The Morphologies of Mudskipper Pelvic Fins in Relation to Terrestrial and Climbing Behaviour

Two species of mudskipper are identified with different behaviours, which are related to their pelvic fin morphologies. Periophthalmus variabilis has unfused pelvic fins and is capable to climb on the vertical substrate. Another species, Boleophthalmus boddarti has fused pelvic fins which supports the fish mobility across the muddy substrate. In context of anatomy, both pelvic fins are composed of a frenum which covers the pelvic girdle and pelvic fin rays (lepidotrichia). The unfused pelvic fin of P. variabilis has split rays that are not interconnected, whereas the pelvic fins of B. boddarti are fused completely and the fin rays are merged to the skin. The pelvic fin ray bones of B. boddarti are composed of large bone structure, allowing it to function as a strong sitting pad on a semi-terrestrial substrate. Comparatively, in P. variabilis, the ray bones are shorter, hence provides flexibility to grip more surface area for the fish to stick during vertical locomotion. Histologically, the epidermal layer of B. boddarti pelvic fins has lower quantities of mucous cell than in P. variabilis, by 14.33 ± 1.53 and 33.33 ± 1.53 mucous cells per 100 epithelial cells, respectively. This difference reveals that more mucus is produced in P. variabilis, hence possibly giving this fish an enhanced capacity for Stefan adhesion on a terrestrial substrate than in B. boddarti. From this, the terrestrial locomotion and climbing ability are more supported in P. variabilis, compared to the more aquatic B. boddarti.