586 THE BIOLOGY OF MARINE ANIMALS 



it executes a series of manoeuvres to recover its associate, first detaching 

 the anemone from the bottom and then placing it within its pincers. The 

 crab seems to derive most of the benefit from this relationship. 



In the hermit crabs (Paguridea) the instinct to form associations with 

 certain other organisms is particularly well displayed. The co-partners are 

 sometimes placed by the hermit crabs upon the gastropod shells which 

 they inhabit, and include sponges, hydroids, zoanthids, actiniarians and 

 bryozoans. Frequently the association is very regular in that a given species 

 of hermit crab has a particular affinity for some species of sponge or 

 coelenterate. The European hermit crab Eupagurus bernhardus is usually 

 found with an anemone Calliactis parasitica on its shell, and sometimes 

 several may be present. The association in this case is not obligatory for 

 the anemone, which often occurs free-living as well. When the hermit crab 



Fig. 14.3. Melia tessellata from the Hawaiian Is., 

 Bearing Actinians in its Pincers. (After Duerden, 1905.) 



changes its shell, as it does on occasion when the shell becomes too small, 

 it transfers the anemone to the new whelk shell that it adopts. A smaller 

 proportion of hermit crabs are covered by the hydroid Hydractinia instead 

 of Calliactis. In this case the hydrorhizal mat of the hydroid colony often 

 extends beyond the mouth of the shell, thus enlarging the living space 

 available to the crab. In these associations the coelenterates with their 

 batteries of nematocysts confer some protection on the crab, and them- 

 selves benefit by being carried about to new situations and by sharing the 

 food caught by the crab (57, 73, 77). 



More intimate is the relationship between the small hermit crab Eupagu- 

 rus prideauxi and the cloak anemone Adamsia palliata (Fig. 14.4). The latter 

 wraps itself around the shell inhabited by the crab, and as the anemone 

 increases in size it adds to the effective capacity of the shell so that the 

 hermit crab finds increased accommodation for its own growth. The shell 

 in this instance is merely a base upon which the cloak anemone can estab- 

 lish itself, and contributes little towards sheltering the crab. The two 

 animals are only found in association, and their intimate relationship par- 

 takes of the nature of mutualism. The American hermit crab Eupagurus 



