Ultrastructure and the importance of wear in the dentition of the halfbeak (Pisces: Hemiramphidae) pharyngeal mill

To assess how tooth microstructure and composition might facilitate the pharyngeal mill mechanism of halfbeaks, apatite structure and iron content were determined by scanning electron microscopy and energy dispersive X-ray analysis for Hyporhamphus regularis ardelio, Arrhamphus sclerolepis krefftii, and Hemiramphus robustus. Iron was present in developing teeth and was concentrated along the shearing edge of spatulate incisiform teeth, which dominate the occlusive wear zone in all three species. A model based on tooth structure and wear rate is proposed to explain how halfbeaks maintain a fully functional occlusion zone throughout growth and consequent tooth addition and replacement. Replacement teeth erupt and wear rapidly so that a constant occlusion plane is always present. Iron within the tooth tissue reduces the wear rate of the cutting edge while simultaneously maintaining its sharpness and efficiency.