DEEP-SEA FISHES 395 



which the hydrostatic pressure quickly follows any change in that of the 

 water outside the animal. As far as the eye is concerned, the principal 

 adaptations of deep-sea fishes are not to low temperature or high pres- 

 sure, but to the absence of light : 



The transparency of the difFerent seas and oceans varies greatly, chiefly 

 owing to differences in the concentration of the microscopic plankton 

 organisms upon which all marine animal life directly or indirectly 

 depends. A white disc two meters across, lowered parallel to the surface 

 in mid- Atlantic, is just visible from the boat at a depth of 20 to 30 meters. 

 In the North Sea, it is visible at such depths only on the calmest days — 

 ordinarily, it disappears at about ten meters. The light has of course 

 travelled twice this distance, down to the disc and back to the eye of the 

 observer. But even making allowance for that, the water at any depth is 

 dim from the point of view of a fish : he is not looking down at a snow- 

 white disc as big as a table, but at a dark bottom. Or, he may be looking 

 horizontally, at objects which receive their illumination glancingly and 

 reflect very little of it sidewise. 



The clearest of all seas is the Sargasso; and even here, the standard 

 disc can be seen from the surface only when it is less than 66 meters 

 down. At 370 meters in the Mediterranean, there is not enough light to 

 affect a photographic plate. In mid-Atlantic, plates were found to be 

 darkened at 1500 meters — but only after two hours' exposure. An eye, 

 however sensitive to light, can take nothing but snapshots, and must 

 have much more light than a camera whose shutter is left open while the 

 operator goes to lunch. Even well above the 370-meter line, there is in- 

 sufficient sunlight to affect a retina, let alone enough unscattered light 

 by which to see — to distinguish one object from another, discriminate 

 pattern and color, etc. 



The deep-sea vertebrates and invertebrates would seem to be in about 

 the same visual — or non-visual — predicament as the fishes of freshwater 

 caves. It would not be surprising to find them all completely eyeless. Yet, 

 not only do a majority of bathypelagic and benthonic fishes have eyes, 

 but some of them, e.g., Bathylagus, Zenion hololepis, and Epigonus ma- 

 crophthalmus, have (relatively) the largest eyes of any vertebrates. Few 

 bathypelagic fishes, however monstrous they may look in a magazine 

 illustration, are as much as a foot in length; and their eyes never com- 

 pare, in absolute size, with those of large pelagic fishes or with those of 

 large land animals. But it will be recalled (see p. 211) that the sensitivity 

 of an eye does not depend upon its absolute dimensions, but upon the 



