COMMUNITIES OF THE SEA BOTTOM 351 



no echinoderms at all are found. That the great majority of these 

 young individuals never attain full growth, is doubtless primarily due 

 to the fact that the environment is here unfavorable in the long run; 

 the action of the waves, for instance, will at times be too violent; very 

 low water will kill off numbers of the young, as also severe cold in 

 winter, etc., presumably the same factors which account for the ab- 

 sence of echinoderms in the same localities. It is in such places as 

 these that the species of the jMacoma communities can live and thrive 

 continually; they are the only forms that are able to withstand the 

 severe conditions prevalent in a degree sufficient to ensure the mainte- 

 nance of the species. 



"It is remarkable, having in mind the hardiness of these Macoma 

 species, that they should not be found deeper out in the Kattegat, 

 throughout the whole of the Venus area, where we might imagine they 

 would find the most favorable environment of all, and where Mytilus 

 also make their appearance on any buoy set out, but hardly ever live 

 on the bottom itself. It cannot be the depth which keeps the Macoma 

 species away from these areas; we find for instance, Mya arenaria, 

 Cardium edule, Macoma baltica and Hydrobia out in at least 20 

 meters depth in the Baltic where their predominance is undisputed; 

 in the Baltic, however, east of Gedser, there are, as we know, no 

 echinoderms, nor are such found in the low water on the shores of 

 the Kattegat. I must, therefore, suppose that it is just certain echino- 

 derms which prevent the animals of the Macoma community from 

 spreading over larger areas than they occupy in fact." 



The fishes have potent effects in the communities, but these are 

 greatest in determining abundance, life span, and replacement among 

 those community constituents that are able to exist with them, but 

 their effect in eliminating certain forms from the communities en- 

 tirely has been but little investigated (Jensen, 1919). However, the 

 work of Blegvad (1925) shows the coaction of fishes on the bottom- 

 inhabiting species to be sufficiently great to class them among the 

 dominants. All the community constituents influence the bottom, 

 especially w^here this is little disturbed by waves and currents (Moore, 

 1931, a, b) ; succession by reaction is to be expected but cannot be 

 followed without great labor over long periods. Here again, as in 

 fresh water, dominance is as much a matter of coaction as of reac- 

 tion, if not more. 



The relatively short life histories and life span of marine plants 

 and animals and their frequent fluctuations in abundance led Petersen 

 to select single species as indicators of communities. For the selection 



