58 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



striking among both fishes and invertebrates. Among the mollusks, for 

 example, most genera and a great many species have this very wide 

 range and differ strikingly from the fauna of the Atlantic. 3 Of the 

 160 species of the genus of crabs Cyclometopa in the Red Sea, only 

 two occur to the west, while the others extend for the most part far 

 into the Pacific. 4 Stenothermal depth and cold-tolerant animals, on 

 the contrary, have a wholly continuous habitat. The temperature drops 

 rapidly with increasing depth, and below 2000 meters rarely is above 

 3°C, even in the tropics, so that the cold oceans of the two poles are 

 united by the cold waters of the oceanic depths. 



The greater oceanic depths form a barrier to the spread of such 

 animals of the littoral region as are confined within a narrow variation 

 of depth. Such animals may be called stenobathic. They can migrate 

 only along the coast. If they meet a thermal barrier, their distribution 

 may be limited. With the exception of a few pelagic animals, a few of 

 brackish water, and a few of the deep sea, scarcely a species of the 

 marine Metazoa is common to the east and west coasts of tropical 

 Africa. 5 On the other hand, eurythermal pelagic forms are restrained 

 by no barriers, and some of them are accordingly found in all seas. 

 Such forms include the schizopod crustaceans Euphausia pellucida 

 and Eucopia australis, the whale shark, and the above-mentioned 

 whales. 



The salinity of ocean water, in general, varies only between nar- 

 row limits. Where arms of the sea have an abundant inflow of fresh 

 water, and are consequently less saline, many inhabitants of the 

 neighboring ocean are excluded. The freshening of the surface water 

 near the mouths of large rivers may be a barrier to the spread of 

 littoral forms in shallow waters. For example, none of the species of 

 sea urchins of the Patagonian coast are found north of the mouth of 

 La Plata. 6 



There are fewer methods of distribution for marine animals than 

 for terrestrial forms. There are no true flying forms. All are dependent 

 on movements within the water or on the movement of the water 

 itself. But even in the water, many animals are restricted in position, 

 either completely sessile on the bottom like the corals, bryozoans, or 

 oysters, or very limited in their power of movement, like the sea 

 anemones and some mollusks. Many marine animals such as worms or 

 snails, which creep, and crabs which walk or run with more or less 

 speed, are restricted to the bottom, but the majority of all these have 

 free-swimming larval stages. The greatest degree of freedom is en- 

 joyed by those which can rise freely in the water, whether in suspen- 

 sion or by active swimming. The greater the capacity for swimming, 



