258 



MARINE ANIMALS 



forms. In the pelagic animals, the highly developed light organs must 

 have other uses. Many are grouped around the mouth, and suggest a 

 use as food lures. It is probably equally important that their charac- 

 teristic arrangement makes possible a recognition of the sexes, and of 

 the species in gregarious forms. This helps explain the great variety 

 in arrangement of the light organs, and in the color of the light emitted, 

 but scarcely touches the problem of bioluminescence of alcyonarians 

 and other sessile forms. 



The sunless depths of the sea are consequently not without light. 

 Thousands upon thousands of luminescent animals provide torches 

 which light the abyssal depths. This light is not uniformly distributed. 



Oases of light occur where there 

 are forests of gorgonians and 

 meadows of alcyonarians. In com- 

 parison with sunlight, this light is 

 weak at best, and does not reach 

 far into the water, but it does en- 

 able animals to direct their mo- 

 tions by means of their eyes. Some 

 of the relationships are indirect, 

 thus a hermit crab is known which 

 carries luminescent actinians on 

 its shell. 17 The development of 

 bioluminescence explains the fact 

 that degeneration and loss of eyes 

 is less general in the abyssal fauna 

 than in caves. 



Eyes in relation to depth. — 

 There are eyeless animals in the deep sea, but their number in the 

 pelagic formation is small. Eyeless forms and forms with reduced eyes 

 are numerous in the abyssal benthos as they are in the littoral. The 

 deep-sea pectens and Eulima and Fusus are eyeless. Among the 

 benthic crustaceans the eyes are frequently reduced, both in number 

 of facets and in amount of pigment present. Doflein 18 found all crabs 

 from considerable depths with fewer facets in their eyes than the re- 

 lated littoral forms. Alcock reports that 20% of the higher Crustacea 

 in the catch of the Investigator had pigmentless or reduced eyes. 

 Among some crabs, reduction of the eyes with increasing depth is 

 found within the same species. 18 Among fishes and cephalopods com- 

 plete degeneration of the eyes is very rare. Only one eyeless cephaloped, 

 Cirrothauma murrayi, from 1500-m. depth, is known. The Macrurus 

 species of the Challenger collection may be sorted as to depth accord- 



Fig. 68. — a, Macrurus fasciatus from 

 250-m. depth ; b, Bathyonus taenia 

 from 4500-m. depth. After Gunther. 



