HOBSON ET AL.: CREPUSCULAR AND NOCTURNAL ACTIVITIES OF CALIFORNIA FISHES 



last migrating Chromis punctipinnis (Figure 13, 

 event 1). But this apparent overlap can be ex- 

 plained without evoking a more relaxed regime. 

 Although the three species of Californian labrids 

 retire relatively late (even considering the longer 

 twilight at temperate latitudes), they grow larger 

 than most of their tropical relatives, and it is gen- 

 erally true that among diurnal fishes larger indi- 

 viduals retire later (Hobson 1972). Similarly, there 

 are tropical equivalents to the relatively early 

 shift made by juvenile S. serranoides to its noctur- 

 nal mode. In the western tropical Pacific Ocean, for 

 example, the largely transparent nocturnal 

 juveniles of some apogonids move away from shel- 

 ter long before their larger adults. Some of 

 them — many < 30 mm long — move out as early as 

 10 min after sunset (E. S. Hobson unpubl. obs.). 

 This entry into exposed locations at a time when 

 many piscivorous predators hunt most effectively 

 might seem in conflict with the quiet-period con- 

 cept. But in the dim twilight we are not surprised 

 that these inconspicuous little fishes seem to go 

 unseen by the visual hunters that so seriously 

 threaten the more visible adults. Certainly these 

 juveniles go unseen by human eyes at this time, 

 except upon close inspection with a diving light, 

 and so fail to detract from the aura of inactivity 

 that characterizes the quiet period. 



So what might appear to be a more loosely struc- 

 tured sequence of events during twilight in 

 California may instead reflect the lesser number 

 of species that define the transition pattern there. 

 At least some semblance of a quiet period is evi- 

 dent. On the occasions depicted in Figure 13, the 

 numbers of migrating C. punctipinnis declined 

 sharply about 10 min before the juvenile S. ser- 

 ranoides first appeared, and after that only scat- 

 tered small groups passed that way. Considering 

 the tremendous numbers of C punctipinnis in that 

 area (we have never seen a single species so dom- 

 inant on coral reefs), numerous exceptions from 

 the norm should be expected. Furthermore, major 

 species that occupy the water column at night — 

 Xenistius californiensis and Seriphus politus — 

 did not arrive until about 20 min after the vast 

 majority of C punctipinnis had passed through. So 

 although crisp definition is lacking, there is evi- 

 dence of a quiet period in Californian waters from 

 about 15 to 35 min after sunset. 



It remains uncertain whether the dangers small- 

 er fishes face during twilight in southern Califor- 

 nian coastal waters are as intense as those faced 

 on tropical reefs. Limited data from gut contents 



indicate that such major predators as Paralabrax 

 clathratus and Sebastes serriceps are primarily 

 crepuscular when capturing smaller fishes. Unfor- 

 tunately, we can no longer directly observe much of 

 the predation that has influenced the evolution of 

 coastal fishes in southern California. This is be- 

 cause during recent decades populations of the 

 larger predators involved — including the giant 

 seabass, Stereolepis gigas, and the white seabass, 

 Cynoscion nobilis — have been decimated by 

 fishermen. Nevertheless, as we discuss next, the 

 possibility that Californian fishes have faced in- 

 tensified selection pressures during twilight is 

 also indicated by the nature of their scotopic visual 

 pigments. Significantly, despite the varied forms 

 and habits of these fishes, the maximum absorp- 

 tions (Amax) of their scotopic pigments cluster 

 about 500 nm, which indicates strong selection for 

 enhanced photosensitivity over this segment of 

 the spectrum. 



Scotopic Spectral Sensitivity and 

 Ambient Light 



Tropical reef fishes have scotopic pigments that 

 cluster about wavelengths that spectrally match 

 twilight, which underwater is bluer than the light 

 of day or night (Munz and McFarland 1973). In 

 developing their Twilight Hypothesis, Munz and 

 McFarland pointed out that because dawn and 

 dusk are the most dangerous times for fishes on 

 tropical reefs (Hobson 1972), even slightly in- 

 creased photosensitivity during twilight may be 

 crucial. 



The Twilight Hypothesis is a variation of the 

 Sensitivity Hypothesis (Lythgoe 1966), which de- 

 clares that visual sensitivity is improved when 

 absorption of photo pigments matches ambient 

 light. The scotopic pigments of both aquatic and 

 terrestrial vertebrates are known to cluster about 

 narrow wave bands, but at different regions of the 

 spectrum (for review, see McFarland and Munz 

 1975b). Among fishes, the scotopic pigments 

 broadly match the spectral transmission of the 

 water in which the fishes live (Lythgoe 1972). Deep 

 sea fishes, for example, have scotopic pigments 

 that tend to be even more blue sensitive ( Amax from 

 478 to 490 nm: Denton and Warren 1956; Munz 

 1957), than tropical marine fishes (X^ax from 489 to 

 500 nm: Munz and McFarland 1973), and the pig- 

 ments of freshwater fishes are green sensitive 

 (\„„. from 503 to 540 nm: McFarland and Munz 



23 



