COMMUNITIES OF THE SEA BOTTOM 323 



ous of these types is that attached to, or resting upon, the substratum, 

 as exemplified by barnacles, gastropods, large echinoderms, and mus- 

 sels on rock or other hard bottom. These may be called barnacle- 

 gastropod communities. The second life-habit type burrows into soft 

 bottom and includes clamlike mollusks, worms, and a few wormlikc 

 echinoderms.^ These may be called bivalve-annelid communities. A 

 third type is represented by corals, especially those associated with 

 coral reefs; the life forms of these resemble plants, as indicated by 

 the term zoophyte commonly applied to them. The predominants of 

 most communities thus combine two or more sedentary life habits and 

 life forms with those of the strictly motile constituents. 



From a physiological viewpoint, bottom communities are separable 

 into two principal types: (1) those that do not tolerate exposure to 

 the atmosphere and are practically always submerged in water, and 

 (2) those that tolerate or require exposure to the atmosphere and 

 occupy stable and usually hard substrata exposed to the full force 

 of the air with each important fall of tide. The latter ordinarily be- 

 came subtidal only locally, when under peculiar conditions of salinity 

 or temperature (Huntsman, 1920). The former are divisible into two 

 subgroups: (n) those whose habitat (tidal clam beaches) may be par- 

 tially exposed to the atmosphere during low tides without exposing 

 the community constituents, and (6) those that are always well below 

 low tide. 



Aside from the corals, the location and relation of the two remain- 

 ing life-form types and the two physiological types are essential to an 

 understanding of marine bottom communities in general. The physio- 

 logical types are readily separated, superficially at least, into two 

 categories, those requiring and those not requiring or tolerating rhyth- 

 mic exposure to the atmosphere. 



As noted above, strictly speaking, the (clamlike) bivalve-worm 

 communities are never rhythmically exposed to the atmosphere with 

 tidal changes. In the so-called clam beaches, both the bivalves and 

 the worms are restricted to sands having a large water-holding capac- 

 ity (Bruce, 1928). When the tide is out, they retract their fleshy 

 organs and remain in the w^ater held by the sand, and even if this 

 water is partially withdrawn, they are not exposed to the sun and 

 atmosphere. Those bivalve-worm communities that reach above the 



1 The first two life-form types are respectively the "on fauna" and "in fauna" 

 of Petersen. Unfortunately, in describing communities Petersen did not stress 

 the motile influents. The term benthic is often applied to the species or com- 

 munities living on or in the bottom, but it is not used here because it seemed 

 confusing. 



