spawn, after which it dies. The Atlantic salmon, 

 however, has a shorter developmental period and may 

 spawn more than once. Tagged salmon and shad are 

 commonly known to ascend the same streams in 

 which they were hatched, and recent studies indi- 

 cate that they recognize their home waters by chem- 

 ical stimuli (smell), memory of which is retained 

 from very young stages (Hasler 1954). Apparently 

 salmon eggs must be laid in fresh water as otherwise 

 the outer membranes do not harden and the number 

 hatching is reduced (Black 1951). Furthermore, 

 young salmon cannot tolerate sea water until they 

 have developed chloride-secreting cells in the gills. 

 The adult female eel spends 5 to 20 years in the 

 fresh-water streams that drain into the Atlantic 

 Ocean in both the Western Hemisphere and in 

 Europe. They often ascend these streams far into 

 the interior of the continents. The male, however, 

 remains in the brackish water of the bays and estu- 

 aries. It is here that mating takes place as the female 

 returns to the sea to spawn. The journeys of the 

 females have been difficult to follow but apparently 

 both the American and European species spawn in 

 the depths' of the tropical sea northeast of the West 

 Indies. The females then die. It is here that the 

 smallest immature forms occur in mixed popula- 

 tions in the open sea. It is still a mystery how the 

 young of the two species become separated and get to 

 their respective continents. 



ZOOGEOGRAPHY 



There have been several attempts to recog- 

 nize taxonomic and geographic divisions in marine 

 communities, beginning with Petersen (1914), but 

 probably the best and most complete is that of Ekman 

 (1953). Ekman divides marine life first of all into 

 faunas, based partly on temperature and partly on 

 geography, then into regions and subregions (Fig. 

 28-8). Subregions may be still further divided into 

 provinces (Stephenson 1954). Families and genera 

 that are endemic or restricted in their distribution 

 have been most useful for distinguishing the major 

 geographic divisions. These divisions are faunistic 

 ones and each division may contain two or more of 

 the biomes that we have just described. 



Pelagic biome 



There is enough interchange of water between 

 the different oceans to give considerable uniformity 

 in the taxonomic composition of pelagic organisms. 

 Ubiquitous species found in all oceans and in both 

 equatorial and polar regions include species of sipho- 

 nophores, ctenophores, polychaetes, copepods, chae- 

 tognaths, and amphipods. 



The principal division of the epipelagic commu- 

 nity is into a warm-water fauna lying between sum- 

 mer isotherms of 14°-15°C north and south of the 

 equator and into Arctic and Antarctic faunas. A 

 large number of species in the warm-water fauna are 

 worldwide. In general, the fauna of the Indian and 

 Pacific Oceans is richer in species than that of the 

 Atlantic Ocean. 



The arctic and antarctic faunas contain several 

 characteristic and endemic species, some of which 

 may at times become very numerous. The blue and 

 fin whales of the antarctic have long been sought by 

 man for their oil. 



There is essentially only one region of abyssal- 

 benthos with many genera of animals widely dis- 

 tributed. Doubtless this is due to the considerable 

 uniformity of environmental conditions in the various 

 ocean bottoms. Subregions can be recognized, how- 

 ever, on the basis of relative proportions of endemic 

 species. 



Biomes of the continental shelf 

 and coral reefs 



The shelf fauna, made up of benthos and asso- 

 ciated organisms, is divisible into warm-water, tem- 

 perate, Arctic, and Antarctic faunas. The poleward 

 Hniits of the warm-water fauna are correlated with 

 minimum yearly temperatures of 16°-18°C. This 

 fauna may be further divided into tropical and sub- 

 tropical sub-faunas along an isotherm of 20 °C. The 

 Arctic and Antarctic faunas are limited towards the 

 equator by summer isotherms of 4°-7°C. Temperate 

 faunas lie between the warm- and cold-water faunas 

 and in turn are sometimes divided into warm and 

 cold temperate sub-faunas. 



The tropical sub-fauna is by far the richest in 

 species and contains numerous endemic elements 

 which do not penetrate extensively even into the sub- 

 tropical zone. Coral reefs are found only in the 

 tropics. The variety of forms making up each fauna 

 becomes progressively less poleward. The tropics 

 have been the center of origin, differentiation, and 

 dispersal of these cold-blooded organisms, and spe- 

 cies have invaded colder waters only as they have 

 been able to acclimatize to them. 



Many tropical genera and families are circum- 

 tropical in distribution ; that is, they are found in the 

 Indian, Pacific, and Atlantic Oceans, although repre- 

 sented by different species in each area. There are, 

 however, a few species that are also circumtropical, 

 including the brittle-star {Amphipholis sqiiamata), 

 certain crabs (Grapsus grapsus. Planes niinutus, 

 Plagusia depressa), the hammerhead shark (Zygaena 

 )nalleus), the porcupinefish {Diodon hystrix), and 

 nearly all the marine turtles. 



370 Geographic distribution of communities 



