INTRODUCTORY 15 



with 0-39 g/1. in outside sea water. The experimental worker should take 

 these factors into consideration when planning experiments which involve 

 the use of aquarium water (1, 13, 41). 



Physical Properties of Sea Water 



These are outlined below under the headings of temperature, viscosity 

 and density, light and pressure. 



Temperature. The range of temperatures encountered in the sea is not- 

 ably small when compared with conditions over the land surface. This is 

 true whether diurnal variations, seasonal variations or changes with lati- 

 tude are considered. The narrow temperature range of oceanic water 

 results from the steady system of oceanic circulation and the high specific 

 heat of water. The coldest waters are those of antarctic seas, where tem- 

 peratures of l-9°C are encountered at the edge of the ice pack. The 

 warmest waters are found in partially enclosed areas north of the equator 

 — the Red Sea, Gulf of Oman and the Persian Gulf — where temperatures 

 of 35°C are attained, while the temperatures in small tropical lagoons and 

 tidal pools may reach 40°C. In the open oceans, however, temperatures 

 are much lower and rarely reach 30 C C. The annual and latitudinal range 

 of temperatures throughout all temperate and sub-tropical seas lies 

 between and 28 C C, and at no place in the open ocean is the annual range 

 of temperature more than 10°C. 



These are surface temperatures, and it is found that with increasing 

 depth not only does the temperature drop, but seasonal variations become 

 negligible and disappear below depths of 200 metres. Thus in tropical 

 waters, when the surface temperature is say, 25°C, the temperature at 

 200 metres is 20 C, at 1,200 metres is 5 C, below which the temperature 

 continues to fall to minimal values of 1-2°C in abyssal regions (37). 



Average surface temperatures for all the oceans range from about 27 C 

 near the equator to - - 1°C in arctic and antarctic regions. In the northern 

 hemisphere the mean surface temperature is about 3°C higher than in the 

 southern, and there are also differences between the average temperatures 

 of the several oceans. 



It is apparent, therefore, that the oceans furnish a relatively stable 

 thermal environment for the animals occurring therein. Some marine 

 organisms are very sensitive to temperature changes, but the relative uni- 

 formity of water temperatures over vast areas minimizes the effects of this 

 environmental variable. Temperature affects animals in several ways; 

 extremes of temperature establish lethal parameters which can restrict the 

 distribution of a species. By its effect on metabolism, temperature regu- 

 lates spawning and affects development and growth. Because of the inverse 

 relationship between viscosity and temperature, it also influences the rate 

 of sinking of small planktonic organisms, as described in the next section. 



Few animals have a universal distribution, a notable exception being 

 the ctenophore Beroe cucumis. This species ranges from arctic to tropical 

 regions, but there are probably physiological differences between animals 



