The infection mechanism of the cystophorous cercariae of Halipegus occidualis (Digenea : Hemiuridae)

Cystophorous cercariae of Halipegus occidualis perforate the gut wall of fe eding microcrustaceans via the explosive expulsion of a delivery tube from the cercariocyst, resulting in the transfer of the body of the cercaria int o the hemocoel of the second intermediate host. The hypothesis that this di scharge is associated with osmotic changes within the cercariocyst was test ed. Increasing the osmolarity of the solution bathing the cercariocysts inh ibited normal excystment. However, similar to 80% of cercariocysts subseque ntly placed in distilled water excysted normally. Excystment was sufficient ly slowed in 0.25 Osm solutions to reveal at least two separate stages of t he process. First, water drawn into the space between the inner and outer c yst walls upon release of the cercariocyst from the snail host creates suff icient turgor pressure to evert the delivery tube when the integrity of the caudal appendage is breached. Second, asymmetric osmotic influx of water i nto the cercaria forces the swelling body through the delivery tube. Contra ry to earlier reports, the delivery tube is everted through the caudal appe ndage of the cercariocyst, and bears no spine at its tip. The fact that the infection mechanism is osmotically driven has both physiological and evolu tionary implications in fresh-water hemiurids, whose origin is thought to b e the marine environment.