I discovered a yellow slug yesterday (16 July 2017) while poking about in some rotting leaves and sticks in the back yard.
The only yellow slugs I had ever seen previously were banana slugs in Muir Woods in California. Those slugs were huge and beyond surprising. This slug is much, much smaller, but also surprising, mostly because of its color.
Being totally ignorant on the subject of mollusks, I tried looking this one up with Google. After some searching I decided that it’s probably a Dusky Arion (Arion subfuscus). This species was introduced from northwestern Europe long enough ago to have become completely naturalized throughout much of the eastern part of the United States. One source I looked at notes that they “have become more common than native species in many areas, and are one of the most abundant slugs found in gardens, fields, and forests.” They have also been introduced into California.
The nature guides I looked at say that Dusky Arions measure 50 to 70 mm. I think “my” slug is a bit on the small side (I didn’t measure it, because, well, it just didn’t occur to me to do that). But the guides also describe a yellow mucus trail, and “my” slug did leave such a trail. It’s visible on the stick behind the slug in the second picture above.
Click on these links for more information about the Dusky Arion: