FISHERY BULLETIN: VOL. 72, NO. 4 



plankton over reefs in Florida also differs between 

 day and night, a fact undoubtedly related to the 

 diurnal-nocturnal dichotomy among the plank- 

 tivorous fishes. As described above, planktivorous 

 fishes that feed in the water column at night, for 

 example Myripristis and Apogon, have the 

 generalized carnivore's large mouth and prey 

 largely on the relatively large plankters, like crab 

 megalops, that are mostly in the water column 

 above the reef after dark. 



Although a large array of plankters inhabit the 

 water column during the day, generally they seem 

 too small for adults of the large-mouthed noctur- 

 nal planktivores. Significantly, diurnal planktiv- 

 ores all have a small mouth, and their major 

 prey, calanoid copepods, are generally smaller 

 than the prey of their nocturnal counterparts. 

 Moreover, diurnal planktivores among adult reef 

 fishes generally are among the more advanced 

 teleosts, having attained, or passed, the percoid 

 level of development. There are no basal percoids 

 among the prominent diurnal planktivores in 

 Kona, but in the tropical Atlantic certain ser- 

 ranids, lutjanids, and pomadasyids specialized in 

 this habit are numerous (Starck and Davis, 1966; 

 Randall, 1967). Most diurnal planktivores on 

 coral reefs, however, are among the higher Per- 

 ciformes. These include the pomacentrids, which 

 probably include a higher proportion of plank- 

 tivorous species than any other major family of 

 coral-reef fishes. The balistids, order Tetraodon- 

 tiformes, are among the most advanced teleosts 

 and include several specialized diurnal planktiv- 

 ores: species of Melichthys and Xanthichthys 

 ringens are prominent on coral reefs over much of 

 the tropical world. 



Many unrelated species that forage on zoo- 

 plankton in the water column during the day dis- 

 play convergent morphologies. Features charac- 

 teristic of these fishes were identified by Davis and 

 Birdsong (1973), who did not distinguish between 

 diurnal and nocturnal forms, however. Drawdng 

 examples from the tropical Atlantic Ocean, they 

 illustrated certain unrelated planktivorous fishes, 

 for example Paranthias furcifer (a serranid) and 

 Chromis cyanea (a pomacentrid), that, on casual 

 inspection, look more like one another than they 

 do members of their own families that feed on the 

 benthos. The similarity among these unrelated 

 forms is based mainly on their common increased 

 tendency toward a fusiform body, a deeply incised 

 (forked or lunate) caudal fin, and a small, up- 



turned mouth that gives their heads a characteris- 

 tic appearance. Presumably diurnal planktivores 

 that tend toward a more fusiform body and deeply 

 incised caudal fin — both well-known characteris- 

 tics of rapid-swimming oceanic fishes — can swim 

 faster than relatives in which these tendencies are 

 less developed. Considering the many active pred- 

 ators at large during the day, increased speed 

 clearly is adaptive for small reef fishes that swim 

 at that time in open water, high above the shelter- 

 ing reef. The advantage of the upturned mouth 

 may be indirect: Rosenblatt (1967) acknowledged 

 Walter A. Starck II for pointing out that this 

 mouth construction gives the fish a shortened 

 snout, which permits close-range binocular 

 vision — an obvious advantage in capturing tiny 

 organisms in the water column. A number of diur- 

 nal planktivorous fishes in Kona possess one or 

 more of these characteristics, as described and 

 illustrated above (e.g. Figures 38 and 40). 



Significantly, none of the nocturnal planktiv- 

 ores in Kona tend toward having either a more 

 fusiform body, or a more deeply incised caudal fin. 

 In fact, planktivorous squirrelfishes of the genus 

 Myripristis are actually deeper bodied than their 

 bottom-feeding relatives of the genus 

 Holocentrus, and the caudal fins of most are less 

 deeply incised (compare, for example, Figures 11a 

 and 14). If, as suggested above, these features gain 

 selective advantage in the planktivores by provid- 

 ing added speed to elude predators in open water, 

 then their absence among forms that rise into the 

 water column after dark is consistent with the 

 contention (above, and Hobson, 1973) that small 

 free-swimming fishes face a much diminished 

 threat from predators at night. Many of the noc- 

 turnal species, including species of Myripristis, 

 have the sharply upturned mouth; but it is a large 

 structure, as noted above, suited to taking the 

 larger zooplankters that appear in the water col- 

 umn after dark. 



Not all of the diurnal planktivores in Kona tend 

 toward fusiform bodies, deeply incised caudal fins, 

 or sharply upturned mouths. None of these fea- 

 tures occur in the planktivorous chaetodontids, for 

 example Hemitaurichthys zoster (Figure 28a), 

 which nevertheless are well suited to feed on 

 copepods, and other tiny zooplankters in the water 

 column by day. Obviously many conflicting pres- 

 sures have differentially affected the mor- 

 phologies of the various fishes that forage on tiny 

 organisms in the mid-waters. 



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