GROWTH AND LIFE HISTORIES 8l 



drop in the average size of mature females. A diagrammatic inter- 

 pretation of this cvcle is given in tig. 38. The cvcle is not always so 

 straightforward, because ponds are complicated and variable places. 

 It is easy enough to rind populations which have outbursts of sexual 

 reproduction, involving the appearance of males and resting eggs, 

 at intervals throughout the summer. 



The brackish water isopod Sphaeroma hookeri lives for about a 

 year and a half. The annual cvcle of this species near Kiel has 

 been described by Kinne (1954). Most of the young are born in the 

 period from June to the end of August, and two broods can be 

 distinguished. The first brood is laid into the mothers' internal 

 pouches in Mav and emerges at the end of June or the beginning 

 of July. The second brood emerges late in August, with stragglers 

 as late as November. This second brood is produced bv females 

 born in the first brood., so that sexual maturity is reached in a 

 matter of a month or so. After producing a brood a female moults 

 and does not Droduce anv more voun°: until the next vear. In this 

 way the first brood of each vear is produced bv vear-old females, 

 and the second brood bv month-old females. All the females do 

 not mature so quickly, about three-quarters of them only produce 

 their first young the vear after thev were born. The annual cvcle 

 seems to vary with latitude: Jensen (1955) found that in the South 

 Harbour of Copenhagen onlv one reallv effective brood is produced 

 each year. This brood is liberated a little later than the first brood' 

 at Kiel. The vear-old females which produce this brood then moult, 

 losing their brood pouches, but thev regain them in August and lay 

 another brood of eggs. Onlv a small percentage of the females suc- 

 ceed in rearing their second brood. At Copenhagen there does not 

 seem to be anv maturation of females in the summer of their 

 birth. This contrast with the population at Kiel is probablv due to 

 the lower temperature in spring at the Danish capital. 



The common shrimp. Crangon vulgaris, serves as the example of 

 the third tvpe of annual cvcle. The account which follows is based 

 on the study made bv Llovd and Yonge (19471 in the Bristol 

 Channel and Severn Estuarv. Females live for four or perhaps five 

 years, while the males live for about three years. During the winter 

 the population migrates out of the less saline waters of the estuarv. 

 but returns in the spring. The breeding season in the estuarv 

 extends from March to June. This is a shorter season than that 

 found further out in the Bristol Channel, where breeding occurs 

 from January to August. Winter breeding in the estuary is pre- 



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