Light and gravity responses of the oncomiracidium ofEntobdella soleaeand their role in host location

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Abstract
SUMMARY: The behaviour of oncomiracidia of the monogenean skin parasiteEntobdella soleae, after hatching without chemical stimulation, has been studied in vertical, 4 cm high chambers illuminated from above. Most freshly hatched larvae are photo-positive and swim to the top of the vessel, where they remain actively swimming. Occasionally larvae become photo-negative and make a brief excursion to the bottom of the vessel. As the larvae grow older, these photo-negative excursions become more frequent and larvae spend progressively longer in the photo-negative phase of behaviour, but, even after many hours, photo-positive vertical excursions are still occurring. If this pattern of alternating photo-positive and photo-negative vertical movements occurs in the natural environment, these vertical movements, coupled with horizontal transport of larvae by water currents, would provide a search pattern for the host, the diurnally-inactive, bottom-living flatfishSolea solea. When eggs are stimulated to hatch in the chambers with urea, large numbers of larvae emerge within a few min, and it was observed that during the period of about 30 min after stimulation, many larvae are photo-negative. When eggs on the sea bottom are stimulated to hatch by mucus or urine from a nearby host, such behaviour would greatly enhance chances of contact with the fish. When eggs are stimulated to hatch by means of urea in the absence of light, there is evidence that the oncomiracidia are geo-negative. It has been shown that larvae are able to extricate themselves in total darkness from sediment containing buried eggs and that, in an experimental situation illuminated only by an infra-red beam, larvae readily attach themselves to the lower skin of soles within 2 min of hatching. There is evidence that, in aquaria, some invasion of the host takes place from below.