Climbing upstream: Multi-scale structural characterization and underwater adhesion of the Pulin river loach (Sinogastromyzon puliensis)

Hillstream fishes live in the current with high flow velocity and strong water impact. To survive in such environments, they evolved various adhesive apparatus and attachment mechanisms. In this study, we investigated the Pulin river loach (Sinogatromyzon puliensis) known for its ability to climb vertically against rapid torrent. The whole body forms a suction system, consisting of lips, overlapped pectoral and fused pelvic fins. The hierarchical structure of lips and fins was characterized by Scanning Electron Microscopy. Polygonal pad-like protrusions, called unculi, were found on the lips and micro-sized, needle-like unculi (setae) covered both pectoral and pelvic fin rays. Pull-off stress was measured to evaluate the adhesion capability of dead river loach to substrates with varying surface roughness and compared with commercial suction cups. Results showed that river loaches can adhere equally well to surfaces with varying roughness while suction cups fail to adhere to rough surfaces. Underwater adhesion was accomplished by three possible mechanisms: (1) unculi enhance friction force on substrates, (2) setae interlock with irregularities, and (3) setae seal the edge and prevent leakage. The specialized adhesion apparatus and capability of Pulin river loach could provide inspirations and design strategies for novel underwater adhesives and devices.