156 MARINE ANIMALS 



water, and the effective distribution of heat in the depths is accom- 

 plished by currents. The average temperatures at different depths for 

 the oceans as a whole, according to Murray and Hjort, 20 are: 



Meters Degrees Centigrade Meters Degrees Centigrade 

 183 15.95 1097 3.89 



366 10.05 2012 2.28 



549 7.05 2743 1.83 



732 5.44 4023 1.78 



Over 80% of the ocean floor is a mile or more below the surface and has a temperature of 3° or less. 21 



Unlike fresh water, sea water with a salinity of 24.7% or more 

 continues to become heavier until its freezing point is reached. Fresh 

 water becomes lighter below 4°, a property which limits the minimum 

 temperature at the bottoms of lakes to 4°. In the sea, bottom tempera- 

 tures of — 1° may occur in regions with polar currents; temperatures 

 of the abyssal waters are usually slightly above zero. On account of 

 the presence of salt, the freezing point is depressed; for water with a 

 salinity of 35%o, it is approximately —1.9°. 



Special conditions are found in the seas which are connected with 

 the adjacent ocean by more or less shallow straits, such as the Red 

 Sea, the Sulu Sea (west of the Philippines), the Caribbean Sea, the 

 Gulf of Mexico, and the Mediterranean. The straits of Gibraltar have 

 a greatest depth of about 400 m. As the Mediterranean waters have a 

 higher salt content than those of the Atlantic, on account of more rapid 

 evaporation and smaller influx of fresh waters, their greater density 

 causes a sinking of the surface waters, which are replaced by an 

 inflow of the less dense Atlantic water. The counter currents in the 

 straits consist of the broader superficial inflow and a deeper outflow. 

 The deeper waters of the Mediterranean accordingly have a tempera- 

 ture of about 12.9°, corresponding with that of the lowest level of the 

 inflow; colder water does not enter; the temperature from about 160 m. 

 to the deepest part of the sea (3968 m.) is uniform at 12.89° (Fig. 

 13) , 22 In the Sulu Sea the temperature of the water is 10.5° from about 

 750 m. to the bottom. A. Agassiz 23 found the water in the depths below 

 1100-1200 m. in the Caribbean Sea and the Gulf of Mexico to have a 

 uniform temperature of 4.17°. Here the strait between Cuba and Santo 

 Domingo, which keeps out the colder waters of the Atlantic, has a 

 depth of 1300 m. 



The gradation of temperature in the sea is not uniform with increase 

 in the depth. At a certain depth the drop in temperatures is notably 

 greater than in the strata above and below, more than 2° in 25 m., 

 in fact. This level is called the "thermocline." It is found in all warm 



