Impacts of habitat features and predation on distributions of mudflat clams

Though the congeners Nutricola (=Transennella) confusa and Nutricola tantilla occupy an important ecological niche on mudflats, there is little published information concerning their population dynamics. We sought to determine what causational factors influence shifts in the relative distributions of these clams. Twelve cores were taken from Westside Park in Bodega Bay Harbor, California, in areas varying in macroalgal cover by Ulva expansa. Presence of the macroalgae, Ulva, heavily affected both species, but a steeper negative response in N. confusa populations was noted. There was also a significant increase in the relative number of N. confusa disarticulated shells when N. confusa decreased in dominance. Juvenile Cancer magister (Dungeness crabs) are unique predators of these clams in that they live among the Ulva and leave disarticulated shells when foraging. We therefore hypothesize that selective predation by C. magister on N. confusa is responsible for the shifts in relative abundances between Nutricola spp. noted. We examined the transport of dead shells in the field to determine if shell material could be considered an accurate tool in surveys. Dead shell material was not transported by tidal flow or wave action, which means its presence is an accurate representation of clams that previously lived in specific areas on the mudflat. Variations in sediment grain size were also examined in order to eliminate them as a controlling factor between Nutricola spp. distributions. There was no difference in response observed between N. confusa and N. tantilla. Juvenile Cancer magister were used in a laboratory feeding experiment to determine if N. confusa and N. tantilla are treated differently as prey. Though large, juvenile C. magister did not exhibit any selective predation between Nutricola spp., small, juvenile C. magister selectively preyed on N. confusa. Therefore, selective predation by small, juvenile C. magister is most likely limiting the distributions of N. confusa relative to N. tantilla in areas around Ulva.