Purple clams were present throughout a 

 wide range of sediments. Higher biomass and 

 densities were recorded in coarse sand, 

 decreasing with finer sediments. This species 

 was absent from sediments with high silt/clay 

 contents. Littleneck clams occurred in very 

 coarse to fine sediments with highest densities 

 in finer sediments. Maximum biomass and 

 density were found in 15%-20% silt/clay. 

 False mya occurred in all sediment types from 

 very coarse to very fine sand. Biomass and 

 densities were bimodal, with highest values at 

 around 35% silt/clay. California jackknife 

 clams were collected in medium to fine sand. 

 Biomass and density increased with a decrease 

 in sediment size. Biomass was highest at 5%- 

 10% silt/clay; density peaked at 20%-25% 

 silt/clay; and size tended to decrease with 

 higher silt/clay percentages. The bent-nose 

 clam was found in sediments ranging from 

 coarse sand to very fine sand. Biomass and 

 density increased with a decrease in sediment 

 size. Density and biomass were optimum at 

 about 15% silt/clay. The white sand clam 

 (Macoma secta) was found only in medium to 

 fine sandy sediments and had the narrowest 

 grain size distributional range. Both biomass 

 and size of white sand clams were fairly 

 constant with changes in grain size, although 

 density was higher in medium sand. 



Water depth also influences the benthos. 

 In her study of littleneck clams, Smith 

 (1974) concluded that size and density 

 increased with increasing tidal depth. She 

 found average densities as high as 207/0.25 

 m 2 in the subtidal areas compared to 

 20/0.25 m 2 in the intertidal zone. She cited 

 sediment grain size, temperature, salinity, 

 duration of feeding time, and probability of 

 predation as factors affecting survivorship of 

 this species. Increased food availability and 

 substrate stability are also associated with 

 deeper water. 



Niesen (1969) sampled areas near the 

 mouth of Tijuana Estuary to characterize the 

 sand dollar (Dendraster excentricus) 

 population. Densities at that time ranged from 

 60-250/m 2 with a mean of 170/m 2 

 (n=10). This species died out after storms 

 and flooding in 1978 lowered water salinities 



(Dexter, pers. comm.). It was not recorded 

 again until Griswold found small population on 

 a sandy area of the main north channel in 

 1985. The largest individuals recorded were 

 13 mm in diameter. 



The dominant crustacean in the estuary 

 was the ghost shrimp, which was studied by 

 Homziak (1977). As recently as the 1970's, 

 it was commercially harvested as fish bait. 

 Hosmer (1977) investigated its competitive 

 interactions with two sympatric burrowing 

 shrimp of the same family: Callianassa gigas 

 and a Upogebia species. His work suggested 

 that the distribution and abundance of C. 

 californiensis and Upogebia species are 

 determined by specific substrates, while C. 

 gigas appears to be controlled by competitive 

 interaction with the other two species, due to 

 broad overlap in substrate and food 

 exploitation. 



Peterson (1975) compared the benthos at 

 Tijuana Estuary and Mugu Lagoon, 300 km to 

 the north. He examined the subtidal, sandy 

 bottom habitats of the two areas and found that 

 the dominant macroinvertebrates each 

 occupied a characteristic depth within the 

 sediments. Sand dollars occupied the top few 

 centimeters (Figure 3.20) and each had most 

 of its test exposed to the water column. 

 Littleneck clams were found within the top 6 

 cm. Ghost shrimp and its obligate commensal, 

 false mya, occurred together at 0-55 cm. 

 There was some overlap with the purple clam, 

 which was found at 25-55 cm. The California 

 jackknife clam burrowed up to 60 cm deep. 

 Peterson attributed their vertical separation 

 to competition for space, because there was 

 little segregation in food utilization. All but 

 one were suspension feeders; thus, they used 

 the same food source, regardless of burial 

 depth. 



Tijuana Estuary and Mugu Lagoon had 

 similar dominant species, but densities and 

 relative abundances were very different 

 (Table 3.6). The purple clam and California 

 jackknife clam were much more abundant at 

 Tijuana Estuary, which Peterson (1975) 

 thought was due to the rarity of other deep- 

 burrowing competitors. Hydraulic harvesting 



49 



