272 MARINE ANIMALS 



can be separated into Arctic and Boreal communities, both distin- 

 guished by characteristic dominant animals. 



The boundaries between different temperature areas are usually not 

 sharply defined either physically or biotically, yet in the open ocean 

 and apart from regions where opposed currents meet, relatively narrow 

 strips can be located in which the water temperatures and their accom- 

 panying animal communities change more abruptly than in adjoining 

 regions. The boundaries of subdivisions are less distinct and therefore 

 possess less significance. Furthermore, the location of all these bound- 

 aries shifts with the season; the warm streams augment their area 

 in summer and are pushed back by the advancing cold in winter. Such 

 phenomena are particularly noticeable in terminating streams, such as 

 the well-known Gulf Stream, but they apply also to circular currents. 

 The pelagic animals are able to follow these shifts to some extent, 

 but the littoral animals are less motile as a rule and their components 

 accordingly require special adaptations. 



Tropical marine communities. — The influence of temperature on 

 marine animals has already been discussed and needs only a brief 

 summary. The warm waters of the equatorial belt lack that distinct 

 seasonal periodicity found toward the poles. This allows the develop- 

 ment of an extensive warmth-limited stenothermal community. As a 

 result of the continuous high temperatures, metabolism and growth 

 rates are accelerated and generations follow each other in quick suc- 

 cession. This is best seen in the smaller plankton organisms where an 

 absolutely equal amount of plankton at any given time would mean 

 more food for the plankton feeders per week in the tropics than in 

 colder water. The speeding up of life histories favors the appearance 

 of mutations, and perhaps the higher temperatures are less rigidly 

 selective than the low temperatures obtaining in polar seas. At any 

 rate, as has been stated previously, the tropical waters are rich in 

 genera and species, but have a smaller number of individuals per species 

 than are found elsewhere. This condition is especially noticeable 

 among the pelagic animal communities. The tropical waters are there- 

 fore rich in index forms, even among the invertebrates, while the com- 

 munities of the colder waters may be distinguished rather by their 

 negative characters. 2 



We have also seen that a different stratification of animal life oc- 

 curs in the tropical as compared with colder waters, particularly in that 

 the depth penetrated by surface pelagic forms is greater, with a conse- 

 quent lack of concentration in the surface waters. This may well be 

 a basic reason why the number of sea birds which secure their food 

 from the uppermost surface water is so much less in the tropics than in 



