PHYSICAL CONDITIONS IN THE OCEAN 151 



seemed to be confirmed by the investigations of Edward Forbes in the 

 Aegean Sea in 1843. Finds of living animals, however, from more than 

 2000 m. gradually became more frequent, and though none have yet 

 been taken in the greatest known depths, perhaps on account of the 

 difficulty of dredging operations at such depths, the presence of life at 

 depths between 6000 and 7000 m. has been fully confirmed. Any 

 impoverishment of animal life at such depths is due to the scarcity of 

 nourishment and not to the pressure. These animals exist under a 

 pressure of 600 atmospheres, but as the same pressures exist in their 

 body fluids, there is no possibility of their being crushed. 



In deep-sea dredging it is the general experience that almost all 

 the animals from great depths are dead, or at least greatly injured, 

 when they reach the surface. The rapid reduction of pressure seems to 

 have less to do with this than the difference in temperature between 

 the depths and the surface. At any rate, in dredging from 1650 m. in 

 the Mediterranean, where in contrast with the ocean a uniform tem- 

 perature of 12.9° rules from a depth of about 160 m. down to the bot- 

 tom, the animals of the depths reach the surface in good condition. 5 

 Great variations of pressure in short periods are the daily experience 

 of many animals of the open sea. Many plankton animals and pelagic 

 fishes which live by day at depths of 400 m. and more rise vertically 

 at night, often to the surface, to return at daybreak to the greater 

 depths. They are not affected by a 30- or 40-fold variation of pressure, 

 quite in contrast with the air-breathing animals, for which a reduction 

 of the atmospheric pressure by one-half produces extreme injury. 



The assumption that greater amounts of gas are dissolved in the 

 water at great depths, in correlation with the increased pressure, and 

 that this creates changed conditions for animal life, is not borne out. 

 Steel cylinders filled with water from the greater depths did not burst 

 by the expansion of the contained gas with the reduction of the out- 

 ward pressure, as was expected. It is true that, in the bony fishes with- 

 out an opening to the swim bladder, the gas in the bladder expands to 

 a great degree when the fish is brought up from the depths, and may 

 project from the fish's mouth or even explode and burst its body. Such 

 fishes are probably capable only of gradual changes from the level they 

 habitually maintain. 



With the exception of these fishes, the pressure factor apparently 

 does not play an important role in the life of marine animals, and at 

 any rate it does not prevent the existence of life at great depths. Many 

 species of animals accordingly have a great vertical range in the sea, 

 i.e., they are eurybathic. Among 20 annelids which reach depths 

 greater than 1800 m., 12 are to be found also within the 200-m. line. 6 



