('i)in/)i>sili(>n and chararU'ristics 



Important plants in this biomc are the marine 

 eelgrass which is a seed plant, and green algae, par- 

 ticularly the sea-lettuce, which grows in sheets either 

 attached to the substratum or lying fragmented over 

 large areas, and Entcromorpha. which grows in tufts 

 or tangles. Occurring on eelgrass and sea-lettuce may 

 be several kinds of epiphytic algae. These plants 

 form extensive stands and are important to aninials 

 for attachment, shelter, and food. Eelgrass was al- 

 most eliminated from the Atlantic coast in 1931-32, 

 possibly because of a protozoan disease. This dis- 

 turbance had a profoundly deleterious effect on the 

 abundance of many animals, including the brant, a 

 bird that depended on it almost exclusively for food, 

 and on scallop and other coastal fisheries. Twenty 

 years later there was evidence that eelgrass was re- 

 covering much of its former abundance (Cottam and 

 Munro 1954). 



Predominant animals are pelecypods, polychaete 

 worms, particularly Arenicola and Nereis; starfishes, 

 brittle-stars, sea cucumbers, crabs, amphipods, and 

 snails. Populations may run to several thousands of 

 individuals per scjuare meter. A variety of small fish 

 occur here. Birds include sandpipers, plovers, and 

 herons. 



The biome is world-wide in distribution, but the 

 characteristic life-forms are represented by different 

 species locally. Thus a number of secondary com- 

 munities (biociations ) may be recognized (Petersen 

 1914, Jones 1950. Thorson in Hedgpeth 1957). 



Many of the animal constituents in this biome 

 are burrowing forms. The substratum of mud and 

 sand holds considerable water and when the tide is 

 out on exposed flats, pelecypods, worms, and other 

 animal constituents retract their fleshy organs into 

 their burrows or shells and remain in a water satu- 

 rated environment (Hesse, Allee, and Schmidt 1951). 

 They are thus never rhythmically exposed to the 

 atmosphere with changes in the tide even on the 

 shore. Furthermore, most forms are generally toler- 

 ant of low' oxygen and high carbon dioxide con- 

 centrations. In order to maintain respiration when 

 retracted in their underground burrow-s these animals 

 have long siphons, sometimes longer than their 

 bodies, or long tubes or canals that extend to the 

 surface. Through these they maintain a circulation of 

 water, often by means of special pumping organs. 



Burrowing crustaceans have setose appendages, 

 modified for digging, and small eyes. Those that re- 

 main near the surface have long antennae and robust 

 bodies ; those that live in deep burrows have short 

 antennae and slender bodies. For the same reason, 

 burrowing clams that remain near the surface have 

 heavy shells while those that burrow^ deeper have 

 more fragile shells. These clams either have wide. 



slimy feet and small shells ami crawl with ease 

 through the sand, or have a slender foot that can 

 (■x])and at the end to give enough anchorage so 

 that the animal can pull itself downward into the 

 -sand or mud (Pearse et al. 1942). 



The infauna also includes many microscopic 

 forms, such as nematodes, flatworms, co])epods, os- 

 tracods, foraminiferans, and other jjrotozoans. These 

 small organisms may be enormously abundant in 

 number of individuals. In respect to the ciliated pro- 

 tozoans, some species are ubiquitous, but other species 

 are characteristic of this habitat (Faurc-Fremiet 

 1950). Some species of ciliates occur in the inter- 

 granular spaces of the sand and mud, other species 

 are associated with the surface film of diatoms (Webb 

 1956). 



In contrast to the great variety of infauna in this 

 biome, the epifauna is more restricted, although some 

 species are abundant. The fiddler crab browses in 

 great armies on beaches left exposed by the tide, 

 but retreats into its burrow and plugs up the opening 

 when the tide comes in. The characteristic ghost crab 

 of sandy beaches of middle latitudes spends most of 

 its time above high tide level but must return occa- 

 sionally to water to dampen its gill chamber. 



Zonal ion 



Zonation is less conspicuous than on rock)- 

 shores because of the prevalence of burrowing forms 

 and of running and swimming species that move up 

 and down with the tides (Brady 1943). Some evi- 

 dence for zonation occurs on sandy beaches with crus- 

 taceans (Dahl 1953) and with pelecypods and annelids 

 (Stephen 1953). A more pronounced change in spe- 

 cies composition occurs in the sublittoral zone ; in 

 the North Pacific this change comes at a depth of 3 m 

 (Shelford et al. 1935). Pelecypods and annelids still 

 predominate (Holme 1953, Sanders 1956). 



Food roactions and productivity 



Bacteria are more abundant on mud bottoms 

 than in the open sea (ZoBell 1946). On mud flats 

 they may average 10 million cells/per cc of mud, or 

 with a biomass of nearly 40 g/nr*. Bacteria are, of 

 course, vital for the decomposition of organic debris, 

 dead organisms, and wastes, but may also be used 

 directly as food. Assuming that the bacteria divide 

 rapidly enough to increase their biomass 10 times per 

 day, this gives a 24-hour production of -K)0 g/m', 

 mostly concentrated in the 5 cm beneath the surface. 

 Many protozoans and zooplankters feed on bac- 

 teria and detritus, and a large number of mud and 

 sand dwelling invertebrates of larger size, such as 



Marine biomes 365 



