666 Ecology 



Beyond this, to a depth of about 100 meters, are various calcareous sea- 

 weeds ; beyond this, bacteria are the only plants normally present. The 

 bottom fauna of this zone consists of tremendous numbers of the proto- 

 zoans of the order Foraminifera. In addition there are many sponges 

 which in turn are the habitat for innumerable smaller animals: sea 

 urchins, sea cucumbers, crinoids, brittle stars, coelenterates, worms, 

 molluscs, and numerous crustaceans. Although there is some food 

 manufacturing by plants in this zone, most of the animals rely upon 

 food straining or being carnivorous for their subsistence. Above this 

 extensive bottom fauna is the neritic province. Here live many active 

 fish as well as masses of floating plankton. Many of these plankton 

 are algae, which are the basis of the food chains in the ocean. It is 

 in this zone, above the continental shelf, that some of the important com- 

 mercial fish are obtained. 



Archibenthic Zone. — This zone extends from the 200 meter depth 

 to about 2,000 meters. It begins where the continental shell falls off 

 rapidly to the great depth of mid-ocean. Below 300 meters there is 

 only a small amount of blue-violet light, the pressure is more than 

 400 pounds per square inch, and the water temperature is constant. 

 In these areas there is not sufficient light to allow algae to grow ; thus 

 the fish and other organisms dwelling here must rely on food falling 

 from above or being carnivorous. On the bottom are great sponges, 

 some many feet across. Many of these have stiltlike attachments for 

 gaining a foothold in the oozy bottom. Echinoderms, including red- 

 dish sea cucumbers, starfish, and brittle stars, are abundant. There 

 are also many brachiopods and bivalve molluscs. At about 2,000 

 meters, the limits of this zone, the water still has the same temperature, 

 but there is no light and the pressure is about one ton per square 

 inch. Here there is a veritable forest of sponges ; upon these live many 

 other smaller animals including crustaceans, echinoderms, and worms. 

 Pycnogonids walk with their stiltlike legs over the soft bottom. Here 

 many of the organisms are luminescent. 



Abyssal-Benthic Zone. — This last zone extends from the 2,000 

 meter depth to the tremendous depths. Here the water is only slightly 

 above 0° C. in temperature and the pressure is enormous. In it are 

 found the fantastic, voracious fish with their gaping mouths and 

 small bodies. Many have elaborate luminescent organs. The bottom 

 is soft and oozy, and animals must have some means of supporting 

 themselves in the soft sediment. There are many sessile animals includ- 



