The effect of host density on parasite infection: a power-law model and an experimental test with trematode larvae infecting mussels

Host density is an important factor determining transmission success of parasites in marine ecosystems. Here, we suggest a simple model linking mean abundance (the number of parasite individuals found divided by the number of host individuals examined) and parasite density (parasite population per area) with the density of the host for parasites with a complex life cycle, where an infected host cannot be the source of infection for conspecifics. The model predicts a power-law dependence: negative for mean abundance and positive for parasite density. This means that the increase of the host density simultaneously reduces the average load per host individual and increases the transmission success of the parasite. For the case when host density is estimated "per unit area" (e.g., ind. per m.sup.2), our model predicts the exponents of this dependence of - 0.5 for mean abundance and 0.5 for parasite density. We tested the model on our data on accumulation of metacercariae of trematodes Himasthla spp. and Renicola parvicaudatus in second intermediate hosts, mussels Mytilus edulis, at the White Sea intertidal and found a good correspondence between the empirical data and the model (R.sup.2 = 0.7 - 0.9). A positive correlation between the parasite density and the host density suggests that dense host settlements with a lower infection level (prevalence, mean abundance and mean intensity) should play a greater role in the transmission of the parasite than sparse settlements with a high infection level.