350 MARINE BIOTIC COMMUNITIES 



the gastropods, to the relatively small size of the individuals also. 

 The great majority of bivalves and annelids are stationary; they con- 

 stitute the most important constituents as regards competition for 

 space and reaction on the substratum. 



Blegvad (1914) made a study of the nutrition of the bottom in- 

 vertebrates, classifying the foods as follows: 



1. Plant food consists of fresh-growing benthic plants. 



2. Detritus is divided into two classes: 



(a) Plant detritus which is floating plant material; some of it 

 is fresh. 



(b) Bottom detritus, which consists of fine particles of j^lant 

 and animal material settled on the bottom. 



He not only examined the stomach contents of the animals, but 

 also made aquarium experiments and observations. The material 

 studied includes the more important representatives from the sta- 

 tionary and slow-moving bottom constituents of the Macoma-Mya, 

 and Echinocardium-Thyasira biomes. 



The bivalves were found to live entirely upon detritus. A few in- 

 vertebrates are herbivorous detritus-eaters; Rissoa, a small snail 

 abundant on Zostera, is an example. Most Polychaeta are detritus- 

 eaters. All the serpent stars of the genera Ophiopholis and Ophiura 

 (Ophioglypha) , which play a prominent part in the communities, are 

 carnivorous detritus-eaters. Echinocardium belongs here also; it feeds 

 on detritus and young bivalves. 



Petersen (1918:17) has pointed out the scarcity of bivalves where 

 serpent stars are abundant and credits them with the control of com- 

 munities. In the second paragraph of the quotation, by "grounds near 

 land" he refers to those areas occupied by the Macoma community. 

 He states his conclusions in the following terms: "Most common ma- 

 rine animals living on the bottom commence their existence as minute 

 larvae in the water, and sink to the bottom at a very early stage, as 

 for instance the bivalves. And it is remarkable to note how in those 

 communities where the Amphiura spread their arms abroad, forming 

 a network in the bottom (see PI. IV and V), extremely few bivalves 

 are found at all. The young bivalves will here doubtless as a rule be 

 devoured, while still quite small, by the Amphiura, and only a very 

 few individuals of certain species manage to survive. Both in shallow 

 water near the coasts, and farther out where it is deeper, where few 

 or no Amphiura are found, there are quantities of small bivalves (see 

 PI. VI and I, II and III) of many different species, for instance, Mac- 

 tra, Tellina, as also in summer on grounds near land, where few or 



