Which way is up? Asymmetric spectral input along the dorsal–ventral axis influences postural responses in an amphibious annelid

Abstract

Medicinal leeches are predatory annelids that exhibit countershading and reside in aquatic environments where light levels might be variable. They also leave the water and must contend with terrestrial environments. Yet, leeches generally maintain a dorsal upward position despite lacking statocysts. Leeches respond visually to both green and near-ultraviolet (UV) light. I used LEDs to test the hypothesis that ventral, but not dorsal UV would evoke compensatory movements to orient the body. Untethered leeches were tested using LEDs emitting at red (632 nm), green (513 nm), blue (455 nm) and UV (372 nm). UV light evoked responses in 100 % of trials and the leeches often rotated the ventral surface away from it. Visible light evoked no or modest responses (12-15 % of trials) and no body rotation. Electrophysiological recordings showed that ventral sensilla responded best to UV, dorsal sensilla to green. Additionally, a higher order interneuron that is engaged in a variety of parallel networks responded vigorously to UV presented ventrally, and both the visible and UV responses exhibited pronounced light adaptation. These results strongly support the suggestion that a dorsal light reflex in the leech uses spectral comparisons across the dorsal-ventral axis rather than, or in addition to, luminance.

Planarian Phototactic Assay Reveals Differential Behavioral Responses Based on Wavelength

“…To verify that the amount of light gradually decreased from Q1 to Q4, the intensity of light in each quadrant was estimated with a phototransistor. Finally, the assay used easily-constructed LED wands powered by 9 volt batteries, as previously described [30] , [31] , which allowed for some control of the ranges of wavelengths tested. Each wand was also designed to deliver roughly equivalent numbers of photons cm −2 s −1 (flux) [30] , [31] .…”

“…Ambient lighting was generated by directing two 100 watt LED flashlights onto the walls on either side of an otherwise completely dark room to produce diffuse background illumination of 50–55 lux (“Light Meter–Lux Measurement Tool” Version 1.2, iPhone application). LED wands (fixed resistor and RCA plug attached to a 9 volt battery with switch) were constructed as previously described [30] , [31] . Each wand delivered roughly equivalent numbers of photons cm −2 s −1 (flux), with the following nominal wavelength ranges: near IR (700–850 nm), red (615–640 nm), green (515–520 nm), blue (460–470 nm), and 2 wavelengths of near UV (395–405 nm and 360–365 nm).…”

“…Filters used were 25.4 mm diameter nickel chromium coated fused silica (7980) as previously described [30] , [31] . A holder was designed from stiff foam pipe insulation to position the LED wand above the filter such that all emitted light passed through the filter.…”

“…For example, zebrafish larvae will swim toward ultraviolet (UV), blue, and red light but are only weakly attracted to green light [29] . Conversely, leeches detect and exhibit complex negative phototactic responses to UV and green wavelengths, with UV producing the maximal response [30] , [31] . In Drosophila larvae, exposure to blue, violet, and UV wavelengths elicits negative phototaxis, while green and red light produces no behavioral response [32] .…”

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

“…In an elegant follow-up study with more tightly controlled light intensities, Jellies (2014b) found that leeches also respond strongly to UV light projected onto their ventral surface, primarily by locally twisting away. Surprisingly, at the level of the central nervous system, leeches respond more strongly to UV light than to green light when ventral sensilla act as the input pathway.…”