STUDIES ON THE JAPANESE CHAPS, THE IWANA (GENUS SALVELINUS)

In order to clarify some biological problems, such as classification, habit, behavior and the role of the endocrine gland in the life history of the Japanese chars, the specimens were collected and observed. In the present paper, the changes occurring in the thyroid, pituitary, adrenal cortical tissue and gonads of the fish, the Nikko-iwana, Salvelinus leucomaenis pluvius (Hilgendorf), throughout its life-span were examined histologically. The Nikko-iwana were obtained from the mountain tributaries of Seki-gawa River in the south-western part of Niigata Prefecture, the Japan Sea side. Although there could not be seen the prominent hyperfunctioning figure in the thyroid gland of the fish throughout the year, higher activity of the gland in the spring and spawning runners suggests that the function of the gland is correlated with the active locomotion of the fish. The conspicuous changes in the external appearance and cyclic changes in the cyto-histology of the pituitary gland of the fish throughout its life-span were detected. It is intimated that the presence of dilated castration cells with mono-or polynuclei, located chiefly in the proximal zone of pars distalis (=mesoadenohypophysis), is one of the characteristics of the spent fish or old fish. A marked indication of hyperadrenocorticism, accompanying the, maturation of the gonads, was not recognized. It is likely that the fish except of senile one can survive after spawning which forces a severe exhaustion of the fish, since there is no extreme pathologic figure of the endocrine gland nor the dead bodies in the spawning place such as occurred in the postspawning Ayu and Pacific salmon. By examination of the process of gametogenesis and conditions of mature and spent gonads, the breeding season of the Nikko-iwana in the mountain tributaries of Seki-gawa River seems to be the fall, from the latter part of October to early in November. It seems to be probable that the fish can spawn several times throughout its life-span.