FISHERY BULLETIN: VOL. 79, NO. 1 



1975b). The present study shows that fishes as- 

 sociated with Californian kelp forests have pig- 

 ments that are most sensitive to blue-green light 

 (\max from 496 to 506 nm). 



If the adaptive advantage of matching ambient 

 light lies in heightened photosensitivity, then the 

 match should be to light that prevails when selec- 

 tion for improved vision is most intense. We are not 

 surprised, therefore, that the k^^ values for fishes 

 in a given habitat match not the light of day or 

 night, but rather the bluer twilight, and thus 

 the fishes are equipped to meet an intensified 

 threat from crepuscular predators. In clear tropi- 

 cal waters, for example, the X^ax values of the 

 scotopic pigments in reef fishes cluster about 492 

 nm, which in that habitat matches twilight, 

 rather than the greener light of day or night 

 (Munz and McFarland 1977). Of course, the spec- 

 tral position of this match is influenced by the light 

 transmission characteristics of water in that par- 

 ticular habitat. For example, in most fresh waters 

 the match is made above 520 nm, but this position 

 nevertheless approximates the XPsq of twilight in 

 these very green waters and is in fact toward the 

 blue from light that prevails there during day and 

 night (McFarland and Munz 1975c). 



As could have been predicted from the Sensitiv- 

 ity Hypothesis, the scotopic pigments of fishes in 

 the blue-green coastal water of California cluster 

 at wavelengths intermediate between those of 

 fishes on coral reefs and those of fishes in freshwa- 

 ter, at about 500 nm (Tables 1-3). The match with 

 twilight, however, is less clear in Californian 

 waters than in the other two environments be- 

 cause, we believe, photic conditions in Californian 

 waters are more variable. Nevertheless, the tight 

 clustering of scotopic pigments around 500 nm in 

 the Californian fishes better matches ambient 

 light during twilight than at night. Both moon- 

 light and starlight are richer in red light than 

 daylight or twilight (Munz and McFarland 1977), 

 and for all water conditions we encountered at 

 Santa Catalina Island downwelling light at night 

 would have XP^^ values well above 520 nm (Figure 

 5). Only during twilight does ambient light un- 

 derwater shift far enough toward the blue-green 

 region of the spectrum (Figures 3, 4) to produce a 

 close match with the visual pigment X^ax- 



In evaluating the impact of crepuscular pred- 

 ators on the spectral position of scotopic pigments, 

 however, we must not forget that other selection 

 pressures are operating. We would expect scotopic 

 pigments in fishes to be particularly responsive to 



such alternate pressures at night, which is espe- 

 cially "short of light" (Dartnell 1975). Certainly 

 fishes must be sensitive to the emissions of biolum- 

 inescent organisms, because few visual cues could 

 be more apparent than a flash of light in the dark. 



Scotopic Spectral Sensitivity and 

 Bioluminescence 



Bioluminescence often signifies underwater 

 movement. According to Hobson (1966), "In many 

 areas of the sea at night, a moving object is readily 

 observed due to the luminescence of many minute 

 planktonic organisms, mostly protozoans, which 

 light up when disturbed. These organisms are 

 often so numerous that while making observa- 

 tions underwater I have been able to identify, to 

 species, fishes that swam actively among them. 

 This was not because the fish itself was illumi- 

 nated, but rather because there were so many 

 minute luminescent organisms about the fish that 

 its form was essentially traced out in tiny flecks of 

 light." If such cues are evident to human eyes, 

 adapted to a diurnal, terrestrial existence, the ca- 

 pacity to sense and to orient by them must be 

 highly refined in animals like fishes that have 

 evolved in this environment. 



Bioluminescence offers an especially effective 

 way to detect predators or prey because predator- 

 prey interactions generally involve movement, 

 and luminescence by plankton is greatly increased 

 in the turbulent water around moving objects. We 

 believe, as did Burkenroad (1943), that this fact 

 has had enormous impact on the nocturnal tactics 

 of both predators and prey. The motionless at- 

 titude that characterizes nocturnal planktivorous 

 fishes when they hunt probably is enforced by the 

 need to minimize turbulence in the water about 

 them. By minimizing turbulence they minimize 

 the firing of luminescent organisms that would 

 betray their presence, and so hover unseen in the 

 dark, ready to strike when nearby prey advertise 

 their positions by disturbing the plankton. This 

 tactic is in essence an ambush and probably is 

 effective only at short range. Reasons for this limi- 

 tation are two. First, an attack, once launched, is 

 immediately identified by flashing plankton, thus 

 giving prey more than a short distance away time 

 for evasive maneuvers. Second, a long-range at- 

 tack directed at plankton luminescing around a 

 particular prey may be led to the prey's wake, 

 because the targets that elicited the attack are left 

 behind when the prey darts away. Probably pred- 



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