WALTERS: ECOLOGY OF HAWAIIAN SERGESTID SHRIMPS 



with a ventral array of simple photophores for 

 low-intensity ventral countershading. This inter- 

 pretation implies that the transition from half-red 

 to all-red sergestids at 650-725 m marks the upper 

 limit of bioluminescence as an important source of 

 ambient light. The lower limit of all-red sergestids 

 with photophores should then mark the threshold 

 of ventral countershading. Unfortunately, this 

 study produced good daytime vertical distribution 

 data for only one such species, Sergia gardineri. If 

 its lower limit under normal conditions is typical 

 of the other species, then the threshold of ventral 

 countershading should lie at approximately 775 m. 

 This depth is also the approximate upper limit of 

 the two sergestids, Sergia tenuiremis and Pef- 

 alidiuni siispiriosum, that lack photophores 

 (Figures 29, 31). If 775 m is the threshold of ventral 

 countershading, then the threshold for lateral 

 countershading should be 110-120 m higher or 

 about 660 m, approximately the depth of the 

 transition from half-red to all-red sergestids. 



It thus appears that the transition from half-red 

 to all-red sergestids does not mark the absolute 

 lower limit of countershading, but is related to the 

 depth at which lateral countershading becomes 

 ineffective and bioluminescent light forces a 

 change in concealment strategy. Although the 

 all-red color pattern hides the shrimp from 

 bioluminescent flashes, enough of the penetrating 

 light remains directly overhead that ventral coun- 

 tershading continues to be eff"ective more than 100 

 m below the transition zone. The simple lensless 

 photophores of the all-red sergestids presumably 

 produce low levels of light in this dimly lit region. 

 Other mid-water animals that lack lateral coun- 

 tershading mechanisms but have ventral arrays of 

 photophores, such as many of the black stomiatoid 

 fishes, may have evolved the same kind of 

 camouflage. 



Nighttime Vertical Distribution 

 and Migration 



The structure of the sergestid assemblage 

 changes drastically as day gives way to night. All 

 Hawaiian sergestid species except Sergia tenui- 

 remis and Petalidiuni suspiriosum migrate into 

 the upper 300 m of the water column. As figure 34 

 shows, the division into a shallow half-red and a 

 deep all-red mesopelagic sergestid assemblage 

 disappears on moonless nights. The species fall 

 with little overlap into a shallow and a deep 

 migratory group, adults of the shallow group 



NUMBER /IO^m' 

 HAIF-RED ALL-RED 



J50 100 50 50 100 150 



NIGHT 



1200-" 



Figure 34.-Nighttime vertical distribution of half-red and 

 all-red Hawaiian sergestids (moonless conditions). Hachure, etc., 

 as in Figure 33. Light intensity estimated from unpublished 

 daytime data of E. M. Kampa at lat. 28°N, using G. L. Clarke's 

 (1968) values for relative intensity of day vs. night. 



living in the upper 100 m, adults of the deep group 

 living from 125 to 300 m. The shallow group 

 includes Sergestes vigilax, S. consobrimis, Sergia 

 scintillans, and 5. gardineri. The deep group 

 includes Sergestes erectus, S. armatus, S. sargassi, 

 Sergia bigemmea, S. inequalis, and S. bisulcata. 

 The single large nighttime capture of adult Sergia 

 fulgens is at about 175 m, probably placing this 

 species also in the deep group. In addition, Ser- 

 gestes pectinatus is broadly distributed from 25 to 

 250 m, and S. atlanticus may likewise be broadly 

 distributed if the single large catch in the upper 25 

 m is not representative of its normal distribution. 

 T. A. Clarke (1973) found a similar pattern in the 

 nighttime distribution of Hawaiian myctophid 

 fishes, with a shallow group down to 125 m, a deep 

 group at 150-250 m, and a few species broadly 

 distributed in the upper 250 m. Closely related 

 pairs of myctophid species separate into a shallow 

 species and a deep species. Most sergestid species 

 pairs are found at the same nighttime depths, 

 except for Sergestes armatus and S. vigilax, and 

 probably Sergia scintillans and S. fulgens. 



The division of the nighttime sergestid as- 

 semblage at 100-125 m may possibly be related to 



827 



