The Importance of Predation and Intra‐ and Interspecific Competition in the Population Biology of Two Infaunal Suspension‐Feeding Bivalves, Protothaca staminea and Chione undatella
Authors: Peterson Charles H. 
Published: April 2006

To estimate the effects of varying density on individual and population parameters of two infaunal, suspension—feeding bivalves, Protothaca staminea and Chione undatella, densities of both species were varied experimentally from their natural levels (X) in shallow, subtidal field enclosures in Mugu Lagoon, California. A factorial design with densities ranging from ½X to 8X in two sand habitat matrices of enclosures and from ½X to 4X in one mud habitat matrix permitted tests of both intra— and interspecific effects of density on (1) individual growth rate, (2) recruitment rate, and (3) survivorship rate. After each rate was estimated for each of four consecutive experimental periods spanning 24 mo with replacement of missing and dead clams at each sampling, a subsample of adult clams was sacrificed from each density treatment in the mud matrix and dissected to produce estimates of how reproductive effort (4) varied with density. Short—term (2—wk) migration rates (5) were estimated as a function of density in the two habitats in separate experiments involving no enclosures. Characteristic shell damage and results of a predator exclusion experiment that was sampled four times over 24 mo permitted clam mortality rates (6) as a function of density and size to be partitioned broadly by source for both habitats. For both Protothaca and Chione, growth rates was consistently depressed at high intraspecific densities. In mud, Protothaca's linear growth declined by 49% and Chione's by 38% as intraspecific density increased from ½—1X to 2—4X. Interspecific effects of density on growth were usually nonsignificant and were consistently far smaller than these large intraspecific effects. The dry mass of gonadal material per unit shell volume also declined significantly with increasing intraspecific density. Average percentage of total soft tissue mass found in gonads in June 1977 declined progressively by 38% in Protothaca and by 49% in Chione as intraspecific density was increased from ½—1X to 2—4X. In Protothaca, growth rate decreased at the same rate as reproductive effort with increases in density, demonstrating that Protothaca did not reallocate energy differentially among the competing processes of growth and reproduction as competitive stress changed. The strong effects of intraspecific density in the immediate vicinity of control densities implies intense resource limitation on both growth and reproduction in each species. The general lack of significant interspecific effects of density on both growth and reproductive effort implies ecological segregation. Food seems more likely than space to be the limiting resource which is partitioned between species because these clams occupy the same depth stratum, such that interference competition for space would presumably have interspecific as well as intraspecific components. In sand, recruitment rate of Protothaca was never related to adult density, whereas a single location parameter explained 76% in one matrix and 50% in the other of between—enclosure variance in recruitment. In mud, Protothaca recruitment was occasionally depressed at high adult density, but the decline was only °40% as intraspecific adult density changed from 0—1X to 2—4X. Spatial location still explained 25% of total variance in recruitment in the mud. Chione's recruitment rate was lower by over one order of magnitude and much more variable from year to year than Protothaca's. On the one occasion when its recruitment was high enough to test for density effects, Chione recruited less in the presence of higher densities of adult Protothaca but showed no intraspecific effect. Emigration rate over 2 wk was density dependent in Protothaca in the sand habitat. Protothaca showed reduced mobility in the mud habitat such that the density dependence of emigration rate could not be tested. Chione was so sluggish that no emigration took place at high or at low density in either sand or mud. Annual Chione survivorship was quite high (averaging at least 88—96%) in every matrix over the 2—yr density manipulation experiment; however, in the 2nd yr of the predator exclusion experiment 10 cm of sedimentation from a once—in—100—yr rainstorm caused 100% mortality in the mud habitat. Mortality rate in Chione was never a significant function of density, and predation had little or no effect on the Chione used in this experiment. Annual survivorship of Protothaca was substantially lower (16—61%) than that of Chione in every matrix during each of the 2 yr of the density manipulation experiment. Survivorship of Protothaca was often significantly reduced at higher Protothaca densities in both habitats, although the effect was greater in the mud. The predator exclusion experiments demonstrated that both Protothaca's higher mortality rate and this occasional density dependence of mortality were entirely an effect of predation. In sand and mud, most predation was of the type that removed the shell, though characteristic shell damage of empty shells revealed that Polinices reclusianus in sand and, to a lesser degree, Cancer anthonyi in mud also contributed significantly. The importance of various types of predators changed dramatically between "replicate" years. Predation by sources that removed shells fell most heavily on smaller Protothaca in both habitats; in the sand this decline in risk with size was occasionally compensated by Polinices' preference for larger Protothaca. As with Chione, percent Protothaca mortality from all sources which leave undamaged shells (including starvation) never increased with increasing clam density. Thus, despite evident resource limitation on growth and reproductive effort, competition did not produce mortality effects in the range of densities used. The differing susceptibilities of Protothaca and Chione to predation and the contingent life history differences may be based upon differences in shell thickness, size, and ornamentation and may be a consequence of Chione's more tropical evolutionary history.