Light is an important exogenous cue that controls the distribution of a wide range of marine larvae and zooplankton. Parasites with multi-host life cycles can use light to locate specific microhabitats, thereby increasing the probability of encountering their next host. For example, free-swimming cercariae (Trematoda) can perceive and respond to a suite of exogenous factors including light to orient themselves in the water column where they are more likely to find and infect a second intermediate host. Here, we studied light as an initiating and orienting cue for swimming in two species of marine cercariae, Euhaplorchis sp. and Probolocoryphe lanceolata, which initially parasitize the same species of benthic snail, but then utilize different second intermediate hosts located in pelagic and benthic habitats, respectively. We measured photoresponses of cercariae to different light intensities under two types of light fields in order to isolate different aspects of light as a behavioral factor. First, we conducted tests in a simulated underwater light field to quantify light as an initiating factor in swimming behavior. Secondly, we used a horizontally-directed light source to determine if light is an orienting cue for cercarial swimming.
Results/Conclusions
In experiments that simulated downwelling light, dark-adapted Euhaplorchis cercariae swam slowly in darkness, but ascended quickly towards light at quantal intensities >4.0 x 1015 photons m-2 s-1. With a horizontal light source, they oriented towards the light, confirming that light plays both an initiating and orienting role in phototactic behavior resulting in ascent in the water column in order to locate a fish host. Their swimming speed increased significantly with increasing light intensity, exceeding 1.5 mm s-1 under the highest light levels. In contrast, P. lanceolata cercariae exhibited haphazard vertical swimming in darkness, but exhibited downward swimming upon exposure to light intensities >4.0 x 1014 photons m-2 s-1. Such descent can quickly bring cercariae to the benthic environment, facilitating their contact with crabs. They also swam significantly faster (up to 1.08 mm s-1) during the light stimulus compared to darkness. However, P. lanceolata cercariae did not swim in response to a directional light source, suggesting that while light initiated their descent, their orientation may be controlled by another factor, such as pressure or gravity. These light-mediated behavioral responses support the use of light in selecting for microhabitats frequented by potential hosts; an adaptive benefit that may enhance transmission success.