FISHERY BULLETIN: VOL. 74, NO. 4 



the penetrating surface light. The scale at the left 

 of Figure 34 is an estimate, derived by assuming 

 the value of light intensity at the surface on a 

 moonless night to be 10' times fainter than during 

 the day, a figure used by G. L. Clarke (1968), and 

 applying this correction to the daytime light curve 

 of E. M. Kampa (unpubl. data) used in Figure 33. 

 The lower limit of daytime ventral countershad- 

 ing, estimated above as approximately 775 m, is 

 equivalent to a nighttime depth of approximately 

 125-150 m, suggesting that the shallow group, but 

 not the deep group may countershade at night. 

 The lower limit of daytime lateral countershading, 

 estimated as about 660 m, is equivalent to about 50 

 m at night, the approximate upper limit of S. 

 gardineri, the shallowest all-red sergestid at 

 night. Although these figures admittedly pile 

 estimate on estimate, they suggest that light may 

 influence the vertical distribution of sergestids at 

 night as well as during the daytime. 



W. D. Clarke (m Barham 1970:118) and Foxton 

 (1970) have suggested that countershading may 

 occur primarily at night in some mid-water an- 

 imals. While nighttime ventral countershading 

 appears feasible for some species of Hawaiian 

 sergestids, these species all may need to counter- 

 shade during the daytime also (Table 7). A number 

 of species maintain approximately constant illu- 

 mination day and night. Some species live in much 

 brighter waters during the daytime than at night. 

 No species, however, lives in brighter waters at 

 night than during the daytime, as would be ex- 

 pected if countershading were occurring only at 

 night. 



Only two species of Hawaiian sergestids 

 definitely do not migrate. Sergia tenuiremis ap- 

 pears to migrate to 300-400 m when less than 15 



Table 7.-Estimated light intensities for daytime and dark night 

 habitats of Hawaiian sergestids. Numbers are negative loga- 

 rithms of light intensity (smaller numbers mean brighter light). 



mm CL. The adult population spreads upward 

 from an upper limit of 750-800 m during the day to 

 about 600 m at night, although many shrimp 

 remain in the daytime depth range. Petal id ium 

 sHspiriosKm remains below 750-800 m both day 

 and night. Few Hawaiian sergestids occupy the 

 depths between 300 and 600 m at night, in contrast 

 to the Atlantic, where Donaldson (1973) found 

 Sergestes corniculum and Sergia grandis and 

 Foxton (1970) found Sergestes corniculum and 

 Sergia robusta in this depth range. The reasons for 

 this difference are unknown. 



Although considerable evidence links diurnal 

 vertical migration to the diurnal light cycle (e.g., 

 Marshall 1954), the exact relation of light to 

 vertical migration is complex and poorly under- 

 stood. The simplest scheme, merely maintaining a 

 constant light intensity around the clock, is not 

 used by all Hawaiian sergestids, as Table 7 shows. 

 The daytime sergestid assemblage cannot shift en 

 masse to equivalent light levels at night, because 

 the light intensity at the surface on a moonless 

 night is approximately equivalent to that at 600 m 

 during the daytime. Instead, we find species with 

 similar daytime ranges but different nighttime 

 ranges, such as Sergestes armatus and S. conso- 

 brinus; species with similar nighttime ranges but 

 different daytime ranges, such as Sergia scintil- 

 lans and S. gardineri; and species that exchange 

 relative positions, such as Sergestes sargassi and 

 Sergia gardineri. Vertical migration is a more 

 complicated behavior than merely maintaining a 

 constant light level. 



A further complication of the vertical migration 

 mechanism involves the response of sergestids to 

 moonlight. When the moon increases the night- 

 time surface irradiance, the two groups of migra- 

 tors react in different ways. The deep group 

 remains relatively unaffected by moonlight, the 

 young often moving downward to the depth of the 

 adults. Moonlight drastically affects the shallow 

 group, depressing most of the species below 150 m. 

 The two assemblages, which separate by depth on 

 dark nights, mix together on moonlit nights. 



In addition to the normal response of sergestids 

 to moonlight, there appears to be a period of about 

 a week around full moon when some species stop 

 migrating entirely, remaining at their daytime 

 depths. This behavior is poorly shown by the 

 results of the Teuthis cruises, showing up better in 

 the supplementary data from 70-12 and Echo IV. 

 Not all species react the same way to the full moon 

 period. Sergia gardineri and probably Sergestes 



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