COMMUNITY ORGANIZATION: STRATIFICATION 



459 



jacent to the San Juan Islands ( Shelf ord 

 et al, 1935). 



All such sublittoral communities, al- 

 though covering ecologically distinct por- 

 tions of the bottom, and generally distrib- 

 uted in strata in accordance with depth 

 of submergence, may be fed upon by wide- 

 ranging fishes which do not necessarily be- 

 long to a limited sublittoral area. These 

 latter, often of great commercial value, 

 range through waters above the continental 

 shelf (Blegvad, 1916, 1925, 1930; Hesse, 

 Allee, and Schmidt, 1937, p. 195; Clements 

 and Shelf ord, 1939, pp. 349-352). Since 

 these fishes can be assigned to the upper 

 200 meter stratum, but, in part at least, 

 obtain their food from beyond the edge of 

 the continental shelf, the artificiahty of 

 thinking of any single stratum in terms of a 

 biome or formation is apparent. It will be 

 remembered that the larger mammals and 

 birds on land may not be strictly confined 

 even to such a major community as a forest 

 or a prairie; in fact, they may not even be 

 confined to the land, since they take part 

 in fresh-water communities. Such nektonic 

 components as the fishes we are discussing 

 are more comparable to the large herbivores 

 and carnivores of forests. This complete 

 interdependence within the major com- 

 munity of the sea receives documentation 

 in the following chapter. 



Archibenthic Zone 



Where the continental shelf becomes 

 relatively rapidly declivous, near the 200 

 meter line, the sublittoral gives way to 

 the third great horizontal stratum, the 

 archibenthic or deep-sea zone (Fig. 157). 

 This latter area is steep-walled, descending 

 from 200 to about 2000 meters. Its fauna 

 has been well investigated ofiF Norway, 

 and within its depth there is considerable 

 intrazonal stratification, in addition to the 

 expected lateral or geographic variation 

 over the sea floor. This range in depth is 

 accompanied by a similar range in pressure. 

 Two examples will suffice to show the 

 faunal-depth variation within the deep-sea 

 zone, one illustration near either extremity 

 of the stratum. 



In fjords off the Norwegian coast, below 

 300 meters, the water temperature is 

 relatively constant, there is a small amount 

 of blue-violet light, and the pressure is not 

 less than 400 pounds per square inch. The 



fauna is characterized by deep-sea sponges 

 which may have rootlike extensions tor 

 anchorage to rock, or stilthke if in the mud. 

 Geodia, one of these sponges, forms large 

 rounded masses many teet across. Reddish 

 sea cucumbers are common in the mud, and 

 starfish and brittlestars are abundant. 

 Brachiopods are numerous, as are bivalves, 

 including the characteristic Lima excavata. 

 In deeper waters, near or at the bottom 

 of the archibenthic zone off Norway, e.g., 

 at 1600 meters, the water temperature 

 has not changed appreciably, but there is 

 no light, and the pressure has increased 

 to about one ton per square inch. At 

 this depth the characteristic aspect is 

 given by forests of sponges (Cladorhiza). 

 The treehke branches of these sponges 

 support a host of crustaceans, worms, large 

 brittlestars, and other organisms, some of 

 which are luminescent (Sars, cf. Clark, 

 1925). They squirm, walk, and dart 

 through the branches as do arboreal animals 

 of epigean forests. Sea spiders (Pycnogo- 

 nida) stalk over the floor ooze, and suck out 

 the organic fluids of the tree-sponges, 

 paralleUng the sap-sucking habit of terres- 

 trial animals. 



Abyssalbenthic Zone 



Throughout the world, seaward of the 

 archibenthic or upper abyssal is the abyssal- 

 benthic or lower abyssal zone of the sea 

 floor; this region may extend downward to 

 10,000 meters. With the exception of a 

 possibly extensive bacterial flora, the 

 abyssal organisms are all animals; again 

 the life of this zone is not self-sustaining, 

 since the occupants are largely dependent 

 upon the occupants of the photic zone. 



The abyssalbenthic region may be said 

 to have no seasonal variation. The water is 

 relatively still. There is no fight penetrat- 

 ing from above, the area being in total 

 darkness save for faint bioluminescence. The 

 temperature is constant at slightly above 

 zero degree C. Dissolved oxygen is reduced 

 below that of the surface, though it may be 

 more plentiful than at somewhat higher 

 levels. Pressures are tremendous. 



The lower abyssal fauna is novel. There 

 is a general behef that its animals, adjusted 

 to the peculiar conditions of their environ- 

 ment, do not stray into the zone above, or 

 if so, that such movements are rare. The 

 abyssal species are thought to be few in 



