18 UNDERWATER GUIDE TO MARINE LIFE 



used in general in the former sense.) Therefore, the substrate of the sea, in 

 which anu over which the life of the sea hves, is three-dimensional in contrast 

 to the two-dimensional substrate of the terrestrial world. Flying animals, such 

 as birds, insects, and bats, like fishes, live in a three-dimensional substrate, air. 



The ocean is divided into two large provinces, the neritic, over the continental 

 shelf, and the oceanic, over the deep sea. Both of these are subdivided into two 

 subprovinces, the henthic, or sea bottom, and the pelagic, or open water. (The 

 divisions of the provinces of the sea are given in figure 1.) 



Since "pelagic" simply means "living in the open sea," it usually refers to 

 animals of several types, subdivided according to swimming ability. "Planktonic" 

 (from flanktos, meaning "wandering") refers to animals whose swimming 

 powers are not great enough to combat water movements. These usually small 

 or microscopic organisms, about 60 per cent of which are diatoms, are the most 

 important life of the sea. The word "nektonic" is applied to animals that move 

 freely through the water. The highest development of free movement is found 

 in fishes and the re-entrants. The use of these terms is rather simple. For instance, 

 the tuna is largely oceanic-pelagic-nektonic. A snail or a scorpion fish is neritic- 

 benthic. Some animals overlap in zones. A bottle-nose porpoise is pelagic-nektonic 

 but is both oceanic and neritic. Some groups are exclusively pelagic, such as 

 diatoms, radiolarians, jellyfishes, arrow worms, pteropods, and some prochordates. 

 In order of abundance among animal pelagic forms, Hesse and others (1951) 

 list copepod crustaceans, pteropods, siphonophores, arrow worms, octopuses, and 

 squids, then fishes. Diatoms are more numerous than any of these. 



Exclusively henthic groups include sponges, sea squirts, brachiopods and 

 bryozoans. Most echinoderms, worms, clams and oysters, and snails are also 

 henthic. 



THE OCEANIC PROVINCE 



The water beyond the continental shelf is divided into zones according to 

 light. For practical purposes, it is best to consider two major zones to be present 

 rather than three previously set up in the section on light. Thus, there is a 

 lighted (euphotic) zone and a dark or abyssal (aphotic) zone. The dysphotic 

 or dimly lit zone is a transitional zone between these two. 



The Lighted Oceanic-Pelagic Zone. This zone is, of course, not well-defined 

 from the lightless zone and varies greatly in depth. In general, it is never more 

 than 600 feet deep. All nektonic-pelagic animals, whether they are oceanic or 

 neritic, must be alert or protectively colored or both because of a complete 

 absence of hiding places. In keeping with this, it is here that are found the 

 swiftest swimmers. Coloration is usually countershaded or obliterative (Chapter 

 2) and is composed of blues or greens above and white or silvers below. The 

 mackerels and tunas are perfect examples of this. 



The fishes, squids, and mammals are the dominant nektonic-pelagic animals 

 since their powers of swimming are best. There are no large plants and there is 

 no organic debris (detritus) in this zone, so all of the nektonic species depend 

 upon plankton or plankton-eaters either directly or indirectly for food 

 (Chapter 2). 



The nektonic species are slightly heavier than sea water and must swim 

 slowly but constantly to keep from sinking. The swim bladder of the fishes 



