The responses of miracidia and cercariae of Bunodera mediovitellata (Trematoda: Allocreadiidae) to light and to gravity

Abstract

Vertical glass tubes, with various portions blackened, were illuminated by a horizontal beam of white light to determine the photoresponse and georesponse of miracidia and cercariae.Four-arm test chambers were used to determine the lowest light intensity at which miracidia and cercariae showed a phototactic response. This intensity (.0.35 lx) was the same for miracidia and cercariae even though miracidia, whose next host is a bottom-dwelling clam (Pisidium casertanum), are photonegative, and cercariae, with a bottom-dwelling caddisfly larva (Limnephilus sp.) as next host, are photopositive.Two-arm test chambers, illuminated with monochromatic light, were used to determine the wavelengths to which the larval stages were responding. Cercariae showed a single response peak at 550 nm while miracidia showed two peaks, one at 550 nm and the other at 650 nm. The second peak may be a result of a screening effect by the pigment which surrounds the photoreceptor.Miracidia showed a positive georesponse when tested in vertical test tubes. However, the negative photoresponse determined their distribution in a conflicting situation. No conclusive results were obtained when the georesponse of cercariae was tested. These larvae remained on the bottom of test chambers under all conditions.