280 MARINE ANIMALS 



of water melted from ice and snow. The salt content of the ocean and 

 of various bays and land-locked seas has been given previously (p. 

 164), and is summarized for the Baltic Sea by the lines of equal salinity 

 in Fig. 77. 



Temperature distribution in the small seas frequently differs greatly 

 from that of the ocean. The temperature of the surface layer is not 

 much different from that in the neighboring part of the ocean, although 

 the highest surface temperature always occurs in the lesser seas, reach- 

 ing 34.3° in the middle part of the Red Sea and in the Persian Gulf 

 35.6 , 42 whereas in the open ocean the summer temperature in some 

 places rises to 29°, near coasts and in the western Pacific to 32°. 

 Further, the decrease in temperature according to depth is different in 

 many small seas from that in the ocean. The decrease in temperature 

 from the surface to the bottom and the low temperature of the abyssal 

 waters in the ocean is explained by the slow creep of the denser polar 

 waters along the bottom. But if the entrance to a deep sea such as 

 the Mediterranean (see Fig. 13) is closed for the deep water by a bank 

 or shallow strait, the cold water from the depths of the ocean cannot 

 enter and the temperature of the restricted sea bottom remains con- 

 stantly at that of the lower layer of the inflowing ocean water. In 

 the Red Sea, the temperature of 21.5° extends from a depth of 700 m. 

 to its greatest depth at 2190 m., whereas in the ocean at such depths 

 the average temperature is 2.11°. Similarly, in the Sulu Sea, the tem- 

 perature from a depth of 800 m. to the bottom (more than 4000 m.) 

 remains at 10.3°. 



The slight movement of the deep waters of many seas caused by 

 the shallowness of their connection with the ocean leads to the further 

 result that the oxygen content is low and that other substances may 

 accumulate which are lethal to animal life. In the deep basins of the 

 Baltic Sea, in the deep parts of the Mediterranean, especially of the 

 Levantine and Ionian seas, and apparently also in the depths of the 

 Red Sea, the water is poor in oxygen and rich in carbon dioxide. The 

 animal life is therefore sparse. Thus the lack of ptychobranchiate 

 ascidians, 43 the unusual scarcity of Hexactinellida (only two spe- 

 cies) , 44 and the small number of mussels and snails 45 in the depths of 

 the Red Sea is striking. In the depths of the Black Sea, whose con- 

 necting straits are only 50 m. deep in the shallowest part, there is a 

 considerable amount of hydrogen sulphide, amounting to 0.33 cc. 

 per liter even at 180-m. depth, so that from that depth to the bottom 

 all life is absent, with the exception of anaerobic bacteria. 



Furthermore, in the seas, tide movements are often very small. 

 This is not universally true; e.g., the flood tide for the Red Sea is 



