THE BEXTHAL 195 



communities of the Puget Sound benthos by Shelford 15 and his stu- 

 dents, but further investigations are required to determine the extent 

 of their validity. 



The coastal conditions are not confined to the immediate vicinity 

 of the coast but extend into shallow water far from shore, frequently to 

 the edge of the continental shelf. The bottom of the North Sea 1G is 

 predominantly loose, composed of sand, sandy ooze, and clay. Its life 

 is characterized largely by the abundance of echinoderms; starfishes 

 (Asterias rubens, Astropecten irregularis) occur at all depths, as do 

 the irregular sea urchins, Echinocardium and Spatangus; the ophiuran 

 Ophiura ciliaris may literally cover the surface. Certain snails 

 (Neptunea, Sipho, and Buccinum) are abundant and widely dis- 

 tributed. The higher Crustacea are few in kinds and in individuals, 

 though the smaller amphipods and Cumacea are abundant, affording 

 rich food for fishes, especially juvenile fishes, whose stomachs are often 

 crammed with them. Where shells of mollusks are heaped up on the 

 loose bottom, sessile animals take up residence, especially hydroid 

 polyps, with their associates, small caprellids and nudibranchs, adding 

 to the variety of animal life in this habitat. The fish fauna is rich in 

 this region of shallow water, much more so in general than over deep 

 water. Most so-called coastal fishes are caught above the continental 

 shelf. One can gain an insight into the wealth of the fisheries of a 

 country by merely examining a bathymetric chart.* 17 



Fauna of the tidal zone. — From low tide mark upward the lit- 

 toral fauna becomes more and more impoverished (Fig. 29) , on account 

 of less favorable conditions of life. The tides uncover a belt of varying 

 width twice daily. Animals of this area must be able to withstand the 

 temporary withdrawal of water; rains subject them to fresh water at 

 times; they are exposed to the heat of the sun in the summer and to 

 extremes of cold in winter. Food supply is also much less abundant 

 than in and below the zone of plant growth. In consequence, only 

 euryhaline and eurythermal animals are able to live within this belt 

 of emergence. 



A number of animals maintain themselves in this situation, the 

 more easily as the stress of competition is reduced by the existence of 

 physical and chemical restrictions. A number of lamellibranchs dig 

 themselves in between tide marks, so Tellina, Scrobicularia, Mactra, 

 Mya, and the excessively resistant Cardium edule. 



* A number of European food fishes attain their greatest abundance in the 

 North Sea, as for example Gaclus aeglefinus and G. merlangus, Pleuronectes 

 plalessa and P. microcephalics, and Rhombus maximus.™ 



