Sticking to it: testing passive pull-off forces in waterfall-climbing fishes across challenging substrates

Abstract

The pelvic sucker of Hawaiian waterfall climbing gobies allows these fishes to attach to substrates while climbing waterfalls tens to hundreds of meters tall. Climbing ability varies by species and may be further modulated by the physical characteristics of the waterfall substrate. In this study, we investigated the influence of surface wettability (hydrophobic versus hydrophilic surface charges) and substrate roughness on the passive adhesive system of four species of gobies with different climbing abilities. Overall, passive adhesive performance varied by species and substrate, with the strongest climbers showing the highest shear pull-off forces, particularly on rough surfaces. Thus, differences in passive adhesive performance may help to explain the ability of some species to migrate further upstream than others and contribute to their ability to invade new habitats.

Stickiness in shear: stiffness, shape, and sealing in bioinspired suction cups affect shear performance on diverse surfaces

“…This exceptional performance is attributed to the hierarchically structured papillae around the edge of disc margin, which interlock with rough asperities and seal the disc as it is pulled [4,13,15]. Similar research has been conducted within different groups, including gobies [5,16], remora [17,18] and loaches [19,20], with each study highlighting the importance of contact area, sealing, and friction for performance on diverse surfaces.…”

“…For shear performance, we considered the peak force attained during the shear pull, corresponding to a static load reached as the cup slid (figure 4(a)). Previous shear studies using fish [5] and bioinspired discs [24] used similar methods of considering peak force. Shear stress (kPa) was calculated from the peak force (N) divided by the area of the fully-engaged suction cup (cm 2 ).…”

“…Attachment mechanisms such as suction, bioadhesives, and frictional elements can stabilize these organisms against forces introduced through crashing waves, strong directional flows, and predation [1][2][3]. In particular, the use of suction organs for aquatic adhesion has been described in multiple groups including various fish [2,4,5], cephalopods [6,7], and even freshwater insects [8][9][10][11].…”

“…Despite the great interest in the adhesive capabilities of these organisms, there are still avenues of research that remain largely unexplored. For example, it is primarily understood that bony elements within the disc provide rigidity, derived from modified structures such as paired pelvic and pectoral girdles (clingfish) [14,15], pelvic fin rays (gobies) [5,21], and dorsal fins (remoras) [17,22]. However, less is known about how the morphological diversity of such structures affects adhesion.…”

“…stiffness, shape) may affect adhesive performance. However, comparative studies focusing on these requirements within aquatic adhesion has been somewhat limited, with only more recent studies exploring performance across multiple species [5,14,16].…”