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

Abstract

Medicinal leeches are aquatic predators that inhabit surface waters during daylight and also leave the water where they might be exposed to less screened light. Whereas the leech visual system has been shown to respond to visible light, leeches in the genus Hirudo do not appear to be as negatively phototactic as one might expect in order to avoid potential ultraviolet radiation (UVR)-induced damage. I used high intensity light emitting diodes to test the hypothesis that leeches could detect and specifically avoid near UVR (395-405 nm). Groups of unfed juvenile leeches exhibited a robust negative phototaxis to UVR, but had no behavioral response to blue or red and only a slight negative phototaxis to green and white light. Individual leeches also exhibited a vigorous negative phototaxis to UVR; responding in 100% of trials compared with modest negative responses to visible light (responding in ~8% of the trials). The responses in fed and unfed leeches were comparable for UVR stimuli. The responses depended upon the stimulus site: leeches shortened away from UV light to the head, and extended away from UV light to the tail. Electrophysiological nerve recordings showed that the cephalic eyes responded vigorously to UVR. Additionally, individual leech photoreceptors also showed strong responses to UVR, and a higher-order neuron associated with shortening and rapid behavioral responses, the S-cell, was activated by UVR, on both the head and tail. These results demonstrate that the leech can detect UVR and is able to discriminate behaviorally between UVR and visible light.

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

“…Little has been known about processing of visual information in the leech, and less about processing of information relating to water movement. It was known, however, that one particular interneuron, the S cell, responds to both visual and mechanical stimuli (Bagnoli et al, 1973;Gardner-Medwin et al, 1973;Kretz et al, 1976;Sahley et al, 1994;Jellies, 2014). By presenting nearly intact leeches with natural water waves in complete darkness, we demonstrated that the S cell responds readily to purely mechanically cued water waves (Fig.…”

“…As mentioned, the vast majority of S cell spikes recorded near the middle of the leech (in segments 10 and 14) traveled in a caudorostral direction (Figs 3D,E and 4A). This may be caused by the same mechanism that causes light flashes onto the tail of a leech to elicit more S cell spikes than light flashes onto the head (Jellies, 2014;Wagenaar and Stowasser, 2016). One possible explanation of these effects could be that posterior sensilla may be more sensitive to light and water motion than the anterior sensilla.…”

A classic model animal in the 21st century: recent lessons from the leech nervous system

“…Despite the fact that the leech has been the subject of biological investigation for over 100 years, studies performed over the past decade have led to the identification of two newly described behaviors, which I discuss in turn: one is the first social behavior described in the leech (Bisson and Torre, 2011), the other is a newly discovered sensitivity to UV light (Jellies, 2014a).…”

“…However, invertebrate opsins that are sensitive to green light commonly have a substantial secondary sensitivity peak at UV wavelengths (Stavenga, 2010), suggesting that this assumption might be false. Indeed, Jellies (2014a) found that leeches retract or move away from bright UV light (400 nm) projected onto their dorsal surfaces. Such responses were rarely seen to visible light.…”

UV-induced DNA damage in coral reef fish: damage levels and protection mechanisms

“…Avoidance behaviours to UV radiation have been shown in a range of aquatic organisms, and include the movements of bacteria (Bebout & Garcia-Pichel 1995), negative phototaxis in zooplankton (Pennington & Emlet 1986, Alonso et al 2004, Hansson et al 2007), movement in leeches (Jellies 2014), settlement in coral larvae (Gleason et al 2006) and covering behaviours of sea urchin (Adams 2001, Verling et al 2002, Dumont et al 2007). …”