Sucker Shapes, Skeletons, and Bioinspiration: How Hard and Soft Tissue Morphology Generates Adhesive Performance in Waterfall Climbing Goby Fishes

Abstract

Many teleost fishes, such as gobies, have fused their paired pelvic fins into an adhesive disc. Gobies can use their pelvic suckers to generate passive adhesive forces (as in engineered suction cups) and different species exhibit a range of adhesive performance, with some even able to climb waterfalls. Previous studies have documented that, in the Hawaiian Islands, species capable of climbing higher waterfalls produce the highest passive pull-off forces, and species found at higher elevation sites are likely to have more rounded suction discs than those found in the lowest stream segments. Morphology of the pelvic girdle also varies between species, with more robust skeletons in taxa with superior passive adhesion. To investigate what factors impact the passive adhesive performance of waterfall climbing gobies, we tested biomimetic suction cups designed with a range of shapes and embedded bioinspired “skeletons” based on micro-CT scans of goby pelvic girdles. We found that while the presence of an internal skeleton may provide some support against failure, the performance of suction cups may be more strongly affected by their external shape. Nonetheless, factors besides external shape and skeletal morphology may still have a stronger influence on sucker tenacity. Our results suggest that the relationship between suction disc morphology and adhesive performance may be influenced by a variety of physical factors, and live animal performance likely is further complicated by muscle activation and climbing behavior. These results have implications for the evolution of suction disc shape in adhesive fishes and for improving the design of biomimetic suction cups.

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

“…Once the seal breaks, the pressure differential between the suction chamber and the external environment is lost [4,15]. Therefore there is a tradeoff-having sufficient compliance in the cup wall for increased ability to conform to the surface and maintain a seal, though not too much compliance which will limit the cup's ability to resist deformation and slipping to failure [21,25,29]. In terms of shape, previous work by Palecek et al [21] did test an oval-shaped design under normal loading conditions.…”

“…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.…”

“…In addition to providing rigidity, the arrangement of these bony structures can change the overall shape of the disc, producing forms that are more elliptical or ovate [3,21]. This shape can be further defined by additional features such as the presence of a soft rim, which may affect the overall design of the contacting disc margin (e.g.…”

“…For example, Gamel, et al produced bioinspired remora discs to test how increasing the number of lamellae affects frictional performance during shear [24]. Palacek, et al embedded 3D-printed pelvic girdles from gobies within silicone cups to test performance under normal pull conditions [21]. Huie, et al tested stiff suction cups with soft rims on various compliant surfaces to evaluate sealing on these substrates [25].…”

“…To achieve variable stiffness, we fabricated elliptical cups, which resulted in shorter walls along the minor axis. This produced higher taper angles along the sides of the cup [14,21], and the lower aspect ratio of the shorter walls would create regions of higher stiffness. Based on recent studies of skeletal structures within adhesive discs [14,21], we hypothesized these regions of stiffness would provide the cup with additional structural integrity needed to maintain the suction chamber under load, namely, a cup which was stiffer on the minor axis would have a higher resistance to shear applied on the major axis.…”