Mucus Matters: The Slippery and Complex Surfaces of Fish

Abstract

Teleost scales are extremely diverse in morphology, with different cate- gories (cycloid, crenate, spinoid, ctenoid) once used to define major groups of fish. We describe these different classical categories of scales and discuss the structure and potential function of small features of scale morphology such as spines, ctenii, radii, and circuli. Modern techniques now make analysis of scale morphology using three-dimensional quantitative data possible. This ability is crucial because many of the hydrodynamic and protective hypotheses concerning the function of scales are dependent on three-dimensional structure. We discuss different techniques to inves- tigate and image the structure of fish scales and skin, and we highlight gel-based surface profilometry as a new valuable tool for studying fish skin. In addition to bony scales, fish skin is also covered by an epidermis that secretes mucus that can coat the exterior of scales. Fish scales are often studied in isolation with the epider- mis removed; here we present topographic, three-dimensional, analyses of fish skin surfaces from seven species with the mucus, epidermis, and relative positions of scales intact. We compare these images qualitatively and quantitatively to the same individuals with the epidermis and mucus removed to show a previously unexplored axis of diversity in fish: how mucus and epidermis interact with scale morphology to create surface texture. The three-dimensional structure of fish skin has important implications for hydrodynamic function during locomotion, but this remains a largely unexplored area. 10.1