Swimming behavior of marine cercariae: Effects of gravity and hydrostatic pressure

Background/Question/Methods

Many species of marine larvae use exogenous cues, such as gravity and pressure, for orientation and dispersal.  However, the role of these cues on the swimming behavior of marine parasites is less clear.  Using two species of marine cercariae, Euhaplorchis sp. and Probolocoryphe lanceolata, which utilize the same first intermediate host (mangrove snail), we tested the hypothesis that cercariae use gravity and pressure to position themselves in the water column to facilitate transmission to their second intermediate host (fish and crabs, respectively).  Under dark laboratory conditions, the geotactic behavior of cercariae were measured in a sealed test chamber and recorded with a closed-circuit video system.  Cercariae observed in the top section of the chamber were recorded as displaying negative geotaxis, while those at the bottom were identified as exhibiting positive geotaxis.  Under similar conditions, barokinesis was tested by exposing cercariae to eleven increasing pressures, ranging from 3 to 300 mbar, and returning to atmospheric pressure between each treatment.  Using recorded videos, cercariae were tracked using motion detection software to determine swimming speed, X-Y position, and direction (ascending, descending) at each experimental and control pressure.

Results/Conclusions

We found that Euhaplorchis cercariae displayed negative geotaxis by swimming towards the top of the chamber, increasing the probability of encountering their second intermediate host, killifish.  In contrast, Probolocoryphe lanceolata cercariae swam downwards, towards the bottom of the chamber.  This positive geotactic behavior would place them in a benthic microhabitat where their second intermediate host, burrowing crabs, are found.  While both species displayed a strong geotactic response, increases in hydrostatic pressure did not evoke a strong behavioral response.  We found no significant differences in swimming speed or direction (ascent or descent) between atmospheric pressure and the higher pressure treatments for both trematode species.  Our results suggest that while cercariae are unresponsive to changes in hydrostatic pressure, their strong geotactic response would position them in a preferred microhabitat of their second intermediate host.  This finding is consistent with published photobehavioral studies in which Euphaplorchis swam upwards toward light and P.lanceolata swam downward towards the bottom of the chamber upon light exposure.   These complementary phototactic and geotactic behaviors optimize host contact, facilitating transmission to their second intermediate hosts.