Traits

Similarly to certain plants and plankton, this trait expresses itself in the form of glowing spots, markings, or body parts on the leech! 

It's noted that the glow is stronger in well-fed leeches. The glow can be turned on and off at will, a recent development previously thought impossible because observed pets preferred to keep their lights on even as they slept.
Wild leeches use it as a lure to draw in all manner of prey, and tend to have lightning-fast reflexes to quickly snatch up their food.