GROWTH RATE OF THE SAND CRAB, EMERITA ANALOGA, 

 (HIPPIDAE) IN TWO DIFFERENT ENVIRONMENTS 



Craig Fusaro* 

 ABSTRACT 



The field growth rate of Emerita analoga was estimated under two different sets of environmental 

 conditions. One beach location on the southern California coastline near Santa Barbara was compared 

 with a beach on Santa Cruz Island, only 42 km distant. The island population of £. analoga experienced 

 colder water which contained less suspended solids than the mainland population studied. The crabs on 

 the island beach were found to grow at about one-third the rate of those on the mainland, as measured 

 by the "instantaneous growth rate" technique. 



The growth rate of a crustacean species is of con- 

 siderable ecological interest for at least two 

 reasons. First, the usual positive relationship be- 

 tween body size and fecundity means that an in- 

 creased growth rate may increase reproductive 

 output. Second, the proportion of animals which 

 are mature within any specified population will 

 depend on the growth rate (to maturity) of its 

 members. These relationships indicate that a 

 measure of individual growth, averaged for a 

 population, may give information about reproduc- 

 tive success and, therefore, about the persistence 

 of a population under its particular set of en- 

 vironmental circumstances. Further, comparison 

 of the growth rates for populations of the same 

 species under differing environmental regimes 

 may indicate how the species responds to its en- 

 vironmental conditions in terms of growth. 



The proximity of the Channel Islands to the 

 mainland near Santa Barbara provides excellent 

 opportunity for comparative studies. The differing 

 oceanographic conditions between some beaches 

 of Santa Cruz Island and nearby Goleta Bay on the 

 mainland allow an analysis of growth as a func- 

 tion of different environmental characteristics. 

 This is the intent of the present work. 



This study is an analysis of the growth rate in 

 two nearby populations of the sand crab, Emerita 

 analoga. It is an anomuran crab which inhabits 

 beaches of the eastern North Pacific from southern 

 Canada to central Baja California. There also are 

 populations of the species in the Gulf of California 

 and the eastern South Pacific, but these popula- 



'Marine Science Institute, University of California, Santa 

 Barbara, Santa Barbara, CA 93106. 



Manuscript accepted July 1977. 



FISHERY BULLETIN: VOL. 76, NO. 2, 1978. 



tions are not considered here. The crab normally 

 lives between high and low tide marks buried in 

 sand on sandy beaches, and follows the water's 

 edge up and down the beach with the tides. The 

 crab is somewhat unusual for its family in that it 

 procures its food by filtering seawater that washes 

 over its finely setose second antennae as waves 

 rush onto and ebb from the beach face. Emerita 

 analoga can often be found in dense aggregations 

 on beaches of southern California during spring 

 and summer. This species was chosen for compara- 

 tive growth study because: 1) it is seasonally very 

 abundant and easily captured in great numbers; 2) 

 it is of a size which facilitates the handling and 

 treatment of such numbers; and 3) it inhabits 

 beaches of the mainland and beaches on nearby 

 Santa Cruz Island. 



Several methods have been used in the past to 

 measure the incremental type of growth experi- 

 enced by crustaceans, all of which have merits and 

 drawbacks. These methods include: field caging, 

 laboratory confinement, mark and recapture 

 methods, and modal size class analysis. The best of 

 these for the estimation of field growth rate is field 

 caging. This method, unfortunately, cannot be 

 applied to sand crabs living in their shifting sand 

 habitat. For reasons presented in detail in the 

 methods, a different approach has been used to 

 measure growth for E. analoga The "instantane- 

 ous growth rate" method used here takes advan- 

 tage of certain aspects of the crustacean molt cycle 

 to minimize handling effects while estimating 

 field growth rate. Both molt increment and molt 

 frequency measures are taken, which are com- 

 bined to give an instantaneous growth rate for an 

 average individual of each population. By this 

 method the response of grovd;h to different en- 



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