Several days ago one night, following an afternoon rain, I went out into the backyard to check out the creatures. While the isopods were loitering in the bird bath, this small Arion subfuscus was crawling on a rock covered with green algae (cyanobacteria). Was the slug eating the algae? There was only one way to find out.
I brought the slug in and placed it in an empty plastic container with plenty of water to make sure it wasn't going to dry up, but, without anything to eat. About 24 hours later, there was plenty of slug poop in the box. I removed the poop and returned the slug to the backyard.
So what had the slug eaten for dinner? Its feces teased apart in a little drop of glycerol provided a partial answer. One needs to be a botanist and turn this into a major research project before every bit of matter in a slug's feces can be identified. I will do my best.
The above picture is an overall view of a part of the sample. As is typical with these slugs, this one had eaten a lot of different things, including both live (green) and dead (brown) matter. The long, brown twisted string-like material (arrow) is probably what is referred to as the liver string in the literature1. They are present in almost every fecal sample from slugs. When teased apart, liver strings do not yield anything identifiable, because, unlike the rest of the material, they are more or less amorphous and homogenous.
The four "large" objects in this picture are pollen (they are actually about 66 µm long). I can't tell which plant they came from. The much smaller and abundant brown things (red arrows) are probably fungal spores.
Here are 2 close-up shots of what were live plants before they were eaten. The fragment on the left appears to be a piece from a leaf.
If the slug had eaten algae from the rock it was on, what would they look like in its feces? To help me answer that question, I scraped some algae off the rock and examined them under the microscope. Fortunately, the cells of cyanobacteria are easily distinguishable from plant remains, because the former are spherical. The picture below shows one such cell (diameter: ~16 µm) from the rock sample.
There were many similar looking cells in the feces of the slug. Compare them with the plant material in the picture above where the fragments have more irregular shapes. Also visible in these photos what appeared to be fungal hyphae. These slugs indeed feed on whatever comes their way, even dead earthworms.
1Runham & Hunter. 1970. Terrestrial Slugs. See p. 57.