366 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 8 



The males come first and remain through the whole winter while the 

 females come later, mate, and start immediately afterward to move on 

 down to the sea. The eggs are laid within 24 hours after mating and 

 are fastened to the small hairs on the pleopods on the underside of the 

 abdomen with a cementlike substance which hardens in salt water. 

 This cementlike substance hardens only in water that has a salt con- 

 tent of more than 2.5 percent, according to F. Buhk. The females, 

 burdened with the weight of the eggs on the pleopods under their abdo- 

 mens, choose to stay on in the deep water outside the river mouths 

 through the winter. As soon as it gets warm in the spring, the tiny 

 larvae escape from the eggs to start to drift about free (pi. 2). In all 

 probability the females hunt up particularly brackish water for this 

 purpose. In June or July, after all the larvae have left the eggs, both 

 males and females set out for the river banks at the mouths of the 

 rivers, where they gradually perish. 



The intermittent stay in fresh water, and these long journeys far 

 inland between birth and death, which both take place in salt water, 

 bring about the peculiar character of the life cycle of these mitten 

 crabs. They cannot repeat these long journeys to reproduce every 

 year or two, which other crawfishes do, because the distances are too 

 great. Breeding has, therefore, been put off to the last part of their 

 life span. But under normal circumstances this single breeding period 

 is compensated by an enormous egg production. The crabs, males and 

 females alike, are therefore completely exhausted and worn out after 

 mating, and waste away gradually. It is a sign of their generally 

 fading strength that they are so covered with barnacles (Balanidae) 

 (pi. 3) in the summer during their stay in the North Sea shallows, 

 that they hardly move about at all; indeed they often cannot move even 

 their mouth parts. They lack the strength to shed their shells which 

 would enable them to get rid of these cumbersome barnacles. 



Whereas the eggs need pure salt water to mature, the larvae leave 

 the eggs in very brackish water. The prezoea, a free-drifting larva, 

 leaves the egg and develops immediately into a 1.7-mm zoea (pi. 2, a). 

 Subsequently, three additional larval stages follow. These larvae 

 probably move gradually into less brackish water. The last zoea 

 develops into the final larval stage, the 3-4-mm-long megalopa (pi. 2, 

 6). The change from the free-drifting life of the zoea, with long sus- 

 pended thorns and a rudderlike tail, to the more uneventful life of the 

 crab on the bottom takes place in the megalopa. The megalopa are 

 brought into fresh water with high tide and develop there into tiny 

 mitten crabs, 2.5 to 3 mm long, the first stage of bottom life (pi. 2, c). 



This migration from salt to fresh water in the larval stage and from 

 fresh to salt water as adults toward the end of their life, is a distin- 

 guishing habit of the mitten crab, and, when considered biologically, 

 the mitten crab appears to be in the act of becoming a fresh-water 



