396 ADAPTATIONS TO MEDIA AND SUBSTRATES 



proportioning of its dioptric parts to its receptor surface. The eyes of 

 deep-sea fishes are probably by far the most sensitive in existence. Some 

 of them have been claimed to have as many as 25,000,000 rods per 

 square millimeter of retinal area. 



For their ability to retain their eyes and get good use out of them, the 

 deep-sea fishes can thank their stars. Not their astrological ones, but the 

 stars that stud their own heads and lie in galaxies along their sides : the 

 light-producing organs, or 'photophores' (Figs. 137, 139c; pp. 401, 404). 

 If bioluminescence — the production of cold light by living organisms — 

 had never been evolved in the animal kingdom, the deep sea would cer- 

 tainly not be fishless; but its fishes would assuredly be as eyeless as those 

 of the caves. Excepting occasionally at the surface when there is a great 

 congregation of luminescent plankton, there is never enough organismai 

 luminescence to light up the ocean. The great depths, if we could visit 

 them in a bathysphere, would hardly look to us like a moonlit landscape. 

 We would be fortunate indeed to see as many 'stars' as are visible on a 

 foggy night. But when one fish sees from afar the dots or blobs of light 

 produced by another organism, the recognition of an enemy, or of prey, 

 or of its own kind — even of its opposite sex — ^may be greatly facilitated. 



Great numbers of marine invertebrates are luminous. Of all the species 

 of cephalopods, about half emit light. Some shallow-water fishes have 

 illuminant organs, which are sometimes (as in Anomalops and Photo- 

 blepharon) associated with the eyes (Fig. 134), though whether they aid 

 the vision of their possessors is questionable. Though the light is perma- 

 nent, being produced by bacteria confined in a palisade of tiny tubules, 

 it can be concealed at will by the fish. The pelagic Anomalops swim in 

 schools, flashing their lights like so many fireflies. Despite the proximity 

 of the organ to the eye, it is probably only a social signal. 



Animal luminescence, as a biological phenomenon, certainly did not 

 originate in the deep sea; but it has reached its zenith of development 

 among the deep-sea vertebrates and invertebrates. Beebe has computed 

 that about two-thirds of all bathypelagic fish species — embracing about 

 96.5% of all individuals — are luminous. We can be sure that as any one 

 species of fish worked its way down the continental slope, or slowly 

 descended from the pelagic zone to the bathypelagic, it would have lost 

 its eyes but for one thing : in the lightless realm it was invading, there 

 were luminous organisms which had gotten there before it. The most 

 ancient of these, at least, must have taken their luminosity down with 

 them from the surface. In the depths, they found their light-organs val- 



