pelecypods, annelids, and crustaceans, are deposit- 

 feeders, or have special straining devices in the form 

 of sieves, brushes, and hairy or mucous nets for col- 

 lecting bacteria, organic particles, and small plankton 

 organisms suspended in the water (Blegvad 1914, 

 Yonge 1953). Deposit-feeders tend to predominate 

 where the bottom is composed of fine sediments, 

 while suspension-feeders predominate where the bot- 

 tom is made of coarser materials (Sanders 1936). 

 Snails, amphipods, and others feed on the plant tis- 

 sues of eelgrass, sea lettuce, and algae. Crabs, echino- 

 derms, and fish feed partly on organic debris and 

 partly on the smaller invertebrates. Fish feed on the 

 invertebrates and larger fish feed on smaller ones. 

 On sand and mud flats on the coast of California, 

 only 5 per cent of all animal species are strictly car- 

 nivorous. Only a few species of animals feed directly 

 on plants, but plant tissue becomes more available 

 after its death and partial decomposition. The prin- 

 cipal food chain is plants : detritus and bacteria : 

 detritus and bacteria feeders : animal feeders : birds 

 (MacGinitie 1935). 



At low tide, feeding and other activities are at a 

 minimum as the bottom forms retract into their bur- 

 rows and motile forms retreat seaward. However, 

 some of the snails continue to feed on plants, and 

 insects and birds come into the exposed area in search 

 of debris and small animals for food. As the tide 

 returns, the insects and birds retreat landward, but 

 pelecypods extend their siphons, annelids rise from 

 their burrows, shrimps, crabs, and fish move about 

 over the surface, and the whole community becomes 

 a scene of bustling activity. 



Petersen (1918) and Jensen (1919) of Denmark 

 were concerned with measuring the productivity of 

 invertebrates in the sea, especially benthos, as a 

 source of food for commercially important fish. More 

 accurate calculations in the English Channel (Harvey 

 1950) give the ratio between the mean annual dry 

 organic weight of bottom invertebrates and the bot- 

 tom-dwelling fish as 15 :1. Annual productivity of the 

 invertebrate fauna is twice as great as that of the 

 fish, so the productivity ratio between the two groups 

 is 30:1. 



Dominance 



In contrast to rock-bottom communities, compe- 

 tition for space is not an important factor in mud- 

 and sand-bottom communities. Actual evidence pur- 

 porting dominance to exist is not convincing 

 (MacGinitie 1939). Eelgrass and algae do not ap- 

 preciably react on the physical characteristics of the 

 habitat except to increase the supply of oxygen. 



Starfishes and brittle-stars may exert control to 

 a limited extent where they are numerous by feeding 



on and preventing pelecypods from becoming estab- 

 lished in the community (Clements and Shelford 

 1939). It is possible that fishes may at times modify 

 the species composition of an area, but for the most 

 part it appears that the presence and distribution of 

 species is controlled directly by the physical condi- 

 tions of the habitat (Jones 1950) and that only popu- 

 lation size is modified by predation and competition. 



CORAL REEF BIOME 



Coral reefs are formed by the accumulation of 

 the calcareous skeletons of myriads of organisms. 

 They extend from the sea bottom at depths of 46 m, 

 or rarely 74 m, to slightly above low tide level. The 

 best formation of coral reefs is confined to warm 

 waters above 18°C, although individual species may 

 extend into colder regions (Vaughan 1919, Wells in 

 Hedgpeth 1957). 



Predominant organisms involved are commonly 

 the anthozoan stony corals and organ corals and 

 the hydrozoan milliporids. Some reefs, however, are 

 formed principally by Foraminifera and still others 

 by calcareous algae. All massive coral structures 

 employ calcareous algae as cement. These algae not 

 only thrive in the pounding surf on the windward 

 side of the reef, but by their growth are able to 

 repair damage to the reef caused by storms. Most 

 typical reef-building animals are colonial and of 

 shapes varying from closely compact, globose, or 

 encrusting, to loosely branched or dendritic, depend- 

 ing in part on their exposure to wave action. Each 

 polyp in a colony secretes its own calcareous skeleton 

 and when it dies the next generation builds on top of 

 the old so that the accumulation of a lime structure 

 is fairly rapid. 



The bright yellow or red colors of corals near 

 enough to the water's surface for adequate light 

 penetration are the result of algae, the zooxanthellae, 

 which are either embedded in the body wall or free in 

 the internal cavities. In addition, there are bands of 

 green filamentous algae growing to a depth of 2 or 

 3 cm in the pores of the inert coral skeleton that may 

 have a biomass sixteen times that of the zooxan- 

 thellae. In their photosynthesis, these algae probably 

 absorb carbon dioxide and nutrients from the animal 

 tissues of the coral and liberate oxygen and provide 

 nourishment of value to the animals (Odum and 

 Odum 1955), although this has been disputed. Per- 

 haps because of this symbiotic relationship which re- 

 quires solar radiation, living corals are largely con- 

 fined to the upper, shallower waters. Coral animals 

 actively ingest zooplankton, but apparently not phyto- 

 plankton, from the surrounding water (Hand 1956, 

 Yonge 1958). 



Reefs may be either fringing, barrier, or atoll 



366 Geographic distribution of communities 



