M. L. Lenhardt
Biomed. Eng. Prog., Virginia Commonwealth Univ., Richmond, VA 23298-0168
Virginia Inst. of Marine Sci., Gloucester Point, VA 23062
Sea lampreys (Petromyzon marinus) are one of only two species of living jawless fish. The brains and ears of lampreys are remarkably similar to that of fossil Ostracoderms, a group of fish that existed over 500 millions years ago. Jawed fish are derived from jawless fish which in turn radiated into primitive bony fish as gars (Lepisosteus) and teleosts and into the line leading to development of tetrapods. Marine turtles (Caretta c. caretta; Lepidochelys kempi) are considered primitive in that they share many characteristics with stem reptilian stock from which birds and mammals also radiated. The three species responded behaviorally and electrophysiologically to very low frequencies in a similar fashion despite their obvious morphological differences. All appear to share an ability to detect substrate vibrations, suggesting that the primitive vertebrate ear was also a low-frequency vibration sensor. It is hypothesized that the first vertebrates evolved an otic organ for both equilibrium and vibration and each function was not separated until jawed vertebrates. In the primitive forms as in living lampreys, the brain must inhibit the ear function when detection of movement and gravity is needed.