LUMINOUS ORGANS 741 



irritated exhibits a wave of photogenic activity which merits the popular 

 name "phosphorescent fire -flame " (Polimanti, 1911). Among Fishes, 

 there are many kiminous examples, both Selachians and Teleosts, most 

 of AA'hich inhabit the deep sea or the ocean bed ; it is interesting that 

 luminous organs are unknown among cave-fishes or fresh-water fish.^ Some 

 shallow-water fishes luminesce but it is in the darkness of the bathypelagic 

 and the absolute night of the benthonic zones that bioluminescence has 

 reached the zenith of its development. Here, far beneath the level of 

 the plankton, the luminous organs of the molluscs and fishes are the only 

 source of light, and Beebe (1934) has computed that two-thirds of bathy- 

 pelagic species of fish including 96-5% of all individuals are luminous. 

 Indeed, to catch these pale gleams of light would seem to be the only reason 



Fig. 892. — The Hatchet Fish, AeGyRop£LEcr.<. (reproduced from Dahlgren, from a 

 drawing by Brure Horsfall ; E. X. Harvej-'s Living Light). 



for the development of the enormous eyes which characterize some of these 

 inhabitants of the great depths." Curiously, in bathypelagic molluscs and 

 fishes the vast majority of these lights are directed dowaiwards ; some, 

 differing between the two sexes, point horizontally and are obviously sexual 

 recognition marks, but luminous organs situated dorsally are invariably 

 minute or degenerate (Hubbs, 1938) (Figs. 892 and 895). 



The biological purpose of bioluminescence is sometimes clear, but often 

 obscure. It would seem that the light is never employed as a search-light 

 whereby to see. but always as a signal-lantern as a lure, a label or a means of 

 dazzling ; for the most j)art they are social or sexual signals. Luminous 

 organs of great complexity thus occur in dee^J-sea fishes in which the eyes are 

 degenerate or even absent (c.(/., Ijmojjs^). Their sexual value as an aid to 

 courtship is the most securely proven. 



Two examples will make this matter clear. The female fire-wormi of Bermuda 

 [Odontosyllis) at mating time seeks the surface of the sea where she circles luminescing 

 brilliantly for 10 to 20 seconds ; the male swimming in the deeper water makes for 



1 The onlv fresh-water luminescent animal described is an acjuatic glow-worm. 



2 p. 322. ^ 3 p. 724. 



