Food 57 



animals are plankton, including minute types such as diatoms and 

 desmids; organic debris, which may form bottom deposits or travel 

 about in the water itself; organisms brought in by rivers; and jet- 

 sam such as accumulates along the drift line. Plant food in the 

 ocean is usually comparatively limited except in cold seas (Harms, 

 1929) . Among invertebrates, littoral marine animals feed largely 

 on plankton, algae, or mud, but those which have taken up life on 

 land are generally vegetarians (Borradaile, 1903; Pearse, 1929, 

 1932a; Watson, 1928). Land vertebrates are often carnivorous 

 (Pearse, 1932a) . The intertidal zone often furnishes an abundance 

 of food and is invaded by both terrestrial and marine animals. In 

 such migrations the vegetarians and scavengers usually precede the 

 carnivores. 



A sand beach at first glance appears to be a barren waste, but it 

 contains many animals that are adapted to burrow and feed on 

 plankton, and myriads of bacteria are continually converting or- 

 ganic debris into soluble foods that are returned to the sea (Pearse, 

 Humm, & Wharton, 1942) . On mud beaches burrowing animals 

 commonly feed on bacteria (MacGinitie, 1935; Zobell & Feltham, 

 1938) . It requires 10,000 pounds of plants to produce 100 pounds 

 of clams (MacGinitie, 1935). At Tortugas, where there is con- 

 stant high temperature and abundant light, Riley (1938) estimates 

 that the amount of plankton is less than 4*^ r of that in temperate 

 regions, and states that nitrate is the chief limiting factor in food 

 production. Putter's (1907) claim that dissolved food substances 

 are absorbed through external membranes by aquatic animals has 

 been discredited (Krogh, 1931). Such materials appear to be ab- 

 sorbed through the membranes of the digestive system. 



There appears to be adequate evidence that various animals have 

 left the ocean in order to take advantage of food resources on land. 

 Along beaches today scavenger crabs which have recently come 

 from the ocean contest for the carcasses of animals along the drift 



