198 THE BIOLOGY OF THE SEA-SHORE 



(Ashcroft, 1899, ^^^ others). The molluscs Tellina halthica 

 and Cardium edule, generally so abundant together on the 

 cockle-beds, are both of very great importance in furnishing 

 food for the fry of the commercial flat-fish at a time when 

 these first become ground-feeders and leave off eating 

 Copepods (Herdman, 1893). The writers themselves have 

 evidence that excessive commercial exploitation of shell- 

 fish beds in estuaries tends to spoil the fishing in those 

 areas, the fish no longer coming in as they once did. 



The food relationships of shore animals are too complex 

 to be satisfactorily exhibited in a diagram, but an attempt 

 is made in the table on p. 197 to indicate some of the most 

 typical food sequences. Three main sources of proximate 

 foodstuffs are shown : the plankton, fixed seaweeds, and 

 general organic debris, any one of which may be the starting 

 point of a series of upward transformations, A forming the 

 food of B, B of C, and so on. The foundation on which the 

 whole structure rests is, of course, the ultimate foodstuffs : 

 the carbon dioxide of the atmosphere and the various sahs 

 enumerated at the beginning of this chapter. 



Retrospect.— Some of the most important conclusions to 

 be drawn from this chapter may now be summarised as 

 follows : — 



The littoral waters constitute the richest food-producing 

 area in the whole sea. This is due mainly to the constant 

 addition, by drainage from the land, of various substances 

 capable of serving directly as food for plants. Not only 

 are the inshore waters a nursery for the plankton of the whole 

 sea, but they support a bottom vegetation of large seaweeds 

 such as cannot exist elsewhere owing to insufficient illumina- 

 tion. The abundance of plant-life in this area is one of the 

 chief reasons for its characteristic wealth and variety of 

 animal forms, though, of course, other factors are concerned 

 as well. In the tidal zone proper, the advantages of this 

 double supply of fixed and floating plants are partly neutralised 

 by the periodic withdrawal of the water. Nevertheless, 

 feeding may still go on in favoured situations. The function 



