The significance of pelvic fin flexibility for tree climbing fish

In this article, we compare the characteristics of biomechanical attachment exhibited by two morphologically different mudskipper species, Boleophthalmus boddarti (with fused pelvic fins) and Periophthalmus variabilis (with unfused pelvic fins). P. variabilis is a tree and rock climber while B. boddarti dwells in the muddy shallows and is unable to climb. Our aim in this article is to determine whether it is predominantly chemical or morphological properties of the pelvic fins from each species that may allow P. variabilis to climb trees whilst preventing B. boddarti from doing the same. To fulfil our objective we perform friction and suction resistance tests, Fourier transform infrared spectroscopy of the mucosal secretions under the fins, direct geometrical measurements and finite element modelling. We find that B. boddarti has considerable resistance to pull-off forces, while P. variabilis has greater frictional resistance. Both species produce mucopolysaccaharides that we suggest aid Stefan adhesion to different substrates. P. variabilis fins are, nevertheless, considerably more flexible than those of B. boddarti and we conclude that P. variabilis is consequently able to maintain a more intimate surface area of contact with underlying material, which aids Stefan adhesion, increases frictional resistance, and helps it to climb trees.