Figure 3.20. The channels suport a wide variety of animals. Illustrated are mullet in the water 

 column and benthic animals, from left to right: the horn snail, sea hare (Aplysia), mud snails 

 (Nassarius), sand dollars, hermit crabs (Pagurus), egg cockle (Laevicardium), sea cucumber 

 (Molpadia) and its commensal pea crab (Pinnixia barnharti), and the crab (Cancer productus) 

 with attached mussels (Mytilus edulis). Burrowing in the sediments are, from left to right: the 

 mud-flat brachiopod (Glottidia albida), bent nose clam, California jackknife clam {Tagelus 

 claifornianus), wavy chione (Chione undatella), ghost shrimp, and spionid worms. Mclntire 

 collection, © 1986 by Zedler. 



of callianassid shrimp was common at Tijuana 

 Estuary during the 1960's and 1970's; their 

 removal, according to Peterson, allowed the 

 clams to expand their populations with little 

 effect on other dominant species. 

 Manipulative field experiments supported his 

 interpretation of the data. 



During the winters of 1977-78, 1978- 

 79, and 1979-80, San Diego County had 

 unusually heavy rains, culminating in the 

 flood of 1980 when approximately twice the 

 normal precipitation and 28 times the mean 

 annual streamflow were recorded. The influx 

 of fresh water lowered channel salinity to 

 zero ppt, and the increased streamflow 



significantly altered the sediment structure of 

 the channel bottoms. Sediment grain size 

 decreased at three of five sampling sites and 

 increased at two (Rehse 1981). 



These shifts in grain size allowed a test of 

 three predictions from Hosmer's (1977) 

 study: 1) The purple clam should be most 

 affected, because it was confined to coarser 

 sediments; 2) the littleneck clam should be 

 less affected because its highest density and 

 biomass were at 15%-20% silt/clay; and 3) 

 the false mya should be least affected, because 

 their maximum biomass and densities were at 

 35% silt/clay. 



50 



