FISHERY BULLETIN: VOL. 74. NO. 4 



numbers of Sergia gardineri, the most abundant 

 all-red sergestid. To see if this overlap is real and 

 not an artifact produced by vertical excursions of 

 the trawl or seasonal variations in the position of a 

 sharper transition depth, Teuthis XIX extensively 

 sampled the 600- to 800-m zone in November 1972, 

 using depth telemtery to try and maintain the 

 trawl within a 25- to 50-m depth range. One tow 

 between 630 and 680 m took 139 Sergesfes erectus 

 and 31 Sergia gardiyieri, another from 650 to 730 m 

 took 157 Sergesfes erectus and 289 Sergia gardin- 

 eri, and a third from 700 to 740 m took 19 Sergestes 

 erectus and 312 Sergia gardineri. On this occasion, 

 at least, substantial numbers of both color pat- 

 terns were living between 650 and 725 m. 



Other investigators have found similar transi- 

 tion zones. In Hawaii, Riggs (pers. commun.) has 

 found that the all-red species Gennadas propin- 

 quiis (Penaeidae, Benthesicymae) lives as shallow- 

 as 600 m, with maximum numbers at 650-675 m. 

 Ziemann (1975) obtained similar results for an- 

 other all-red shrimp, Systellaspis debilis (Caridea, 

 Oplophoridae), 75% of the adult population being 

 found above 650 m on one occasion. In the Atlantic, 

 Foxton's (1970) data show the half-red Sergestes 

 coruiculiirn (closely related to S. erectus) extend- 

 ing to at least 800 m, overlapping the ranges of the 

 all-red species Sergia rohusta and Systellaspis 

 debilis (although most of the catch of the latter 

 species were lighly pigmented juveniles). 

 Donaldson's (1975) data show a transition zone 

 from 650 to 800 m occupied by the half-red Ser- 

 gestes atlanticus and S. corniculum and the all-red 

 Sergia grandis. In view of this extensive overlap 

 in the distribution of half-red and all-red 

 decapods, it is necessary to review the conditions 

 under which countershading is an effective con- 

 cealment strategy and, in particular, Foxton's 

 conclusion that only half-red decapods counter- 

 shade. 



The angular distribution of light in the meso- 

 pelagic environment is independent of solar 

 elevation and depth (Denton and Nicol 1965). At 

 any given point, the background light intensity is 

 highest directly overhead, falling off rapidly to the 

 sides, with a very low light intensity of back- 

 scattered light from below. The intensity of the 

 background light 90° from the vertical is only 3-4% 

 of the zenith value, decreasing to 0.3-0.5% at 180° 

 from the zenith (Tyler and Preisendorfer 1962). 

 Changes in surface irradiance or depth change the 

 intensity but not its angular distribution. Coun- 

 tershading mechanisms match the animal to this 



background pattern; thus mid-water fishes use a 

 dark dorsal surface, silvery sides, and ventral 

 photophores for countershading (W. D. Clarke 

 1963; Nicol 1967; Badcock 1970). Foxton (1970) 

 concluded that the half-red coloration of shallow 

 mesopelagic decapods is a countershading mech- 

 anism using transparency rather than reflectors 

 for lateral countershading. I propose that some 

 deep mesopelagic all-red decapods also counter- 

 shade ventrally and that ventral countershading 

 can be effective below the transition zone from 

 half-red to all-red decapods. 



As depth increases and the intensity of the 

 penetrating light dwindles, bioluminescence 

 becomes relatively more and more important as a 

 source of light in the mesopelagic environment. 

 Bioluminescent light has a much different tempo- 

 ral and spatial distribution from the penetrating 

 surface light. The bioluminescent light field is the 

 sum of glows and flashes from many point sources 

 whose angular distribution is more or less random. 

 Countershading is an ineffective concealment 

 strategy against bioluminescence; the silvery 

 sides which camouflage a mid-water fish against 

 the penetrating sunlight may in deeper water 

 reflect a bioluminescent flash and reveal the fish 

 against a black background. The best strategy of 

 concealment in an environment lit only by random 

 flashes is to be as nonreflective as possible. The 

 dark brown or black fishes and all-red Crustacea of 

 the deep mesopelagic zone reflect blue light poorly 

 (Nicol 1958), presumably indicating their use of 

 this strategy. 



Another effect of increasing depth is that the 

 penetrating light eventually becomes too dim 

 to be seen. The absolute visual threshold for 

 deepsea fishes has been estimated as about 

 3 X 10-20 ^w / cm2 by Clarke and Denton (1962), a 

 figure that undoubtedly varies in other groups of 

 animals correlated with the degree of develop- 

 ment of the eye. A slightly higher intensity is 

 required before countershading becomes neces- 

 sary. The maximum depth of effective counter- 

 shading depends on the angular distribution of the 

 penetrating light; thus in Hawaiian waters the 

 threshold of lateral countershading is reached 

 110-120 m higher in the water column than the 

 threshold for ventral countershading. Between 

 these two depths lateral countershading is not 

 needed but ventral countershading can still be 

 effective. 



The all-red sergestids with photophores appear 

 to combine an antibioluminescent color pattern 



826 



