Biological and robophysical experiments of terrestrial fish locomotion on mud of controlled, variable strength

Many amphibious fishes can make forays onto land. The water-land interface often has wet deformable substrate like mud and wet sand/gravel, whose strength varies with water content, which challenges locomotion. Most previous studies of fish terrestrial locomotion focused on kinematics, muscle control, and functional morphology. To fully understand how these locomotor features interact with the environment to permit performance, we need to further quantify the interaction with wet deformable substrates. Here, we used controlled mud as a model wet deformable substrate and developed methods to prepare mud into spatially uniform states and temporally stable states and tools to characterize its strength. We studied the Atlantic mudskipper (Periophthalmus barbarus) moving on a stronger and a weaker mud. The animal performed similar “crutching” walks on mud of both strengths, with only slightly slower speed on thinner mud (from 0.4 ± 0.12 to 0.31 ± 0.13 body length/s, P < 0.05, ANOVA). However, it jumped more frequently (from 1.4 ± 0.97 to 3.2 ± 1.6 times per minute, P < 0.05, ANOVA). This may be to mitigate the higher stickiness of the weaker mud. For more repeatable and controlled experiments, we created a robophysical model. The robot has two fins that rotate in phase to raise the body and “crutch” forward on mud like the mudskipper. We are currently testing the animal on even weaker and stronger mud and refining the robot for systematic experiments on varying mud strengths with varying fin morphology and kinematics.