FUSARO: GROWTH RATE OF THE SAND CRAB 



pagurus. From this description it is clear that a 

 crustacean actually spends a great deal of its time 

 involved with the molting process. Once the 

 specific stimulus to molt has occurred, environ- 

 mental changes seem to have little effect in mod- 

 ifying the process (Green and Neff 1972). In other 

 words, a crab which is about to molt will molt. In 

 fact, keeping £^. analoga in running seawater ta- 

 bles for a very short portion of their molt cycle does 

 not alter a relatively high value for molt frequency 

 and increment. For example. Table 1 shows data 

 from a 14-day period in which E. analoga were 

 kept in running seawater tables. These data 

 suggest that for a period of 5 days or so£'. analoga 

 may be relatively unaffected by impoundment in 

 running seawater tables, but that confinement for 

 longer periods reduces both the increment at molt 

 and molt frequency. 



From the above, it follows that keeping E. 

 analoga in running ambient seawater tables for a 

 period of 5 days generates relatively accurate "in- 

 stantaneous" estimates of field growth rate for 

 these crabs. This allowed me to collect sand crabs 

 from the field, place them in seawater tables 

 within 2 or 3 h after collection, and monitor them 

 daily. Molt increment was measured directly as 

 animals within each size class molted, and molt 

 frequency was calculated by the formula: /'„, = tlp^ 

 where f^ = molt frequency in days, t = total 

 number of days held, and p, = proportion of ani- 

 mals which molted in t days. For instance, if half 

 the crabs molted in 5 days, the average molt fre- 

 quency would be 10 days. 



Table l. — Molt records for Emerita analoga kept in running 

 seawater tables for 14 consecutive days. 



Environmental Differences 



Water temperature showed a consistent annual 

 difference between Goleta Bay and Santa Cruz 

 Island for 1974 and 1975 (Figure 1), though the 

 peak water temperature for 1975 was the same for 



22 



21- 

 20 

 19 



18 

 17 

 °C 16 

 I& 

 14 

 13 

 12 

 I I 

 10 



X GOLETA BAY 



SANTA CRUZ ISLAND 



/\ \ 



■i — I — I — I — I — I — I — t- 



-+— I — I — I — t- 



FMAMJJ ASONDJ 



1974 



F M fl M J J 



1975 



-t 1 — t— t— H 



A S N D 



Figure l. — Annual surface temperature variation at Goleta 

 Bay and the western tip of Santa Cruz Island for several years. 

 Data from March to October 1974 are from direct observation. 

 All other data are from U.S. Coast Guard sea surface isotherm 

 charts for Point Conception south. 



both locations. During June-October 1974, the 

 mean surface temperature difference between 

 sites was 2.9°C. Peak water temperature was 

 maintained for about 2 mo (mid- June to mid- 

 August) at Goleta Bay in 1974 while Santa Cruz 

 Island experienced peak temperature for a month 

 or less. 



Four water samples, taken at each location 

 monthly between July and October 1975, showed a 

 large significant difference (P<0.001, t = 6.36, df 

 = 14) between the amount of filterable solids in 

 the water washing onto the two beaches. Goleta 

 Bay averaged 0.11321 ± 0.02933 g/1 of filterable 

 solids while Santa Cruz Island averaged 0.04416 

 ± 0.00925 g/1. This means that about 21/2 times as 

 much filterable material was potentially available 

 for E. analoga at Goleta Bay as at Santa Cruz 

 Island. Unfortunately, organic content of these 

 samples was not measured. This would have been 

 a more reliable index of food availability than 

 filterable solids. 



Growth 



Comparison of data from Goleta Bay in 1974 and 

 1975 indicates that the instantaneous growth rate 

 method produces repeatable results and that the 

 growth rates at Goleta Bay for the two time 

 periods were about the same (Table 2). Compari- 

 son of data from Goleta Bay and Santa Cruz Island 

 in 1974 indicates that sand crabs from the island 

 site were molting about half as often, on the aver- 

 age, as those crabs from Goleta Bay during the 



371 



