FISHERY BULLETIN: VOL. 79, NO. 1 



520 - 



510 



\ 



max 



500 



490 - 



2 3 4 5 



RANGE IN METERS 



Figure 15. — Calculated optimum K^^^ position for absorption 

 of luminescent emissions of Noctiluca miliaris at increasing 

 range in waters of differing clarity. Water types I, 1, 5, and 7 of 

 Jerlov (1968). Solid lines = coastal waters of increasing turbid- 

 ity; dashed line = clear tropical water. 



invisible beyond 2-3 m. Obviously under such con- 

 ditions bioluminescence, too, would be invisible at 

 these distances. Probably most meaningful in- 

 teractions involving fishes and the bioluminescent 

 emissions of plankton occur over distances of just a 

 fev^^ meters or less, and to see bioluminescence at 

 these ranges in the waters of varying visibility 

 that surround Santa Catalina fishes probably 

 would best have visual pigments with k^^^ values 

 at a little above 500 nm (Figure 15). 



One might expect diurnal fishes to be less sensi- 

 tive than nocturnal or crepuscular fishes to selec- 

 tion pressures on scotopic pigments because they 

 are less active under low light. But among species 

 studied at Santa Catalina the X^^ values scarcely 

 differ between the two groups (diurnal feeders: x = 

 499.1 nm, range = 496.1-505.8— Table 1; noctur- 

 nal feeders: x = 501 nm, range = 496.1-505.1 — 

 Table 2). Perhaps there is little difference here 



because many of these diurnal feeders are exposed 

 at night and need means to detect threatening 

 predators. In fact, if the fishes are grouped accord- 

 ing to their relative exposure after dark, rather 

 than by how active they are at this time, their X^ax 

 values show a clear pattern (Table 5). Those fishes 

 that are exposed at night, compared with those 

 that are sheltered, tend to have scotopic pigments 

 with spectral sensitivities closer to what we con- 

 sider optimum for detecting bioluminescence in 

 Californian coastal waters. This would mean that, 

 despite their apparent quiescence, at least many 

 diurnal fishes remain visually alert for potential 

 threats during the night. 



So the similar X^ax values in the diurnally feed- 

 ing A^/iermops afflnis (505.8 nm) and the noctur- 

 nally feeding Hyperprosopon argenteum (505.1 

 nm) may answer the similar threats both face 

 while exposed in the water column at night. We 

 assume the increased sensitivity to biolumines- 

 cence would also benefit H. argenteum in feeding, 

 but suspect this advantage would be less forceful. 

 The need to evade predators should be sharper 

 than the need to capture prey. Both needs are criti- 

 cal, but there is less tolerance for error on defense 

 than on offense. A fish as prey is likely to be elimi- 

 nated the first time it errs in responding defen- 

 sively to an attack, but the same fish as an at- 

 tacker may err many times without serious con- 

 sequences. 



Bioluminescence, triggered by movement, rep- 

 resents a well-defined indicator of immediate 

 danger that can effectively focus selection pres- 

 sures on a narrowly defined adaptive response. 

 Moonlight and starlight would seem less suited for 

 this because neither so effectively identifies 

 specific threats and because the impact of both 



Table 5. — Relative exposure at night, andAj^j,^ position of scotopic visual pigments in certain southern Californian 



marine fishes.' 



Relative exposure at night 



Species 



'^max "^ean 



''max''a"9e 



I. Fully exposed in 

 water column 



Fully exposed on or 



near seafloor, often 



on open sand 

 Partially or fully 



sfieltered by rocks 



or algae 



Atherinops affinis 

 Sebastes serranoides 

 Xenistius californiensis 

 Seriphus politus 

 Umbrina roncador 

 Damalichthys vacca 

 Embiotoca jacksoni 

 Scorpaena guttata 

 Sebastes atrovirens^ 

 S. serriceps 

 Paralabrax clathratus 

 Girella nigricans 

 Ctiromis punctipinnis 



Brachyistius frenatus 502.1 



Cymatogaster aggregata 

 Hyperprosopon argenteum 



Leiocottus tiirundo 501.1 



Pleuronichtttys coenosus 



Hypsypops rubicunda 497.3 



Semicossyphus pulctier 

 Alloclinus holder! 

 Heterostichus rostratus 

 Coryphopterus nichoisi 



500.4-505.8 



500.0-503.1 



496.1-499.7 



'Of species studied, Medialuna californiensis is excluded because its nocturnal fiabits remain uncertain (see text), and Halichoeres semiclnctus 

 and Oxyjulis californica are excluded because tfiey have a different type of visual pigment (see Table 1, footnote 3). 

 ^Even though S. atrovirens feeds in the water column at night, these activities occur close to rising kelp stipes or just beneath the surface canopy 



26 



