Lordosis in fish is an abnormal ventral curvature of the vertebral column, accompanied by abnormal calcification of the afflicted vertebrae. Incidences of lordosis are a major problem in aquaculture and often correlate with increased swimming activity. To understand the biomechanical causes and consequences of lordosis, we mapped the morphological changes that occur in the vertebrae of European sea bass during their development from larva to juvenile. Our micro-CT analysis of lordotic and non-lordotic vertebrae revealed significant differences in their micro-architecture. Lordotic vertebrae have a larger bone volume, flattened dorsal zygapophyses and extra lateral ridges. They also have a larger second moment of area (both lateral and dorso-ventral) than non-lordotic vertebrae. This morphology suggests lordotic vertebrae to be adapted to an increased bending moment, caused by the axial musculature during increased swimming activity. We hypothesize the increase in swimming activity to have a two-fold effect in animals that become lordotic. The first effect is buckling failure of the axial skeleton due to an increased compressive load. The second effect is extra bone deposition as an adaptive response of the vertebrae at the cellular level, caused by an increased strain and strain rate in these vertebrae. Lordosis thus comprises both a buckling failure of the vertebral column and a molecular response that adapts the lordotic vertebrae to a new loading regime.