In order to elucidate how the crystal texture of the fish otolith and its growth are influenced by water temperature, its concentration, the amount of food and the season, the present writer chose marine teleosts, Lateolabrax and Mylio. They were reared through all seasons in tanks and the temperature of the sea water, its concentration and the amount of food were variously changed and experimented. For the observation of the crystal texture of the otolith surface, the electron microscope was used and details concerning the formation of the otolith zone were studied through the autoradiographs. Radioactive isotope Ca45 was used as a tracer in order to measure the amount of calcium deposited upon the otolith surface, and, concerning the crystal texture of the annual rings, Argyrosomus was used instead of Lateolabrax and Mylio for convenience' sake, and their Laue photographs were taken by the micro-beam X-rays. As the results, the followings were obtained.
(1) Calcium dissolved in the sea water is deposited on the surface of the otolith, and micro-crystal grains of CaC03 which are the main constituent of the otolith are thus formed. It goes without saying that the growth of the otolith is mainly due to the formation of micro-crystal grains of CaC03.
(2) In summer and autumn, in which the fish grows rapidly, a large amount of calcium is deposited on ·the otolith surface, and the micro-crystal grains of CaC03 are formed abundantly. A very little protein fills the groove between crystal grains. In this way, the wide translucent zone which contains a small amount of protein is formed.
(3) In winter and spring, in which the fish grows slowly, a small amount of calcium is deposited and the micro-crystal grains are formed very slowly. Much protein fills the groove between crystal grains. This is the way in which the opaque zone,containing much protein is formed.
(4) Concerning Lateolabrax and Mylio, the concentration of the sea water or the amount of the dissolved calcium in this water cannot have influence upon the formation of micro-crystal grain of CaC03. In llther words, the growing velocity of the otolith is always unchanged. But, on the other hand, the formation of the micro-crystal grains is influenced much by the water temperature and the amount of food taken by the fish. The micro-crystal grains of CaC03 are formed abundantly and the otolith grows rapidly when the environment of the fish is favorable and its physiological condition is good independently of the concentration of the water. On the contrary, when their environment is unfavorable, the micro-crystal grains are formed only a little and the otolith grows slowly. The good environment for the fish in this case means that they are living in the warm water and taking plentiful food, and the contrary means that they are living in the cold water and taking only insufficient food, and both environments can be realized either in summer and autumn or in winter and spring.
(5) As is mentioned above, the formation of the zone in the otolith is closely connected with the history of the fish and the zone mark once formed in the otolith is preserved ever after as it is. Accordingly it can be used as an effective key to investigate the history of the individual fish.