soil. Some animals were transferred from the ocean 

 by land elevations which isolated them in bodies of 

 water which gradually became fresh. . . . Emigra- 

 tion from the sea did not take place at any one time. 

 It has occurred many times in the past and is slowly 

 progressing on many shores today. . . . The most 

 successful animal colonisers of the land have been: 



(1) the arthropods, which have in many cases de- 

 veloped book-lungs or tracheae for breathing air; 



(2) the vertebrates, with lungs and dry skins; and 



(3) the snails, with slime and spirally coiled shells 

 to prevent desiccation. . . . There are at present 

 many examples of animals which are in the midst of 

 their transformation from marine to fresh-water ani- 

 mals, or from marine or fresh-water into land ani- 

 mals. Not only have plants and animals emigrated 

 from sea to land, but there are countless instances 

 ivhen migrations have taken, and are taking place 

 in the opposite direction. Grasses, insects, reptiles, 

 birds, and mammals have left the land for the 

 sea. . . . Fishes began in fresh-water, but now range 

 throuqh the ocean at all depths (Pearse 1950: pp. 

 9-10," 14). 



On rocky shores and cliffs there is a splash or 

 supralittoral zone above high tide level. Green algae 

 occur here and scattered individuals of marine snails, 

 acorn barnacles, limpets, amphipod sandfleas, and 

 flatworms, as well as insects, especially Diptera and 

 other forms that come from the land. Above the in- 

 fluence of splashing, the rocks may be covered with 

 lichens and mosses, representing the initial stages in 

 the terrestrial rock sere. However, salt spray is often 

 blown inland a considerable distance to affect con- 

 spicuously the development of normal terrestrial 

 vegetation and its accompanying animal life. Cliffs 

 along the ocean, as well as sandy beaches and islands, 

 are favorite nesting places for large numbers of 

 pelagic birds. 



Above water action on sandy shores, the wind 

 may blow the sand into dunes with the consequent 

 development of the dune sere. On muddy flats there 

 is typically a development of salt marshes, particu- 

 larly in protected embayments or along the margins 

 of outflowing rivers. The high marshes are flooded 

 completely only during the spring tides, but the 

 ground water is more or less continuously saline. As 

 sediment accumulates the marsh eventually becomes 

 dry land (Steers 1959). 



The seashore snails (Littorina), the marsh snail 

 (Melampus), mussels (Brachidontcs, Mytiliis). crabs 

 (Carcinides, Cancer), amphipods (Gammarus, Or- 

 chestia), and isopods (Philoscia) occur through the 

 extensive salt marshes on the Atlantic Coast of North 

 America and there are numerous flies and mosqui- 

 toes. Killifishes are abundant and devour many 

 mosquito larvae. Herons, plovers, sandpipers, ducks, 

 rails, bitterns, redwinged blackbirds, marsh wrens. 



and sharp-tailed sparrows feed or nest. Muskrats 

 and meadow voles, as well as other species of mam- 

 mals, occur in salt marshes but are not particularly 

 characteristic of them (McAtee 1939). 



In tropical regions mangroves may develop in- 

 stead of marshes on mucky, poorly aerated bottoms. 

 The red mangrove has an extensive prop root system 

 and grows in deep water not ordinarily exposed even 

 at low tide. The mangroves protect the shore from 

 erosion and aid in the accumulation of deposits of 

 peat and mud that build up the shore and form 

 islands. The black mangrove at higher levels usually 

 produces erect roots that stick up through the mud 

 and serve as pneumatophores. Mangroves are usu- 

 ually heavily populated beneath by crabs and other 

 marine species. 



SUCCESSION TO FRESH WATER 



Where rivers flow into the ocean on low 

 coastal plains and there are extensive embayments or 

 estuaries, as along the Atlantic Coast, there is a very 

 gradual change from salt water to brackish water 

 (salinity: O.S-30°/00) to entirely fresh water. This 

 habitat gradient fluctuates back and forth with the 

 tides. Since fresh water is less dense and often 

 warmer, it flows over the top of the salt water with 

 the result that strata with different physical charac- 

 teristics are formed and these different strata are in- 

 habited by different kinds of fish and other organisms. 



Species of marine organisms extend towards 

 fresh water as far as permitted by their tolerance of 

 reduced salinity. Since this tolerance varies between 

 species, the marine flora and fauna become impover- 

 ished as the fresh-water flora and fauna become 

 enriched. There are, however, many more marine 

 species than fresh-water species in estuaries, although 

 productivity in brackish water is considerably less 

 than in the sea. A few species find optimum environ- 

 mental conditions in brackish waters and decrease in 

 abundance both toward fresh water and towards the 

 open sea. Economically important brackish and shal- 

 low water species on the Atlantic coast are the blue 

 crab, lobster, American oyster, scallops, hard-shell 

 clam, soft-shell clam, and such fishes as the Atlantic 

 croaker, striped bass, American shad, scup, weakfish, 

 and others. 



Of special interest are fish that perform long mi- 

 grations between fresh and salt water for spawning 

 purposes. Anadromous fish, principally salmon, shad, 

 striped bass, and some trout, come from the ocean 

 into fresh-water streams ; catadromoiis fishes, like the 

 fresh-water eels, reverse the process. The chum 

 salmon spends several years in the sea until it be- 

 comes sexually mature, then it ascends fresh-water 

 streams to their cool, gravelly-bottom headwaters to 



368 Geographic distribution of communities 



