66 



A BIOLOGY OF CRUSTACEA 



I9SI 1952 



ACE IN YEARS 



Fig. 31. Growth curves of Crustacea. A, a single 

 female of Daphnia magna, reared in the labora- 

 tory. B, Balanus crenatus (Cirripedia) grown in 

 conditions of continual submergence. Note the 

 stoppage of growth in winter of the specimens 

 settling in April, and the way in which the 

 specimens settling in September catch up with 

 those settling earlier (after Barnes and Powell, 

 1953). C, Cancer pagurus (Decapoda, Brach- 

 yura) the edible crab of Europe. (Based on data 

 given by Pearson, 1908). 



The actual amount of 

 growth which occurs at 

 each moult varies with 

 the age of the crusta- 

 cean. Sometimes no 

 growth occurs at all; 

 rarely, as in some indi- 

 viduals of Daphnia 

 longispina, a decrease in 

 size is found towards the 

 end of life. By measur- 

 ing the increase in size 

 at each moult, or by 

 measuring an individual 

 at intervals throughout 

 its life, a curve may be 

 drawn which shows how 

 growth varies during a 

 lifetime. Three examples 

 are given in figs. 31 a-c. 

 It is important to notice 

 the difference in the 

 time scales, and to realise 

 that curves of this sort 

 can be considerably 

 modified by environ- 

 mental conditions. 



The growth of barn- 

 acles, once they have 

 settled, presents some 

 interesting differences 

 from the growth of 

 more normal Crustacea. 

 The tough outer shell is 

 not cast off, but is added 

 to at its base, so that the 

 shell increases in height 

 and basal diameter. The 

 more delicate coverings 

 of the limbs and body 

 within the shell are cast 



