TESTER and CAREY: INSTAR IDENTIFICATION OF SPIDER CRABS 



calculated for each instar, and when it was clear 

 from the equal sex ratio that juvenile males and 

 females were not dimorphic, the percent increase 

 in carapace width per molt was computed as 



Percent increase in CW (mm) = 



Postmolt CW (mm) - Premolt CW (mm) 

 Premolt CW (mm) 



x 100 



In one series of size-frequency histograms from 

 samples collected in June, July, and August 1974 

 and January and March 1975, the progression of 

 modes (representative of instars of the small 

 juveniles from fish gut contents) was used to 

 estimate growth rate. The next larger instar (CW 

 = 10 mm) was the first to be consistently sampled 

 by the trawl gear. Starting with the 10 mm CW in- 

 star from April 1973, growth of juvenile tanner 

 crabs was followed through August, October, and 

 November 1973 and March 1974. 



Carapace Condition 



Detailed observations were made of the carapace 

 on each specimen and included hardness, amount 

 of attached fauna, and general condition. Darkened 

 and softened or weakened areas on the carapaces 

 were similar to those caused by chitinoclastic bac- 

 teria (Sindermann 1970) and were thought to be 

 associated with age. Adult female C. tanneri were 

 especially subject to carapace deterioration. 



RESULTS AND DISCUSSION 



Since a high degree of correlation between gonad 

 maturity and external morphology has been shown 

 for the genus Chionoecetes (Brown and Powell 1972; 

 Donaldson et al. 1981), a plot of carapace width and 

 chela length (Fig. 2) was used to define adult males. 

 Specimens with chelae longer than 85 mm (corre- 

 sponding to carapace width >118 mm) were as- 

 sumed to be sexually mature males. Those females 



E 

 E 



D) 



C 

 CD 



CO 



D 



T5 



O 



a 

 o 



05 



CD 



.C 



O 



140 



130 



120 



110 



100 



90 



80 



70 



60 



50 



40 



30 



20 



10 







N=760 



.**■"' 



10 20 30 40 50 60 70 80 90 100 1 10 120 130 140 150 160 170 180 



Carapace Width (mm) 



Figure 2.— Relationship between carapace width and chelar propodus length for juvenile and adult male Chionoecetes 



tanneri. 



975 



