Epidermal microstructures on the paired fins of marine sculpins suggest new functional hypotheses supporting benthic station-holding

Harsh environments, such as those with breaking waves and turbulent flows, present extreme challenges to organismal survival. Many animals exploiting these habitats possess adaptations to maintain position under dynamic flow conditions, such as reversible or permanent attachment systems. However, some station-holding fishes (e.g., sculpins) instead rely on morphological and behavioral modifications of their pectoral fins to increase friction with the substrate and combat drag. Despite epidermal microstructures on the fins of other benthic fishes, little exploration of pectoral fin surfaces at the microscopic scale has been undertaken in sculpins. Using scanning electron microscopy, we discovered microscopic, fibrillar projections contained within single cells on the ventral surfaces of the paired fin rays of two intertidal and two subtidal species of marine sculpins. In contrast to subtidal species, the intertidal species possessed epidermal cells with discrete channels separating groups of fibrillar projections. These features bear a striking resemblance to epidermal microstructures described in other fishes but have distinct morphological differences. We suggest the hypothesis that these previously overlooked features contribute to sculpin station-holding performance via enhanced mechanical interactions with the substrate, suggesting new taxa within which to explore potential mechanisms of underwater friction enhancement and adhesion.

Methods
Three specimens from each of four species within the Superfamily Cottoidei were examined: intertidal Artedius lateralis and Oligocottus maculosus, and subtidal Myoxycephalus polyacanthocephalus and Leptocottus armatus. Two individuals of each species were collected on San Juan Island, WA during the summer of 2022. One additional individual per species was provided by the University of Washington Burke Museum ichthyology collection. However, epidermal quality of specimens obtained from the Museum was poor, rendering data to be excluded from formal analysis. Right pectoral and pelvic fins were dissected at the base and prepared for imaging with a Hitachi S3000N or Thermo Scientific Scios 2 DualBeam scanning electron microscope (SEM). Images were taken at 1500X at the approximate midpoint along the free margin of each of the 5 ventral-most regionalized fin pectoral fin rays and the first pelvic fin ray. Some fin rays required multiple images because of the three-dimensional complexity of fin ray shape. For Oligocottus maculosus, each fin ray was imaged along multiple regions of the free margin to explore morphological variability within a single species and individual. Additional images were taken at lower magnifications to verify orientation and document other nearby features, but these were not used for analysis.

Following imaging, FIJI (https://fiji.sc/) was used to divide each picture into quadrants of 1000 mm each (Figure S2) to quantify cell density and protruded area from at least 4 quadrants per fin ray. Cell density was calculated by counting the total number of cells per quadrant, including partial cells along the edges. Protruded area was determined by tracing the outline of the protruded portion (containing the fibrils) of 3 cells per quadrant. To explore the morphometrics of the fibrils covering the surface of cells as well as the width of the channels between the protruded area of cells, we selected the single highest quality image of the pectoral fin ray surface for each individual at 1500x magnification and made estimates of fibril density, diameter, and length, as well as channel width. Three protruded areas (or cells) per image per specimen and three fibrils per cell were selected for measurement. Fibril density was estimated by counting the number of fibrils within a specified area. Fibril diameter was estimated by measuring the diameter of the fibril’s tip, while fibril length was estimated by measuring the length of a line from the base to the tip of the fibril.

This dataset includes: All raw analyzed photos, raw measurement data, and a powerpoint file indicating the position of each photograph along each fin ray for O. maculosus. Manipulations including averages were performed external to these files.