Musculoskeletal comparison of the pectoral fin in mudskippers (Gobiidae: Oxudercinae)

Shifting of the locomotor mode from an aquatic thrust to a ground reaction force enabled vertebrates moving on land, which happened at the same time as fin-to-limb evolution. Early stages of this evolution are little known due to scattered fossil records. Oxudercine gobies, exhibiting the full spectrum of terrestriality and terrestrial locomotor behavior, provide a unique window to understand how the form and function of the pectoral fins were modified during the water-to-land transition. In the present study, the musculoskeletal system of pectoral fins of three mudskippers [Oxuderces nexipinnis (low terrestrial), Scartelaos histophorus (moderate terrestrial), and Periophthalmodon septemradiatus (high terrestrial)] and the out-group species (Oxyeleotris urophthalmus) were examined. There are some anatomical modifications endowing crutching on land including: the robust and triangular cleithrum; the stiff and elongated radials and fin rays; the development of the coracoid process, the radial process, and the coraco-radialis; the two hinge-like structures; the differentiated muscular system inclining in various angles; and well-developed adductor muscles. These configurations are correlated with the degree of terrestriality. Of these anatomical modifications, the presence of the coracoid process, the radial process, and the coraco-radialis could endow terrestrial movement in early oxudercine gobies. In addition, fin rays of early oxudercine gobies could prevent them from sinking into the unconsolidated substrates which could be analogous to the condition in sarcopterygian fishes. In general, this study shows the gradual evolution of the muscular and skeletal systems of the pectoral fins in the oxudercine gobies during water-to-land transitions.