The locomotive behaviours of the tree-climbing fish, Periophthalmus variabilis: a cross-disciplinary approach to bioinspired design

Mudskippers constitute a group of amphibious fish that have adapted for terrestrial locomotion. Some species in this group possess a tree-climbing ability. This study looks at the biomechanics behind this unique locomotion to assess their prospects for engineering applications. Our main objective in this study was to characterise the mudskippers’ body features that relate to the terrestrial behaviour. We achieved this by developing simulation-based models to replicate the locomotive functions for engineering purposes.

We sampled the mudskippers through on-site (in situ) recording — employing cameras to capture both moving and still images — in order to conduct mechanical and kinematical analyses. We also collected off-site (ex situ) data, recording pelvic fin features to examine individual fin flexibility through finite element modelling (FEM). Additionally, we dissected fish to acquire data for an FEM focused on the skeletal system and musculature of the pectoral and pelvic fins. Furthermore, we obtained and analysed mucus samples from the mudskippers using Fourier transformation infrared (FTIR) spectroscopy. Following FTIR, we modelled the mucus chemical component through molecular dynamics simulations to assess its biomolecular interactions with the substrates on which the fish are commonly found. We conducted these simulations to test the hypothesis that the mucus provides the mudskipper with additional adherence support during vertical substrate attachment (i.e., tree-climbing).

The results of the molecular dynamics simulations show that the mucus chemical compound (hyaluronic acid [HA]) attracts the substrate compounds (calcium carbonate of the limestone, silica, and plant cell wall components: cellulose, hemicellulose, and lignin), indicating that the mucus positively supports mudskippers’ adhesion. Complementing the mucus, the high-structural-flexibility pelvic fins enable the fish to grip the surface of the substrate. The mudskippers are unique in that their scales are covered by skin and mucus—most fish have skin covered by scales. Furthermore, their pelvic fins can be dispatched downward, like a piston passively using the inward push of pectoral fins as an efficient energy-saving method. Finally, the mudskipper can hop over water surfaces. It is able to conserve its kinetic energy throughout the water-hopping sequence to perform a long, efficient hop as an escape mechanism.

Learning from the results of this study, the biomechanics of the mudskipper can be modelled into various applications, some of which are detailed in this study. First, we consider the development of a controlled adhesion surface by applying the Stefan adhesion used by the mudskippers during climbing. The adhesion could be activated through mucus production at the interface area and deactivated using water, as the adhesion is governed by fluid viscosity. Second, we consider the creation of a mudskipper-inspired robot/drone capable of walking on land and sticking to inclined surfaces. Third, we consider the development of elastic materials inspired by the skin-covered scales, though this would require further examination of mudskippers.