Comparative biological and robophysical study of amphibious fishes moving on mud of variable strength*

Amphibious fishes often encounter mud as they move across the water-land interface. Mud is challenging to move on, because it can either stay solid or flow like a fluid and the yield strength at which it goes through solid-fluid transition varies with how wet it is. Here we prepared mud of controlled, variable wetness states/strengths and systematically studied the mudskipper moving on them, which uses both pectoral fins in phase to “crutch” the body forward on hard ground. We also collected preliminary data for the bichir, which uses both pectoral fins alternately while laterally undulating the body on hard ground, and the ropefish, which relies on body undulation on hard ground. As mud weakens, all species sank more deeply and had a longer body section contacting mud during locomotion. The mudskipper’s performance (forward displacement per cycle) reduced, and it bent its tail to propel to compensate. The bichir’s and the ropefish’s performance did not change, and they lifted parts of the body, likely to reduce drag. To understand these observations, we are developing a robophysical model. Its body can laterally bend to propel against mud and vertically lift to control contact. Its two fins can move in or out of phase or be removed, to study a diversity of body-fin coordination.