POPULATION-DYNAMICS OF CHAETOGASTER-LIMNAEI-LIMNAEI (OLIGOCHAETA) AS AFFECTED BY A TREMATODE PARASITE IN HELISOMA-ANCEPS (GASTROPODA)

The population dynamics of Chaetogaster limnaei limnaei, a commensal and predatory oligochaete living in the mantle cavity of the pulmonate snail, Helisoma anceps, were studied. The densities of the oligochaete infrapopulations were positively correlated with water temperature, declining in the autumn, reaching their lowest levels in winter and rising in spring and summer to their highest levels. This pattern was also influenced by the annual reproduction and mortality cycle within the snail population. Most importantly, however, in snails shedding cercariae of the hemiurid trematode, Halipegus occidualis, densities of the oligochaete were significantly higher than those in uninfected snails. The continually accessible and nonmotile cercariae of this trematode were especially vulnerable to predation by Ch. l. limnaei which enhanced the rate of asexual reproduction (by building) of the oligochaete.