The relationship between parasite load, crawling behaviour, and growth rate of Macoma balthica (L.) (Mollusca, Pelecypoda) from Hudson Bay, Canada

Abstract

At ebb tide Macoma balthica makes crawling tracks on the intertidal sand flats near Churchill, Manitoba, on Hudson Bay. Clams from two tidal levels, mean low water and 1.0 m above mean low water, were sampled to compare the parasite load and growth rate of crawling versus buried Macoma. For each clam the number of trematode metacercariae present were counted and the growth rate was determined by the measurement of annual growth rings. Clams were infected by more metacercariae at the higher than at the lower tidal level, larger clams more than smaller ones and crawling clams more than buried ones. Increased exposure of the clams at the higher tidal level to shorebirds, the final host of the trematodes, is proposed as the reason for the difference in parasite load between the tide levels. High-tide clams (more parasitized) grew faster than low-tide ones (less parasitized), and crawlers (more parasitized) grew faster than the buried (less parasitized) clams. Enhanced somatic growth as a result of parasitic castration is proposed to be the most logical explanation to account for the faster growth of the parasitized clams.