160 MARINE ANIMALS 



ciated with such eggs, are by no means confined to cold waters; but 

 the frequency with which these phenomena appear among the most 

 diverse groups in arctic and antarctic seas indicates a convergent 

 association with low temperatures. 48 " 61 



Light. — The amount of light in the different strata of sea water 

 determines primarily the development of the plant life, and thus in- 

 fluences the animal life secondarily. It has also a primary influence 

 on the coloration of animals and upon their organs of vision. The 

 amount of reflected light which falls on the surface of the sea and is 

 reflected back reaches a minimum of 3 to 4% in Buzzards Bay near 

 noon on clear calm days. It increases a few units when there are 

 waves and a few more on cloudy days. As the sea's surface becomes 

 more disturbed, the "surface loss" of light becomes greater although 

 the reflection is increased but little. Apparently, under these condi- 

 tions the amount of light absorbed in the upper meter or so of depth 

 becomes much greater. Recent measurements 62 have shown that the 

 surface loss is usually about 15% although it has often been reported 

 as being much greater. 63 



The amount reflected from the surface is the greater with a greater 

 slant of the rays. To this must be added the fact that the absorption 

 occurs in the direction of the ray, so that light passing in a slanting 

 direction through the water does not reach as great a depth as that 

 which strikes more nearly perpendicularly. Thus the amount of light 

 that reaches a given depth at a given place varies with different time 

 of day and year, and at a given time, varies with the latitude of 

 the place. Monaco observed that the "day" in the water at the harbor 

 of Funchal (March, 1889) had a length of 11 hours at a depth of 

 20 m., 5 hours at 30 m., and only about a quarter of an hour at 40 m. 

 Toward the poles the light falls at a lower angle with the surface of 

 the water and thus in general reaches a lesser depth. The vision depth, 

 i.e., the depth at which a Sechi disk disappears, is about 50 m. at 33° 

 N. latitude, 40 m. at 50°, and 25 m. at 67°. With the same position 

 of the sun (i.e., at the same moment of time) the light at 800-m. depth 

 at 33° N. is about equal to that at 500 m. at 50° and to that at 

 200 m. at 67°. 64 



Traces of light can be detected at a depth of 1000 m. in the open 

 ocean in the subtropical zone by means of photographic plates. The 

 amount of light at this depth is one three-millionth of the amount at 

 a depth of 1 m. At 1700 m. the photographic plates are unchanged 

 after two hours' exposure. Beebe, 65 diving in a bathysphere near the 

 Bermudas, found light visible to the human eye at 579 m.; at 610 m. 

 all evidence of sunlight had vanished. The composition of light, in 



