Influence of trematode parasitism on the growth of a bivalve host in the field.

Trematode-induced gigantism of molluscs, enhanced growth of trematode-parasitised individuals, has been observed in many laboratory studies. This study reports the effect of the sterilising trematode, Rhipidicotyle fennica, on the growth of the freshwater clam Anodonta piscinalis under field conditions. In addition to single infections (prevalence 44%), a few clams (3%) were infected with both R. fennica and Rhipidicotyle campanula. Parasite-induced gigantism was not found; parasites lowered host growth. The decreased in growth was correlated with the quantity of parasite material. Clams with double infections grew the least, although they did not differ significantly from hosts with a heavy single species infection. The growth of the experimental clams was lower than that of undisturbed control clams. Trematode-induced gigantism of molluscs in the field, in general, remains undemonstrated.