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

• Geotactic and barokinetic behaviors were measured for two species of marine cercariae.
• Euhaplorchis cercariae ascended in darkness, demonstrating negative geotaxis.
• Probolocoryphe lanceolata cercariae swam downwards, displaying positive geotaxis.
• Geotaxis allows cercariae to increase the probability of contacting their next host.
• Cercariae were unresponsive to increases in hydrostatic pressure.

Many species of marine larvae use exogenous cues, such as gravity and pressure, for orientation and dispersal, but it is unclear if parasitic larvae exhibit comparable geotactic and barokinetic behaviors. Using two species of marine cercariae, Euhaplorchis sp. and Probolocoryphe lanceolata, which utilize the same first intermediate host, the mangrove snail Cerithidea scalariformis, the hypothesis that they use gravity and pressure to position themselves in the water column to increase the probability of contacting their respective second intermediate hosts was tested. In a test chamber under dark laboratory conditions, Euhaplorchis sp. cercariae ascended, and therefore displayed negative geotaxis, increasing the probability of finding their second intermediate host, salt marsh killifish (Fundulus spp.). In contrast, P. lanceoloata cercariae swam downwards towards the bottom of the chamber, and this positive geotactic behavior would place them in a benthic microhabitat consistent with their second intermediate host, fiddler crabs (Uca spp.). In a subsequent experiment testing swimming activity in response to step increases in pressure, no effect of pressure changes over the range tested (0.3–30 kPa) was observed for either species. Collectively, these results suggest that while cercariae are unresponsive to changes in pressure, they are able to utilize gravity for orientation, placing them in the preferred microhabitat of the second intermediate host. This finding is consistent with published photobehavioral studies in which Euhaplorchis swam upwards towards light and P. lanceolata swam downward towards the bottom upon light stimulation. These complementary phototactic and geotactic behaviors optimize host contact, and thus completion of their complex life cycle.