I went slug watching last Saturday morning. This is something that I normally do in the spring, but Saturday's weather was indeed spring-like. Moreover, it had rained all day Friday and throughout the nite. So, on Saturday morning the slugs that would normally have been hiding away from the cold, the snow or the ice this time of the year were instead out on the wet trees feasting on the fungi and the algae.
This particular individual was on a small branch on the ground. It is probably a Megapallifera, one of the eastern U.S. natives. They are usually hard to identify without dissection. They leave characteristic feeding tracks on beech trees.
If you watch a slug carefully, you can tell when it is scraping something with its radula, because its head will rock back and forth. This one was doing that when I photographed it.
After I removed the slug to a nearby tree, I broke the section of the branch it had been on and took it with me. Later, under the microscope I could see that the surface of the branch was covered with some sort of crust fungus. There were also lighter colored areas where the top layer of the fungus had been removed (arrows in the picture below). I believe those are the marks left by the slug's radula.
Ruler is in millimeters.