LIBIINESCENCE IN MARINE ORGANISMS — NICOL 455 



data already considered, we can estimate tliat the maximal distance at 

 which the members of a school can be dispersed and still see one 

 another is about 5 m. 



Another function of luminescence is certainly connected with re- 

 production. In deep-sea crustaceans, squid, and fishes, the two sexes 

 sometimes have characteristic and distinct patterns of light organs. 

 These enable the species and sex of any animal to be recognized, and 

 attract the males to the females for spawning purposes. We must also 

 recognize, however, that many deep-sea animals, including fishes, are 

 solitary hunters. The light organs of these species may be concerned 

 with repelling the members of the species from one another, and 

 keeping them spread out in limiting territories, the limits of which are 

 regulated by the distances at which their lights can be recognized. 



Voyagers who have sailed over tropical seas have given many ac- 

 counts of great "phosphorescent" displays which they have observed, 

 when large areas of the sea surface appear to glow with light. These 

 great displays of luminescence are produced by dense aggregations of 

 animals such as Noctilucae, comb-jellies, and Pyrosomae (pi. 4, fig. 1). 

 We may well wonder what the biological significance of such lumi- 

 nescent displays may be. The intensity of light from a very large sur- 

 face area diminishes with distance according to the extinction coeffi- 

 cient of sea water, in contrast to a point source, the intensity of which 

 diminishes in proportion to the square of the distance and according to 

 the extinction coefficient. Clear oceanic water is very transmissive, 

 passing about 95 percent of blue light per meter depth. The light from 

 a luminescent sea, having an intensity of, say, 1 X 10"^ microwatt at 

 the surface would be reduced to about 1 X 10"^ microwatt per cm. square 

 receptor surface at 100 m. Tliese intensities are well above the thresh- 

 old of vision ; moreover they appear to lie in the range of light in- 

 tensities which affect the vertical migrations of marine animals. JSIany 

 free-swimming animals, especially crustaceans, squid, and fish, make 

 these vertical movements, coming toward the surface at dusk and 

 descending into the depths at dawn. The chief factor controlling the 

 movements appears to be light intensity, the animals following some 

 optimal light intensity or isolume. Many years ago Prof. A. C. 

 Hardy pointed out that animals tend to avoid dense surface aggrega- 

 tions of certain kinds of plankton, and he advanced a hypothesis of 

 "mutual exclusion." 'Wlien the light beneath a surface display of 

 luminescence is bright enough, the ascent of migrating animals may be 

 halted. Differences in the directions of surface and deeper currents 

 may then bring about further separation of the animal populations. 



When W. Beebe and O. Barton made their famous descent in a 

 Bathysphere 26 years ago, they observed a wealtli of luminous animals 

 in the ocean depths, often as abundant as the stars in the sky on a 



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