11 June 2007

What the slug had eaten for dinner

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.


Pete Krull said...

Aydin; This slug is obviously a "scraper". That is, it scrapes food off of a hard surface like a rock, piece of bark etc. Many snails are scrapers as well. One of the more well known ones is Liguus which feed off of the stuff on tree trunks and branches. These snails, when feeding leave a trail so it is obvious as to what they are eating(you could study it without waiting for it to pass through the snail's intestines). It's also one way of finding these snails, by following their feeding trail. I've never really liked slugs but I wonder if they also leave a trail as they graze? I don't see one in the photo. By the way, your posts are always interesting.

Megan said...

When I was last doing slug fecal analysis, I think I figured out if chlorophyll was from algae or plant leaves based on the presence of continuous pieces of "plant tissue," but I didn't realize you could distinguish algal cells too based on their shape. That's a neat and useful observation. Great photos, too.


Pete: Slugs too leave feeding tracks behind them. Here are 2 posts about them:

Tree hugging slugs

Saturday's slug

Michael said...

Hello Aydin! The four large pollen grains in your picture are so called bisaccate pollen which are produced by pine and fer trees.


Michael, thanks for the comment. We do have a couple of pines in the backyard. I will see if I can get a pollen sample from them.

umit said...

In Lıkharev and Rammelmeier slugs and snails tend to be mushroom lovers, but they couldnt digest the intermediate algae zone cells within lichens. i think your spherical cella mass belongs to these.

Nuthatch said...

As an ecologist I've done a lot of strange things, and not just a few involving poop, but sorting through slug feces -- just for the hell of it -- man, you've got me beat.

I can tell we'd really get along.


Hey, if I do enough of them, I might get a paper out of it.

Anonymous said...

I can't believe that five people posted on a blog post about dissecting slug poop.

(six now. fucking google)

Anonymous said...

hey, I just want to say that I very much appreciate this article, and snailstales in general. I'm a new snail collector and it is very hard to find information about slugs or snails. Just want to let you know I appreciate your contribution to this understudied area, and to encourage you to keep the flow of information going, because it is worthiwhile to a lot of people to sit here and read about slug feces.