SPATIAL VARIATION IN INFECTION BY DIGENETIC TREMATODES IN A POPULATION OF FRESH-WATER SNAILS (POTAMOPYRGUS-ANTIPODARUM)

Larval digenetic trematodes commonly castrate their first intermediate hosts, and should therefore impose strong selection on the timing and mode of host reproduction. Here we examine spatial variation in infec tion by trematodes in the freshwater snail Potamopyrgus antipodarum. S nails were collected at 11 different sites at Lake Alexandrina on the South Island of New Zealand from transects that ran perpendicular to t he shore and across several different habitat types (from 0 to 8 m dee p). Logistic regression was used to analyze the relationships between the frequency of trematode infection and snail size, habitat type, and transect location. On average, the probability of infection increased 3.3 times with each 1 mm increase in shell length. Prevalence of infe ction by the most common species of trematode, Microphallus sp., was h ighest in the shallow-water habitats where its final hosts (waterfowl) spend most of their time. Prevalence of infection by another parasite , Telogaster ophistorchis (final host: eels) increased with depth, but because Microphallus was much more common, total infection by all tre matodes decreased with depth. The effects of transect location were mi nor for Telogaster, but there was significant variation in Microphallu s prevalence among transects, especially in the shore-bank habitat. Ta ken together, these results suggest that the risk of infection is spat ially variable, but generally higher in shallow-water habitats, which may explain the greater frequency of sexual individuals as well as ear lier reproduction among individuals near shore.