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it's either arion vulgaris or arion rufus, depending on ths size. adults of arion vulgaris get up to 12 cm in length and arion rufus even 15 cm. concerning it is allmost black, it could be even arion ater, whose size would be between the above mentioned. the only thing i can say for sure, is, that it is a arion "something". and gross to touch.
I think it's the yellow bellied slider because that's just the kind of turtle someone buys, outgrows the aquarium and is then released "to the wild". Here in Vienna we already have a population of red-eared slider and yellow-bellied sliders in the Prater because people always released them in the same spot.
the dark spots on the lower house is soil i wasn't able to wash out, the yellow colour that seems to be a spot was actually the snails real colour.
as i understand it, snails use to chew on empty houses of "former" snails because they need a calcium source for building up theirs, and as it looks like, the topp one has been chewed on a lot. i have also a spotting of a different colour-morphing of capea hortensis with allmost the same drawing (thus, same or close population) where you can see this "bleaching" effect even better.
i looked it up in the german wikipedia, and there it says, that you can't really distinguish them by color, only that spanish slug is smaller (8-12 cm for adults) and red slug is bigger(12-15 cm for adults) and has bigger genitals
i had them in my garden in large quantities always, just not this year because we had a pretty cold winter. i think the main difference is, that the red slug has a more agressive, intensive red that looks almost poisonous.