CHAPTER XVIII 

 MORPHOLOGICAL EFFECTS OF CROWDING 



In addition to its effect upon primary sexual characters through 

 sex determination, which has been discussed in the preceding chap- 

 ter, crowding may produce morphological changes in secondary 

 sexual characters especially noticeable in animals such as Bonellia, 

 which have a strong sexual dimorphism. Crowding also exerts de- 

 cided influences upon structures, entirely apart from its effect upon 

 sex. Casual observation shows that a tree grown in the open coun- 

 try has a different growth form from that shown by the same sort 

 of tree grown in a forest. Similarly, when sessile marine animals, 

 such as barnacles, ascidians, corals, sea anemones and sea mussels, 

 grow in closely packed masses upon a rock or wharf piling, their 

 growth form differs from that shown if they grow separately. With 

 these sessile animals the changes associated with crowding appear 

 to be due largely to the limitations imposed by the physical contacts 

 estabhshed under conditions of close aggregation. 



The effect of crowding upon physical form is not limited to sessile 

 organisms. There is the well-known case of the free-swimming pelagic 

 tunicate^ Salpa, with its alternation of the asexual solitary, casklike 

 form with the quite differently shaped aggregated form of the mem- 

 bers of a Salpa chain. These latter are sexual individuals that, ap- 

 parently because of crowding, have lost their regular barrel-like ap- 

 pearance and are rather rounded, ovoid, or fusiform. 



With non-sessile animals which lack the physical connections of 

 a Salpa chain, even though the crowding be less dense, there are 

 cases of marked morphological changes other than mere decrease 

 in size. Whitefield (1882) observed that Lymnaea, a snail, when 

 grown in a small volume of water for three successive generations, 

 had not only become much reduced in size but had suffered other 

 physical changes as well: the male organs did not develop, and the 

 liver was also much smaller in proportion to the other organs, as 



311 



