174 THE BIOLOGY OF THE SEA-SHORE 



economic importance the fact that they are very active 

 forms leads them further afield and renders their migrations 

 more likely to be remarked. 



Scientific investigation has now confirmed the fact, 

 long known to fishermen, that the adult crab undertakes 

 definite migrations of considerable extent. In September 

 both hard and soft crabs begin to move outward from the 

 tidal area to deeper water, travelling to a depth which varies 

 with the locaUty, but is generally between twenty and thirty 

 Inshore Migraltbn 



ApriJ 



Off5hore niarattori 

 Fig. 13. — Migration of edible crab (after Pearson). 



fathoms. The crabs remain in the offshore waters from 

 December to February, and it is here that the females 

 spawn (Pearson, 1908). The return migration begins in 

 February and by May the majority of the crabs are probably 

 close to the shore again. In the warm inshore waters the 

 larvae are hatched, and in the following autumn the cycle 

 recommences. The above diagram, which we reproduce 

 from Pearson, summarises the main facts of the cycle. 



Williamson (1900) considers the reasons for the migration 

 to lie in (i) the influence of temperature, and (2) the necessity 



