is not surprising that Owenia is at times a significant component of the 

 subtidal macrofauna. This species ranked fifth in abundance at the 6.1- 

 meter stations (Tables 11 and 15). Fager (1964) reported that newly 

 settled Owenia appeared throughout the year in southern California. A 

 maximum density of 15,000 per square meter was reported, but average 

 densities within aggregations were 500 to 1,000 per square meter. 

 Owenia appeared sporadically in Imperial Beach samples and occurred in 

 comparatively large numbers at station A, 6.1 meters and station C, 3.7 

 meters during survey IV following beach replenishment. The density of 

 Owenia tubes within these areas was 4,097 and 373 per square meter, 

 respectively. This survey was preceded by a period of relatively calm 

 wave activity (Fig. 7) which allowed successful settlement in the shallow 

 water. By survey V after storm activity (November 1977), this species 

 was absent from station C, 3.7 meters and densities had been reduced to 

 13 per square meter at station A, 6.1 meters. The ephemeral nature of a 

 large population buildup of Owenia and its subsequent decline suggests 

 that some factor other than beach replenishment was instrumental in the 

 successful recruitment of this worm. Such rapid population declines are 

 typical of opportunistic species with short lifespans (Grassle and 

 Grassle, 1974). 



d. Nematodes . Roundworms are one of the most numerous and wide- 

 spread of all multicellular organisms, but their taxonomy and role in 

 marine sediments are poorly understood. This assemblage ranked third in 

 abundance at the 3.7- and 6.1-meter stations (Table 11). Marine nema- 

 todes are small organisms, with the majority able to pass through a 

 0.5-millimeter screen (Reish, 1959; Warwick and Gage, 1975). This study 

 was not designed to sample this group, because such a design is not 

 cost-effective for the purpose of the study. Estimates of numbers based 

 on the nematodes that were retained on a 0.5-millimeter screen showed 

 densities approaching 3,000 per square meter with numbers fluctuating 

 greatly (Tables 14 and 15). 



e. Mollusks. Mollusks, which are primarily planktonic in their 

 larval form before settling in the sediments, are major infaunal com- 

 ponents of the biomass. 



(1) TiDela stultorwn (Pismo Clam) . The Pismo Clam is the 

 only large bivalve in the surf zone along the southern California coast. 

 This thick-shelled clam may reach a length of 12 centimeters or more and 

 live for 7 years or more (Fitch, 1950). Individuals up to 6 centimeters 

 long were collected at Imperial Beach, indicating successful recruitment 

 within the previous 1 to 2 years. Population density estimated from 

 combined intertidal stations and surveys was 2.1 per square meter. A 

 comparison of clams collected at each station showed a significant de- 

 crease (p<.05) in average density (1.17 +0.97 to 0.14 +0.27) following 

 beach replenishment. The deposited sediments may have affected density, 

 though the estimates were based on very few individuals. A special 

 sampling program would have to be designed to obtain good estimates. 

 Populations of large, relatively low density species cannot be estimated 

 unless extensive dredging is employed to obtain large samples (see 

 Loesch, 1974). 



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