FIG. 2S 7 Three types of coral reef 

 origin, accordir)g to Charles Darwi 

 reef, cl atoll. 



and their possible manner of 

 a) fringing reef, b) barrier 



(Fig. 28-7). Fringing reefs are in direct contact with 

 the shore : barrier reefs are separated from the shore 

 by a lagoon of varying width ; atolls are annular or 

 horseshoe-shape, surrounding a lagoon that does not 

 contain any central land mass. The Great Barrier 

 Reef that extends for great distances ofif the east 

 coast of Australia is a good example of the second 

 type of reef, and atolls are numerous in the .South 

 Pacific. According to a theory first proposed by 

 Darwin, barrier and atoll reefs form from fringing 

 reefs either as the land subsides or the water level 

 rises (\'aughan 1919). Atoll islands are formed 

 either when the water level falls or when waves break 

 off and pile up chunks of coral limestone to build the 

 reefs a few feet above the high tide level. As the 

 stope dissolves or becomes pulverized, it forms a soil 

 on which plants can grow and terrestrial communities 

 of animals invade. Reefs are often not continuous 

 because the organisms are intolerant of fresh-water 

 brought down by streams, and because they are very 

 sensitive to smothering by mud or sand. On the ex- 

 posed ocean side there is generally a zonation of 

 different species from the shore outward. (Odum 

 and Odum 1955. Goreau 1959, Wells in Hedgpeth 

 1957). 



The coral organisms, particularly the algae and 

 the coelenterates, are true dominants in this biome 

 since they build the substratum that makes possible 

 the development of the community and the occurrence 

 of other organisms. Competition for space, light, and 

 protection from wave action is keen. 



There is a great variety of secondary species asso- 

 ciated with the corals. These include many kinds of 

 alcyonarians : numerous brittle stars, crinoids, and 

 holothurians ; a great variety of chaetopod. echiurid, 

 and sipunculid worms ; crustaceans, including hermit 

 crabs ; mollusks ; and large numbers of brilliantly 

 colored, strikingly-marked fish. The many crevices, 

 holes, and cavities in and between the coral provide 

 excellent hiding places and refuge from predators so 

 that the impressive development of color among the 

 fishes may be due, in part, to lack of predation pres- 



sure. The fishes have a variety of food habits and are 

 represented in all consumer trophic levels (Hiatt 

 and Strasburg 1960). 



At the Hniwetok atoll in the .South Pacific, the 

 average dry weight biomass of the living pliotosyn- 

 thetic plant material is estimated at 703 g/nr, that of 

 the herbivorous and carnivorous animals at 132 g 

 and 1 1 g, respectively. These weights exclude the 

 dead skeletal materials associated with the proto- 

 plasm. The ratio between plants and herbivores is 

 5.3:1, between herbivores and carnivores 11 :1, or a 

 composite ration of 64:12:1. The total primary pro- 

 ductivity per year as the result of photosynthesis 

 was estimated at 12.5 times the biomass of the stand- 

 ing crop. This is sufficient to balance approximately 

 the total plant and animal energy needs of the reef 

 and thus render the coral reef a self-contained steady- 

 state ecosystem (Odum and Odum 1955). 



SUCCESSION TO LAND 



The three great biocycles — ocean, fresh- 

 water, and land — come into contact with each other 

 around the margins of the seas. The change in the 

 physical nature of the habitat from salt water to 

 freshwater is a drastic one, but not more drastic than 

 the change from salt water to land. The transition 

 of animal and plant life is abrupt, and a zonation or 

 physiographic succession of communities can be 

 recognized. This transition from the ocean to fresh- 

 water and from the ocean to land as we see it today 

 is of special interest since it parallels the probable 

 evolution and dispersal of life in past ages. 



Life is generally believed to have originated in 

 the littoral region. Apparently no great groups 

 [phyla] of animals originated except in the ocean. 

 The routes by which animals probably left the ocean 

 and reached fresh-zvater and land have been various. 

 Some animals probably migrated directly across 

 sea beaches; others probably ascended rivers, passed 

 through marshes and swamps, or bnrrozved through 



Marine biomes 367 



