ZONES OF THE SEA-WHERE THEY LIVE 19 



helps keep them near the surface, but oddly enough some pelagic fishes such 

 as mackerels have no swim bladder. Oil and fat help the mammals rise to the 

 surface, but even they almost never cease moving during the whole of 

 their lives. 



Plankton, on the other hand, is largely dependent upon currents. Most plank- 

 tonic animals and all planktonic plants are microscopic or small, like crustaceans, 

 pelagic tunicates, various larvae, arrow worms, protozoans, pteropods, and 

 diatoms, but some are very large, such as the giant lion's-mane jellyfish, Cyanea, 

 the basking shark, and the ocean sunfish (Hesse, and others, 1951). These 

 large planktonic animals are good swimmers, but they usually just drift with 

 the currents. All of the small plankton depend heavily on flotation mechanisms 

 to keep from sinking. The specific gravity of sea water is 1.02 to 1.03 and that 

 of living matter is 1.02 to 1.06. The flotation mechanisms that compensate for 

 this slight overweight are several. Most protozoa, many larvae, and many 

 crustaceans have extensions of their body or skeletons so that they off^er great 

 resistance to water. These extensions work best in water of rather high viscosity 

 and are found only on very small animals. 



Other animals and plants reduce their specific gravity by various means, such 

 as reducing the weight of their skeletons (pteropods, protozoa, and crustaceans), 

 taking on extra body water (jellyfish), storing oil droplets (fish eggs, basking 

 shark, penguins, whales, ocean sunfish), or by storing gases (Portuguese man- 

 of-war, bony fishes with an air bladder, chambered nautilus). 



The Ahyssal-Pelagic Zone. The animals of this zone depend entirely upon the 

 rain of dead plankton falling from above or on other members of their community 

 for food. There are no herbivores, and there is usually a direct relationship 

 between the abundance of plankton at the surface and the abundance of life 

 in this zone. Fishes and cephalopods predominate, and these are usually small 

 although exceptions to this rule occur. For instance, the giant squid is the 

 largest of cephalopods. Some protozoa are larger at depths, but this is related 

 to increased viscosity of deep cold waters. 



This is a very stable and uniform zone where changes in light and temperature 

 and the movement of water are very slight. Since there is very little water motion, 

 animals with fragile bodies, such as the oarfish (ribbonfish), are common. Many 

 of the fishes are elongate in shape (the chimaera and the frilled shark). Low 

 temperatures in this zone as well as high pressures prevent the efficient formation 

 of lime (calcium carbonate) so skeletons and shells are weak. Pressure also 

 causes protoplasm to take up more water so tissues are gelatinous. Archaic 

 animals are common because of a comparative lack of competition probably. 



All over the world, the abyssal zones are rather uniform resulting in the 

 wide distribution of the deep-sea animals. 



The Abyssal-Benthic Zone. This is perhaps the least well-known zone of the 

 sea where cold, lightless, almost motionless waters exist. In a few places, a rock 

 bottom is found, but most of the bottom is covered with clay or ooze, the latter 

 being composed of the countless skeletons of planktonic animals and plants. 

 In shallower places, the protozoan oozes cover huge portions of ocean bottom 

 (Chapter 7). This is particularly true of the North Atlantic where the protozoan 

 Glohigerina, holds forth. A red clay bottom is typical of much of the Pacific and 

 is a very sterile medium for life. 



