Famed for their medicinal uses in ages past, leeches love nothing more than a juicy blood meal. But before they can sink their jaws into a succulent snack, most leeches have first to find a victim. Cynthia Harley and colleagues, Javier Cienfuegos and Daniel Wagenaar, from the California Institute of Technology, USA, explain that many aquatic predators, including leeches, locate prey using the tell-tale water waves produced by the victim's movements. But other things make waves too, like rain and falling objects, so how do hungry leeches distinguish between distracting environmental noise and signs of tasty life? According to Harley, many aquatic predators only react to specific water wave frequencies, which are characteristic of their prey of choice. However, leeches are known to switch their dining preference as they mature, gorging on fish and amphibians when young while opting for more nutritious mammalian blood as adults. So, do leeches have a water wave frequency preference and, if so, which senses do they use to detect the vibrations and does their use alter with age? Intrigued, the trio began analysing leeches' responses to water waves ().
Harley explains that leeches can sense water waves in one of two ways: either with vibration-sensitive hairs on their bodies or by using simple light-sensitive eyes that pick up the shadows of passing waves. So the team devised a series of tests where they could generate real waves and the optical illusion of waves while monitoring whether or not hungry juvenile, adolescent and adult leeches headed toward the wave's source.
Generating waves using a plastic disc driven by a loudspeaker vibrating at frequencies ranging from 2 to 24 Hz, the trio filmed the leeches in natural light and found that the adults responded most strongly to mid-range frequencies between 8 and 12 Hz. However, the juveniles seemed to prefer 2 Hz, although they reacted to every frequency to some extent. So, the leeches did have preferences, and they changed as the animals aged.
Next, Harley and her colleagues tested the leeches' reactions to the sensation of waves alone, and then to the visual effects of the passing wave shadows in still water, to find out whether individual senses were tuned to particular wave frequencies.
First, they switched off the lights and filmed the animals using infrared. Deprived of the waves' passing shadows, and depending on mechanical sensation alone, all of the age groups responded most strongly to 12 Hz waves and headed toward the vibrating disc. However, when the team suspended a dummy tank above the leeches' tank and cast shadows from the dummy tank's waves over the leeches in calm water, the leeches reacted most to the 2 Hz waves.
The leeches seem to use their visual sense for picking up low frequency water waves and vibration-sensitive hairs to pick up higher frequency passing waves. And when the team tried out-foxing the leeches by generating 12 Hz mechanical waves at one location while showing them 2 Hz wave shadows originating from a different point, they found that the adults only homed in on the mechanical source of the waves, while the juveniles were equally happy to focus on both the mechanical and the visual wave sources.
So their reliance on the sensory systems shifts with age: juvenile leeches depend on both senses, while adults place more emphasis on their mechanical sense to locate a wave's origin and hopefully a tasty mammalian meal.
