LUMINESCENCE IN ]VIARINE ORGANISMS — NICOL 453 



(pi. 3, fig. 1), Pyrosoma (a colonial animal of the plankton; see pi. 4, 

 fig. 1) and some deep-sea fish, the luminescence of which ranges from 

 1 X 10'* to 1 X 10"^ microwatt per cm. square of light- receptor surface 

 at 1 cm. distance. 



To give tliese values some meaning in terms of human vision, it 

 may be noted that the lowest intensity which man can see is of the 

 order of 1 X 10"" microwatt falling on 1 cm. square. Deep-sea fish 

 certainly have eyes that are as sensitive as ours, perhaps more so; 

 those of crustaceans are probably less efficient. From these values 

 we can estimate how far these lights can be seen by other animals in 

 sea water. The light from a single fish or other animal decreases 

 with distance according to the inverse square law. The light from a 

 single lantern fish can be seen by another fish at about 10 meters 

 (10 m.) ; that of a marine crustacean such as Euphausia can be seen 

 by another Euphausla at about 5 m. The bright light of a comb- 

 jelly and a Pyrosoma can be seen by fish, or other animals wath 

 equally sensitive eyes, at distances of up to 100 m. 



Many, but by no means all, deep-sea animals have dark liveries and 

 are black or brown: for example deep-sea fish and jellyfish; others 

 are bright scarlet or crimson, such as prawns or shrimps. There is 

 little doubt that these are concealing colors that make i\\Q fish or 

 other animal difficult to see. Light in the ocean depths comes from 

 animal luminescence, and black surfaces serve to reduce telltale gleams 

 to a minimum. The carotenoid pigment responsible for the red color- 

 ation of pelagic Crustacea reflects very little blue light, which forms 

 such a preponderant part of animal luminescence, and is as eifective 

 as a black covering. 



FUNCTIONS OF LUMINESCENCE 



Terrestrial luminescent animals are nocturnal in their habits or 

 live in dark places. Fireflies and glowworms use their lanterns as sig- 

 nals during mating, to bring the sexes together, and at least one lu- 

 minous fungus gnat is carnivorous and attracts its prey by means of 

 its lantern. These animals can be studied close at hand, with relative 

 ease, but far different is the situation regarding luminous marine ani- 

 mals. Many of these, especially from deep waters, have never been 

 seen alive; those that do survive capture rarely live long in captivity. 

 Consequently, our ideas about the functions of luminescence in marine 

 animals are mostly conjectures. Some of these seem reasonable and 

 it should be possible to verify them by various means. 



Luminous animals are of great diversity and habits. Similarly, 

 we may expect their luminescence to be used in many different ways 

 and to perform many roles. One role might be called static advertise- 

 ment. There are many passive members of the plankton, such as jel- 



