Trina Y. Du and Emily M. Standen
The ability of bones to sense and respond to mechanical loading is a central feature of vertebrate skeletons. However, the functional demands imposed on terrestrial and aquatic animals differ vastly. The pectoral girdle of the basal actinopterygian fish Polypterus senegalus was previously shown to exhibit plasticity following terrestrial acclimation, but the pectoral fin itself has yet to be examined. We investigated skeletal plasticity in the pectoral fins of Polypterus after exposure to terrestrial loading. Juvenile fish were divided into three groups: a control group was kept under aquatic conditions without intervention, an exercised group was also kept in water but received daily exercise on land, and a terrestrial group was kept in a chronic semi-terrestrial condition. After five weeks, the pectoral fins were cleared and stained with Alcian blue and Alizarin red to visualize cartilage and bone, allowing measurements of bone length, bone width, ossification, and curvature were taken for the endochondral radial bones. Polypterus fin bones responded most strongly to chronic loading in the terrestrial condition. Fish that were reared in a terrestrial environment had significantly longer bones compared to aquatic controls; wider propterygia and metapterygia; more ossified metapterygia and medial radials; and showed changes in propterygial curvature. Exercised fish also had longer and more ossified medial radials compared to controls. Polypterus fin bones exhibit plasticity in response to novel terrestrial loading. Such plasticity could be relevant for transitions between water and land on evolutionary scales, but key differences between fish and tetrapod bone make direct comparisons challenging.