20 



THE BIOLOGY OF MARINE ANIMALS 



84-5% penetration. Similar results have been obtained elsewhere, and 

 contrasting data for the Sargasso Sea and the English Channel are shown 

 in Figs. 1.7 and 1.8. Very turbid coastal and estuarine waters show much 

 lower values than these, and maximal penetration shifts towards the red 

 end of the spectrum under such conditions (2, 3, 9, 14, 15). 



Earlier estimates of illumination levels in deep waters have been 

 superseded by photo-electric measurements, and data are now available 

 showing how light intensities change during the day at various depths in 

 the water column. As the daylight disappears with depth, the light 

 produced by animals becomes more pronounced. At certain stations in 



o-i 



0-6 a, 

 05 ,, 



0-4 § 



0-3$ 



o 

 0-2 i> 



- 0-1 



Hi 



450 



500 550 600 



Wave- length (mfj) 



650 



Fig. 1.6. Smoothed Graphs Showing the Extinction Coefficents for Light 

 of Different Wave-lengths in Pure Water, and in Different Kinds of 



Sea Water 

 (After Sverdrup, Johnson, and Fleming (37) from Utterback's data) 



the West Atlantic, background light remained constant or increased at 

 depths greater than 300 metres, and individual flashes at 600 metres 

 were as much as 1,000 times brighter than background illumination (9, 

 10, 28c). 



There is little information available for the photo-sensitivity of marine 

 animals in weak illumination, but what there is suggests that some species 

 possess a sensitivity at least equal to that of man, about 1 x 10~ 10 //W/cm 2 

 receptor surface, and special adaptations in deep-sea fishes may result 

 in greater sensitivity (see Chapter 8). The deepest records hitherto ob- 

 tained at about 600 metres show light intensities of at least 1 x 10 -6 

 //W/cm 2 (10). 



Light and Pelagic Animals. Many plankton species in upper pelagic 

 waters exhibit vertical migrations of great magnitude. These are responses 



