Discontinuous locomotion and prey sensing in the leech

Abstract

SUMMARYThe medicinal leech, Hirudo verbana, is an aquatic predator that utilizes water waves to locate its prey. However, to reach their prey, the leeches must move within the same water that they are using to sense prey. This requires that they either move ballistically towards a pre-determined prey location or that they account for their self-movement and continually track prey. We found that leeches do not localize prey ballistically. Instead, they require continual sensory information to track their prey. Indeed, in the event that the prey moves, leeches will approach the preyʼs new location. While leeches need to continually sense water disturbances to update their percept of prey location, their own behavior is discontinuous -approaching prey involves switching between swimming, crawling and non-locomoting. Each of these behaviors may allow for different sensory capabilities and may require different sensory filters. Here, we examined the sensory capabilities of leeches during each of these behaviors. We found that while one could expect the non-locomoting phases to direct subsequent behaviors, crawling phases were more effective than non-locomotor phases for providing direction. During crawling bouts, leeches adjusted their heading so as to become more directed towards the stimulus. This was not observed during swimming. Furthermore, in the presence of prey-like stimuli, leeches crawled more often and for longer periods of time.

Scanning Behavior in the Medicinal Leech Hirudo verbana

“…A previous study determined that leeches orient toward stimuli during crawling bouts [32]. While at first glance two orientation behaviors may seem redundant, it is possible that they have fundamentally different but cooperative functions.…”

“…This foot was placed such that it lay flat on the surface of the water when at rest. This stimulator created waves in the testing arena containing the leech [32].…”

Detection and selective avoidance of near ultraviolet radiation by an aquatic annelid: the medicinal leech

“…Leeches also use the visible spectrum to orient while swimming, and they respond to shadows, or brief, bright flashes (Kretz et al, 1976). In addition, leeches orient to moving bars of visible light when hungry Carlton and McVean, 1993), and a recent set of elegant experiments have shown that leeches use frequency-modulated (moving bars) visible light in combination with tactile vibration (both of which are related to water disturbances) to orient and locate potential prey (Harley et al, 2011;Harley et al, 2013). Thus, it might be suggested that hirudiniid leeches are not particularly averse to visible light, and under some conditions may use it as an appetitive cue.…”

“…Despite this, leeches in the genus Hirudo are only modestly negatively phototactic when tested with artificial sources of visible light (Mann, 1962). This negative phototaxis to visible light is further reduced by feeding status, such that hungry leeches are somewhat positively phototactic, and have been shown to orient toward moving bars of light associated with water waves that might be produced by potential prey (Carlton and McVean, 1993;Dickinson and Lent, 1984;Harley et al, 2011;Harley et al, 2013). Although there have been a number of studies on the visual system of leeches showing sensitivity to visible wavelengths (Kretz et al, 1976;Laverack, 1969;Walz, 1982), as well as studies characterizing some of the interneuronal targets of photoreceptors within the central nervous system (Laverack, 1969;Peterson, 1984b;Peterson, 1985a;Peterson, 1985b), there has been no report of leeches detecting or responding to UVR.…”

Responses to mechanically and visually cued water waves in the nervous system of the medicinal leech

“…Behaviorally, they can use visual cues, mechanical cues, or a combination of the two to infer the presence of water waves and their direction of propagation (Dickinson and Lent, 1984;Carlton and McVean, 1993;Harley et al, 2011Harley et al, , 2013. Little has been known about processing of visual information in the leech, and less about processing of information relating to water movement.…”