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The response of miracidia and cercariae of Bunodera mediovitellata to light and to gravity Kennedy, Murray James


This thesis investigates the effect of light, and to a lesser extent of gravity, on the distribution of the two free-living larval stages (miracidia and cercariae) of the digenetic trematode Bunodera mediovitellata. Test tubes, with various portions blackened, were illuminated by a horizontal 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 reacted. This intensity proved to be the same for both miracidia and cercariae even though miracidia are photonegative and cercariae are photopositive. Two-arm test chambers, illuminated with monochromatic light, were used to determine which wavelength(s) the larval stages were responding to. Cercariae showed a single response peak at 550Anm. while miracidia showed two peak's; one at 550 run. and the other at 650 nm. The second peak may be due to a screening effect by the pigment which surrounds the photoreceptor. The experimental results support the hypothesis that behavioural responses of free-living miracidia and cercariae to environmental stimuli guide? them to the general area of their next host and thereby increase! the chance of host-parasite contact. The photonegative and geopositive behaviour of miracidia of B.mediovitellata would keep them on or near the bottom of the pond and in the general habitat of the next host, Pisidium casertanum. Cercariae are photopositive and either geopositive or are very weak swimmers. Their behaviour would keep them on the bottom of the pond in the prefered habitat of the next probable host, an insect nymph or a crustacean. A model depicting the possible theoretical combinations of photoresponse with georesponse was constructed. A survey of the literature was undertaken to find larval stages with known photo-response and georesponse. The responses of these larvae were tabulated and compared to the model. In each case, the responses of the larvae supported the hypothesis that a complementary response to light and gravity should occur more frequently in nature than antagonistic reponses or a response to either light or gravity alone.

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