Photo- and geo-orientation by Echinostome cercariae results in habitat selection.

The effect of light and gravity on orientation was studied in cercariae of 4 echinostome species: Pseudechinoparyphium echinatum, Echinostoma revolutum, Hypoderaeum conoideum, and Isthmiophora melis. The cercariae were placed into vertical and horizontal cuvettes, illuminated with 2 different light intensities from various directions, and their distribution recorded for 6 hr Each species showed its individual pattern of horizontal photo-orientation and geo-orientation, with distinct changes during the time after emerging. The geo-orientation was controlled differently in each species by the intensity and the direction of light radiation. The different orientation patterns suggest functions such as leaving the habitats of the host-snails emitting the cercariae, dispersal, and frequenting the microhabitats of potential hosts. The high diversity of orientation patterns among the species that originated from the same first intermediate host Lymnaea stagnalis in the same ponds and that invade similar host spectra suggests adaptations to different ecological conditions.

Studies on the attachment response of Isthmiophora melis cercariae (Trematoda: Echinostomatidae).

The cercaria of Isthmiophora melis is relatively specific to suitable intermediate hosts, amphibia and fish, as early as the stage of attachment. Attachment responses are maximally stimulated by intact amphibia, whereas isolated frog tissues have a reduced efficiency. Human urine contains attachment triggering substances. Its effectiveness is reduced, when carbonate systems are removed, and may be restored by addition of carbonate. Aqueous solution systems of dissolved CO2 and H2CO3/HCO-3 stimulate attachment responses, especially at pH 6.0-7.5. The attachment response to frog skin surface seems to be triggered by the buffering system, which uses carbonate solution equilibria as important components.