Black slugs are a common invasive species that are voracious eaters and enemy of cultivators everywhere. This one caught my attention because of the mysterious purplish glow in its belly. What could it be?
Answer from Grok query:
The black slug (Arion ater), a common terrestrial species in Europe and parts of North America, typically has a dark dorsal surface ranging from jet black to brown or gray. However, its ventral side (belly or foot) often appears lighter—usually pale gray, off-white, or yellowish—and can exhibit a subtle iridescent sheen. This iridescence arises from thin-film interference in the mucus layer covering the foot, where microscopic layers of mucus and skin refract light to produce shifting hues like purple, blue, or green under certain angles or lighting conditions. The "glow" is not true bioluminescence (a chemical light emission seen in some deep-sea creatures) but an optical effect, similar to the rainbow sheen on oil slicks or soap bubbles. It's more noticeable when the slug is moving on a reflective surface or in low light, as the mucus spreads thinly and interacts with ambient light.
This trait serves no known specific function beyond the mucus's primary roles: aiding locomotion by reducing friction and protecting against desiccation. Pigmentation variations in black slugs, including lighter bellies, are influenced by genetics, latitude, and temperature, with northern populations tending toward darker overall tones. If you're observing this in a different slug species (e.g., certain invasive leatherleaf slugs like Belocaulus angustipes, which have a faint tan ventral stripe), the effect could be even subtler, but the iridescent purple "glow" description aligns best with the common black slug.
I had no idea... nature always surprises me, and that glow is impressive; it seems to radiate something!
I didn't know this either. They're strange creatures.
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