that 

 had risen from the substrate directly below it. 



Our doubts about these studies, however, were 

 moderated by limitations in our own knowledge of 

 the phenomenon. We had worked extensively with 

 these activity patterns as they relate to fishes 

 (Hobson 1968, 1974; Hobson and Chess 1976, 

 1978) and had made inferences about the daytime 

 modes of nocturnal zooplankters in nearshore 

 habitats. Still, we had not satisfactorily distin- 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



275 



