With gel-based profilometry, you can tune the gel properties to match even very soft surfaces, such as the epidermis and mucus that covers the scales of live fish. Above, we show a bluegill (Lepomis macrochirus) that was imaged with and without mucus. Without mucus, many surface details of scales are obvious, such as the concentric growth lines of each scale, the lateral line, and clear margins made of spiny ctenii. When mucus is present, the surface details are obscured. Below each image we provide tables of common surface parameters including root-mean-square roughness (Sq – http://bit.ly/2Amhpeb), kurtosis (Sku – http://bit.ly/2zUY8ne), and skew (Ssk – http://bit.ly/2zUY8ne). Roughness is much lower on the surface with mucus, demonstrating its smoothing effect. This smoothing effect and the material properties of mucus will likely affect the swimming performance of this fish, and these results show how useful this technique can be for exploring surfaces of live animals. Images: Dylan Wainwright and the Freshwater and Marine Image Bank.