FISHERY BULLETIN: VOL. 77. NO, 4 



meiofaunal predators (defined as organisms that 

 are retained on sieves 0.04-0.1 mm and passed 

 through sieves 0.5-1.0 mm (Mclntyre 1969; Coull 

 1973)1, that were nearly the same size. Swedmark 

 (1964) listed turbellarians, coelenterates, and 

 nematodes as interstitial predators. Thorson 

 (1966) cited studies that have shown turbellar- 

 ians, nematodes, and harpacticoid copepods to be 

 predators on newly settled spat. Although only a 

 few turbellarians and no coelenterates were recov- 

 ered during the study period, a large number of 

 nematodes and harpacticoids were included in 

 each core sampled. In spite of the citation by 

 Thorson, the harpacticoids in this study were not 

 likely to have eaten even the smallest clam spat, 

 as these particular species are considered almost 

 exclusively detritus feeders (Sibert et al. 1977; 

 Illg^). The degree to which nematodes may have 

 accounted for loss in clam spat is unknown. Al- 

 though larger predators (shore crabs, drilling 

 snails, sea stars, fish, birds) may account for sig- 

 nificant predation losses on larger clams, their 

 effect on survival of the newly settled spat was 

 probably low. Large, active predators would not 

 likely have expended the energy to forage for the 

 small spat that would have provided little energy 

 in return. 



Of all the above factors listed, I concluded the 

 major cause for the large loss in spat that I ob- 

 served was due to predation by meiofaunal preda- 

 tors. Since some empty clam shells were found on 

 the beach and vigorous sieving in the laboratory 

 did not damage the shells of the spat, only preda- 

 tion could have accounted for both the high mor- 

 talities and the destruction or removal of shells. I 

 assumed that nematodes were the dominant pred- 

 ator. 



Movement of Clams on the Beach 



Two experiments were performed (November 

 1976 and May 1977) to test for movement of small 

 clams on the beach. In each case, 2 cm of surface 

 gravel was removed from a plot that was 0.25 m^, 

 to insure that no small clams remained. One 

 month aft;er the start of each experiment, core 

 samples were taken and small clams were found 

 in the center of each plot that had been previously 

 clam free. It was not known whether this move- 



■■^Paul lUg, Department of Zoology, University of Washington, 

 Seattle, WA 98195, pers. commun. February 1977. 



ment was active or passive. Active movement im- 

 plies that clams physically moved (probably by 

 foot action) across the beach. Passive movement 

 implies physical transport of clams across the 

 substrate, with or without movement of surface 

 gravel. In the present study, byssus threads were 

 detectable on clams as small as 0.45 mm long. 

 This indicated that they had the ability to attach 

 to the substrate which would decrease their sus- 

 ceptibility to movement by currents. To the con- 

 trary, Sigfurdsson et al. (1976) proposed that some 

 postplanktonic bivalve larvae use their byssus 

 threads as a method for dispersal. The method of 

 transport would be analogous to the gossamer 

 flight by young spiders. Either of these two 

 methods for utilization of byssus threads may 

 have been used by some of the calm spat at the 

 study site. 



Baggerman (1953), with C. edule, found that 

 transportation of clams over the substrate may 

 have been an important factor in the final dis- 

 tribution of clams. In this study, transportation 

 may have played an important part in the growth 

 and survival of the spat. Growth was shown to be 

 significantly greater in treatments without adult 

 clams than in treatments with adult clams. The 

 adults thus may have directly caused a decrease 

 in growth of juveniles by decreasing the availabil- 

 ity of food, and/or indirectly they may have 

 influenced clam spat to actively seek new sub- 

 strates in which to resettle to avoid competition. 

 Additionally, in the process of resettlement, clam 

 spat may have become susceptible to a larger 

 number of predators. 



ACKNOWLEDGMENTS 



Thanks to K. K. Chew, J. L. Congleton, P. A. 

 Jumars, and P. L. Illg for their advice and assist- 

 ance. Daniel B. Qualye helped with the identifica- 

 tion of young Manila spat. 



Office space and materials for field work were 

 provided by R. R. Whitney, Leader, Washington 

 Cooperative Research Unit at the University of 

 Washington. Michael Shepard, T. Schink, D. 

 Skidmore, and R. Carter provided assistance with 

 the field work. My especial thanks to J. Taylor 

 who allowed me to use his beach for this study. 

 His wisdom gained from years of commercial 

 clam and oyster harvesting provided me with 

 valuable insight into the biology of the Manila 

 clam. 



898 



