FUSARO: GROWTH RATE OF THE SAND CRAB 



relatively high molt frequency was observed at 

 smaller sizes (about every 7-12 days/molt at 5 mm 

 carapace length), and this value increased gener- 

 ally in larger size classes (to about every 80-90 

 days/molt at 20 mm carapace length). Molt fre- 

 quency averaged over a similar range of premolt 

 carapace lengths did reveal, however, a large dif- 

 ference between the populations on the two differ- 

 ent beaches (Table 2). 



Population structure data reveal differences in 

 several measures which relate to the growth rate 

 of E. analoga (Table 3). The mean size of both 

 males and females was reduced at the Santa Cruz 

 Island site. The proportion of females larger than 

 the mean minimum size of ovigerous females was 

 drastically reduced at the Santa Cruz Island site 

 (2.7% compared with 21.1% at Goleta). In addi- 

 tion, the overall proportion of females which were 

 ovigerous was much smaller at the island site 

 (1.1% compared with 19.4% at Goleta). 



Table 3. — Population structure data from pooled samples of 

 Emerita analoga between June and September 1974. All size 

 values are in millimeters carapace length and sample sizes are in 

 parentheses. 



DISCUSSION 

 Methods for Measuring Growth 



The growth rate of crustaceans in nature, 

 though of considerable research interest, has been 

 difficult to measure for several reasons. Primarily, 

 all of the hard parts of the animal are cast off with 

 the molt, making the marking of them all but 

 impossible until recent years. Wenner et al. ( 1974) 

 discussed the problems associated with measuring 

 growth for crustaceans, and their table 1 sum- 

 marized possible patterns of growth for the Crus- 

 tacea. That table stressed the relative contribu- 

 tion of two factors in crustacean growth: molt 

 increment and molt frequency. Both of these may 

 be responsive to different environmental parame- 

 ters, altering the growth pattern of a species. 



The standard methods for measuring field 

 growth rate for crustaceans (caging, mark and 

 recapture, and analysis of modal size classes with- 

 out corroborative data) are unsatisfactory to com- 



pare field growth rates for different populations of 

 E. analoga. Since none of these methods alone 

 suffices for this kind of comparative measurement 

 with E. analoga, the instantaneous growth rate 

 approach was used in this comparative analysis. 

 The method has qualities common to some of the 

 other methods mentioned, but avoids some of the 

 inherent problems of those methods. This 

 technique allows direct observation of size-specific 

 molt frequency and molt increment, while 

 minimizing the handling effects normally as- 

 sociated with laboratory impoundment. It is likely 

 that molt frequency estimates by this method are 

 more accurate for the larger crabs, for which the 

 5-day holding period is a relatively smaller pro- 

 portion of the intermolt period. The method has 

 allowed comparison of growth factors (molt incre- 

 ment and frequency) in detail {orE. analoga and 

 gave repeatable data such as that found for Goleta 

 Bay in 1974 and 1975. Thus a technique for mea- 

 surement of crustacean growth has been de- 

 veloped here which may hold promise for such 

 comparative studies as this, where field caging is 

 impractical. 



Growth of Emerita analoga 



The large difference in the growth rate of E. 

 analoga between beaches of Goleta Bay and a 

 Santa Cruz Island bay is remarkable in view of 

 their proximity (about 42 km apart) but not in 

 view of the different environmental conditions 

 found at each beach. The combination of colder 

 water and reduced filterable material in suspen- 

 sion in the water appears to have slowed the 

 growth of .E. analoga on Santa Cruz Island. This 

 difference is evidence of the sensitivity of these 

 two factors of sand crab growth to variation in 

 environmental qualities. 



It is tempting to construct growth curves from 

 such data on molt increment and molt frequency, 

 having arrived at estimates for these. Both of 

 these factors, however, have been shown to be 

 highly responsive to environmental conditions. In 

 fact, they vary widely in time and space with no 

 clear pattern emerging as yet. A growth curve 

 constructed from these data would apply only 

 under a specific set of environmental conditions. 

 Certainly these large differences in growth rate in 

 nearby beaches precludes the use of modal size 

 classes from several beaches for the determination 

 of growth for£^. analoga, as Efford ( 1967) did ear- 

 lier. 



373 



