LIGHT 



125 



surface. Similarly, when the sun is near the 

 horizon, the hghted zone is more shallow. 

 Under all conditions and at all latitudes, the 

 length of day decreases with depth. 



Something of the degree of variation of 

 the penetration of light is shown by the 

 fact that, in Wisconsin lakes, the depth at 

 which light is reduced to 1 per cent of that 

 at the surface varies from 1.5 to 29 meters. 

 Similar values for sea water are: 8 meters 

 for the harbor at Woods Hole (Mass.); 32 

 meters for the Gulf of Maine and 149 

 meters in the Sargasso Sea. The last value 

 indicates nearly or quite the most transpar- 

 ent water yet measured and is to be com- 

 pared directly with the value of 29 meters 

 for Crystal Lake, Wisconsin, which is the 

 most transparent body of fresh water that 

 has yet been studied with comparable 

 methods. In Crater Lake, Oregon, tfiick 

 green mats of green mosses, Fontinalis and 

 Drepanocladus, grow over the bottom at a 

 depth of 18 to 60 meters and are found at 

 120 meters. These two genera of mosses 

 have been reported at a depth of 20 meters 

 in Crystal Lake (Hasler, 1938). 



Different wavelengths of Hght show dif- 

 ferential penetration with depth. The usual 

 rule of clear water is that light from the 

 blue end of the spectrum is more pene- 

 trating. In strongly colored waters, the 

 longer wavelengths of the red end pene- 

 trate more readily. In such waters, below 1 

 meter's depth, there is little light present 

 with a wavelength of less than 6000 A. 

 With moderate transparency, such as is 

 found in many lakes and inshore waters of 

 the ocean, maximum penetration shifts to 

 the yellow (about 5500 A). 



Such physical facts are meaningless eco- 

 logically until they have been considered in 

 relation to hving things. It can readily be 

 understood that both the quantity and the 

 quality of light is important in the energy- 

 storing processes connected with photosyn- 

 thesis. Some of the other biological relations 

 will be considered immediately. 



Bioluminescence 



As the sunlight fades in the deeper 

 waters of the ocean, bioluminescence in- 

 creases. This abiUty to produce animal Hght 

 is found among a variety of plankton forms 

 in surface waters, including protozoans, 

 jelly fish, ctenophores, copepods, and tuni- 

 cates. A few burrowing animals of the Ut- 

 toral region also are photogenic— the annehd 



Chaetopterus, for example. A glowworm, 

 AracJinocampa luminosa, excepted (Blakes- 

 lee, 1948), the ability is lacking among 

 cave dwellers and in animals from fresh 

 water, although a number of terrestrial 

 forms are bioluminescent, of which the 

 lampyrid beetles are outstanding examples. 

 In his bathysphere dives, Beebe saw the 

 first animal bioluminescence at 207 meters; 

 below that point there was a slow increase 

 in the number of such forms, down to the 

 greatest depth reached at 924 meters. 

 Light is produced most efficiently by fishes, 

 crustaceans, and cephalopods, some of 

 which have highly speciafized Hght-produc- 

 ing devices. Other animals give off light 

 from diffuse organs scattered over the sur- 

 face of the body (Harvey, 1940). 



Coloration 



The relatively rapid absorption of fight 

 with increasing depth, combined with the 

 distribution of animal life from the hghted 

 surface to the abyssal depths, allows a test 

 of the possible relationship between inten- 

 sity and quahty of illumination and the 

 coloration of animals. In the hghted ben- 

 thal regions, coloration of animals is varied 

 and, at times, related to the colors of the 

 background; flounders, for example, may 

 change their color intensity and pattern and 

 match that of their background. Surface 

 pelagic animals tend to be transparent, or 

 they have blue, greenish, or brown backs 

 with silvery sides and belfies. The pattern 

 may be broken by wavy dark lines as in the 

 mackerel. At some distance below the sur- 

 face, in the so-called mesopelagic region, 

 there is a preponderance of reds of various 

 shades shown by a great variety of animals. 

 The red spectral rays have been screened 

 out, and these red animals must appear as 

 though they were black. Almost all decapod 

 crustaceans below 750 meters in the tropics, 

 500 meters in middle latitudes, or 200 

 meters in polar seas are red in color. Black 

 and violet predominate as deep-sea colors. 

 Some other animals become pale or color- 

 less, and color patterns may or may not be 

 present. Again, there is a marked contrast 

 between these varied colors and the faded- 

 out, white animals from caves. 



Relation to Size of Eyes 



The ratio of eye to head in crustaceans 

 and fishes is called normal in surface-dwell- 



