Mapping an Unseen Landscape: An Analysis of Scale Texture in Two Darter Fishes

Fish scales can vary considerably in surface texture and morphological features, and these attributes may impact energetic performance during swimming or station holding in swiftly flowing waters. We examined the scale texture of two members of the darter family, Etheostoma blennioides (the Greenside Darter) and Nothonotus rufilineatus (the Redline Darter), which differ in body size and habitat specialization. We applied gel-based profilometry, a non-destructive approach to produce high resolution 3D reconstructions of surface features, to measure surface texture in these two species. Texture properties from these scans can be quantified using surface metrological variables used in mechanical engineering and geographic terrain mapping. Using the novel handheld GelSight tool, high resolution scans were taken from preserved specimens and were processed using OmniSurf3D. Through these scans, 11 surface metrology variables were collected from two anatomical locations (caudal and flank scales). We found significant differences in at least seven of those traits in regard to species and body size. Redline Darters possessed proportionately rougher, spikier, and more ornamented scales, and these species are more specialized to high current environments. However, Greenside Darters also showed consistently stronger allometry in texture values, with large specimens showing similar values to Redline Darters. We suggest that these differences may have functional implications for the varying current and turbulent conditions in particular. Such studies are rare for both smaller and rheophilic fishes, and further testing (e.g., macroevolutionary, fluid mechanics) may reveal connections between surface texture and life in turbulent environments.