A mechanical piston action may assist pelvic–pectoral fin antagonism in tree-climbing fish

Abstract

In this research, we compared the anatomy and biomechanics of two species of mudskipper vs an aquatic sandgoby in view of terrestrial locomotion. Of particular interest was the relationship (if any) of pectoral fin movement with pelvic fin movement. We show that the pelvic fins of the terrestrial mudskippers studied herein, are retractable and move antagonistically with the pectoral fins. The pelvic fin of the sandgoby studied here is contrarily non-retractable and drags on any underlying substrate that the sandgoby tries to crawl across. We find that the pelvic and pectoral fin muscles of all fish are separated, but that the pectoral fins of the mudskipper species have bulkier radial muscles than the sandgoby. By coupling a detailed morphological investigation of pectoral-pelvic fins musculature with finite element simulations, we find that unlike sandgobies, the mudskipper species are able to mechanically push the pelvic fins downward as pectoral fins retract. This allows for an instant movement of pelvic fins during the pectoral fin backward stroke and as such the pelvic fins stabilize mudskippers through Stefan attachment of their pelvic fins. This mechanism seems to be efficient and energy saving and we hypothesize that the piston-like action might benefit pelvic–pectoral fin antagonism by facilitating a mechanical down-thrust. Our research on the biomechanics of tree-climbing fish provides ideas and greater potential for the development of energetically more efficient systems of ambulation in biomimetic robots.

Emersion and Terrestrial Locomotion of the Northern Snakehead (Channa argus) on Multiple Substrates

“…Mudskippers ( Periophthal mus spp.) do use both of their pectoral fins simultaneously in a crutching motion that lifts their body off the substrate ( Swanson and Gibb 2004 ; Pace and Gibb 2009 ; Kawano and Blob 2013 ; Wicaksono et al. 2017 ); however, mudskipper crutching is solely appendage-based locomotion, which is likely unfeasible for adult snakeheads to achieve as they are too large to lift their bodies up and support them on their soft pectoral fins that lack stiffened fin rays.…”

Kinematic comparisons between mudskipper fins and salamander limbs during terrestrial locomotion

“…fins rather than the pelvic fins of Periophthalmus mudskippers (Wicaksono et al, 2018), allowing the former to transmit larger forces to the ground to adduct the fin and body and pitch the anterior end of the body upwards (Figures 3a-e and 4a-e). The radials tend to be longer in mudskippers compared to other goby fishes within…”

“…Gobiiformes that have limited terrestrial capabilities (Wicaksono et al, 2018), which results in a longer moment arm that can produce larger movements about the shoulder for a given amount of force.…”

“…However, the pelvic fins serve their own important role during locomotion. The antagonistic movements of the pectoral and pelvic fins during crutching result in the radials of the pectoral fins compressing the body during retraction, which causes the pelvic fins to be pushed towards the substrate and stabilized through Stefan adhesion (Wicaksono et al, 2018). This mechanism was described as a "mechanically driven piston" that is thought to reduce the energetic costs of terrestrial locomotion and provide faster movements because it does not rely entirely on active muscular contractions (Wicaksono et al, 2018).…”

The water-hopping kinematics of the tree-climbing fish, Periophthalmus variabilis

“…Mudskippers are amphibious fishes that have developed a locomotor ability on land, by which means conduct continuous movements known as 'crutching' (Pace and Gibb, 2009). To improve their locomotive abilities on land, they use their pectoral fins antagonistically with their pelvic fins, such that their pelvic fins are deployed as their pectoral fins are pulled back towards the body, and vice versa (Wicaksono et al, 2017). In some cases, mudskippers, such as Periophthalmus variabilis have adapted fin-morphologies enabling them to climb on inclined surfaces, vertical trees trunks and igneous rock faces (Wicaksono et al, 2016).…”