CHINESE MITTEN CRAB — PANNING 367 



animal. This transition demands important readjustment in their 

 (body) system. Their life substance has the same salt concentration 

 as sea water and therefore differs greatly from that of fresh water. 

 Whereas animals in sea water, with equal salt concentration inside as 

 outside the body, are not imperiled through any osmotic action, life in 

 fresh water demands equalization, and the constant absorption of 

 water by osmotic action through unprotected places in the body, as for 

 instance through the gills, is retroacted by increased water outlet 

 through the kidneys. Otherwise the plasma would be destroyed 

 through continuous swelling. The migration of the tender mitten 

 crab larvae from the sea into fresh water with its added requirement 

 for body functions, therefore, implies strong intrinsic power. 



THE BREEDING PERIOD 



Although the time of mating and the laying of eggs is fairly fixed, 

 beginning about the end of October and lasting until January, the 

 larvae's hatching time changes very much depending upon the weather. 

 When springs are warm, which does not happen often, at least not 

 in northwestern Germany, the larvae hatch sometime between the 

 end of March and May or June. But the time for hatching usually 

 comes considerably later and lasts until far into July. Thus their 

 whole development is, of course, delayed. During warm springs, 

 tho megalopae may appear in July or August in the fresh water 

 below Hamburg, and there develop into the first bottom stage, but 

 during unfavorable weather their appearance is delayed until October. 

 In 1933, when it was exceptionally warm, the young mitten crabs 

 reached an average length of 10 mm in October but again in 1935 and 

 1936, when the weather was unfavorable, their average size was only 

 4 to 7 mm when they went into winter rest. 



THE WANDERINGS OF THE MITTEN CRAB 



The migration of the larvae into the fresh water in the upper tidal 

 regions is (probably) aided by the tidal currents. These feeding 

 grounds are very rich, and the wanderings could have continued to 

 end here, as they did up to the beginning of the twenties, if the number 

 of the crabs had not increased so tremendously. It was this enormous 

 increase that forced them to move on farther upstream in their 

 search for food. But the crabs are too small at first to be able to 

 make their way upstream against the strong current. During their 

 first summer in their larval stage they are brought with the tidal 

 current into fresh water, and during their second summer they stay 

 there in the coast regions where the water recedes at ebb tide until 

 they grow sufficiently to enable them to wander on. 



