Goblet Cell Density of Adhesive Structures Correlates With Climbing Ability in Hawaiian Stream Gobies

Hawaiian stream gobies exhibit diverse adhesive abilities that can be used by these fishes to help climb waterfalls. Mucus is recognized as contributing to successful performance in many adhesive systems, but potential specializations of mucus production and composition have not been tested in these fishes. This study examines how anatomical (sucker size and goblet cell density) and biochemical (mucus composition) traits may contribute to adhesive success in climbing gobies. Using histological and spectroscopic analyses, we quantified the density of mucus-producing goblet cells in adhesive structures (lips and pelvic suckers) and assessed differences in mucus chemistry between the pelvic suckers and the body. Goblet cell density in lips and suckers increased with climbing ability, aligning with species distribution across stream elevations. The non-climbing Stenogobius hawaiiensis exhibited the lowest goblet cell densities, while the best climbers (Sicyopterus stimpsoni and Lentipes concolor) showed the highest densities. Among inching climbing gobies that use the mouth to climb especially as juveniles, goblet cell density in the lips was, instead, greater in adult individuals. This suggests that mucus production of the lips may have a broader protective role during interactions with rough substrates, rather than a strict relationship with adhesive performance. Infrared spectroscopy of mucus revealed similar chemical signatures in both sucker and caudal peduncle mucus, suggesting that mucus composition does not change across the body to enhance adhesion. These findings indicate that goblet cell density and, thus, enhanced mucus production (rather than compositional changes) may aid the adhesive performance of climbing gobies, contributing to their ecological success. Understanding these adhesive mechanisms from tissue to whole-animal levels of organization clarifies the specific factors that were specialized during the evolution of the distinctive locomotor behavior of these amphidromous fishes.