Parasitism and host sensitivity to cadmium: an acanthocephalan infection of the freshwater amphipod Gammarus pulex.
Of 600 G. pulex removed as a single sample from the River Teme, Herefordshire, UK, 52% were infected with Pomphorhynchus laevis, 20% contained larvae, and 80% contained 1-6 cystacanths (mean = 1.02). In G. pulex containing one cystacanth, the wet weight of male and female P. laevis was significantly correlated with host wet weight. For any size of host, female P. laevis were significantly heavier than males. The wet weight of G. pulex and the number of P. laevis/host accounted for 48% and 55% of the total variance observed in, respectively, male and female P. laevis wet weight. After 80 days exposure to dilution water, 14% of uninfected and 94% of infected G. pulex were dead. Infected G. pulex exposed to a nominal cadmium concentration of 2.1 µg/litre were significantly more sensitive (LT50 = 17.5 days) to the toxicant than uninfected conspecifics (LT50 = 42 days), whilst at 6 µg/litre, differences in mortality rates between infected and uninfected individuals were not significant. Rates of mortality amongst the infected individuals were not related to parasite sex or the number of parasites/host. Infected G. pulex consumed only 17-21% of the food eaten in 24 h by uninfected G. pulex in dilution water. Cadmium depressed the feeding rate of uninfected G. pulex: at nominal Cd concentrations of 6 µg/litre, infected G. pulex consumed 34-40% of the food eaten in 24 h by uninfected conspecifics. Accumulation of Cd by infected and uninfected G. pulex did not differ significantly but P. laevis accumulated less metal than their hosts. P. laevis was insensitive to Cd: no mortalities were recorded and ability to evert the proboscis both in vitro and in vivo was unimpaired by exposure to Cd. The implications of these findings for the ecology of natural G. pulex populations harbouring P. laevis and at risk from Cd pollution are briefly discussed.