FISHERY BULLETIN: VOL. 59, NO. I 



meter, at 27.5 x magnification. 



The relationship between uropod length and 

 body length was established from a selected ser- 

 ies of 94 animals that had been collected in the 

 field and pi-eserved. The series included ani- 

 mals that ranged in body length from 0.8 mm to 

 7.2 mm, and included late stage larvae, juveniles, 

 immatures, and adults. Both sexes were in- 

 cluded; there was no difi'erence between sexes 

 in this relationship. 



The body length was measured from the end 

 of the last abdominal segment (base of uropod) 

 to the anterior edge of the carapace, behind the 

 insertion of the eyestalk. Mysids tend to curl 

 when preserved, and they can be distorted to 

 appear longer if they are stretched when meas- 

 ured. To avoid this we chose specimens that 

 were at most only slightly curved, and measured 

 the length of the arc through the midline of 

 those that had significant curvature, rather than 

 the straight line distance between head and tail. 



As shown in Figure 1, the relationship be- 

 tween uropod length and body length is linear. 

 The body length is 4.5 times the uropod length. 



Table 1. — Frequency of molting periods observed for 

 Metamysidopsis in the laboratory. 



FiGUKB 1. — Relationship between uropod length and 

 body length of Metamysidopsis. 



Molting Frequency 



Average intermolt periods were estimated 

 from 414 observations, 146 on males and 268 on 

 females. In many cases several observations 

 were made on the same animal. The maximum 

 period of laboratory survival for a single ani- 

 mal was 157 days, and the maximum number of 

 molts observed for a single animal (not the same 



animal) was 21. The molting frequency data 

 for animals reared in the laboratory are sum- 

 marized in Table 1. The sex of the juveniles 

 was established after they had grown large 

 enough to develop obvious morphological dif- 

 ferences. 



Supplementary data on molting frequency in 

 the field population were obtained indirectly. 

 Over a period of 3 days, 1,211 juveniles + im- 

 matures and 2,979 adults were brought into the 

 laboratory late in the day and placed in large 

 aquaria. The following morning all the animals 

 and their molts were collected and counted. Of 

 the juveniles + immatures 218 or 18 % had 

 molted, and of the adults 356 or 12 % had molted. 

 The recii)rocal of the relative number molting 

 is an estimate of molting period. The observed 

 reciprocals were 5.6 for juveniles + immatures 

 and 8.3 for adults. Since these values are mid- 

 way in the ranges shown by laboratory animals 

 (3-8 days for juveniles + immatures and 4-13 

 days for adults) we assume that the laboratory 

 observations are valid estimates of molting fre- 

 quency in the population as a whole. 



Although our observations were made from 

 February to October, and the water tempera- 

 tures in the rearing troughs varied from 14° 

 to 20° C, we were unable to detect any obvious 

 effects of temperature or time of year on molting 



96 



