274 MARINE ANIMALS 



Although the pelagic animals serve adequately to separate the tropi- 

 cal animal communities from those of cooler waters, the warm seas are 

 particularly distinguished by the reef corals. These benthic animals 

 occur in suitable tropical situations in enormous numbers and are 

 entirely lacking in colder waters. They are limited in their distribution 

 to a mean temperature of at least 20°, and their variety increases in 

 higher mean temperatures. The area of distribution of these corals is 

 indicated in Fig. 75 ; the boundaries lie within the limits of the tropical 

 belt; the southern limit coincides somewhat with Ortmann's southern 

 border of the warm-water zone, but the northern limits are independent 

 of it, particularly in the Atlantic. 



Polar marine communities. — A direct result of the winter freez- 

 ing appears in the absence of algae and of sessile animals in the lit- 

 toral formations of the Arctic Sea to a depth of 6 m. ; only bare rocks 

 are to be seen, from which the drifting ice masses have scraped off all 

 life. Below the limits of ice action, on the other hand, plants and 

 animals are present in abundance, often in larger numbers than in the 

 adjacent boreal zone, 5 a little to the southward. 



Because of the low temperature, fertility is reduced, and, correlated 

 with this fact, is an increased size of egg (compare p. 159). The sup- 

 pression of free-swimming larval forms is a result, especially among the 

 benthic animals. The descendants therefore remain in the immediate 

 vicinity of their parents; thus enormous aggregations of single species 

 occur in many places, as is well known in arctic seas. For example, in 

 the sea along east Spitzbergen, sedentary annelids (Scidne lobata, 

 Thelepus cincinnatus) are found together in patches; in the Barents 

 Sea, the Michael Sars brought up more than a ton of the siliceous 

 sponge, Geodia, with a single haul of the dredge, at another time, near 

 Jan Mayen, more than a barrel of a scallop, Pecten groenlandicus, was 

 similarly taken ; other accounts mention large catches of feather stars, 

 Antedon eschrichtii. 6 



The composition of the cold-water fauna is different from that of 

 the tropics in many respects. The number of animal species that can 

 live in the surface water of both the polar and the tropical seas is 

 small. The fauna of the polar seas shows throughout the influence of 

 environmental selection as compared with that of the tropics. Though 

 not so numerous as in the warmth-limited types characteristic of the 

 tropics, still many genera and species find their most favorable condi- 

 tions for existence in these colder waters. Such groups are the Hydro- 

 zoa among the coelenterates and the Holothuria among the echino- 

 derms. Siliceous sponges occur especially in antarctic seas. On the 

 other hand, the scarcity of higher crustaceans is striking; of the 



