Subsequent Changes in the Thymus of Ayu Plecoglossus altivelis Reared from Juveniles with Temperature-induced Differential Thymus Growth

― The temperature of water used to rear juvenile stages of ayu affects the development of the thymus. However, subsequent differences in the thymus in adults after rearing juveniles at different temperatures are unclear. We analyzed thymus development of ayu grown at a constant water temperature of 15 ° C from juveniles reared at different water temperatures. Juvenile ayu weighed at 0.6 g and 1.4 g were reared at different temperatures (10 ° C, 12 ° C, 15 ° C, 18 ° C and 22 ° C) for 64 days and 54 days, respectively, and then held at 15 ° C for analysis of the thymus development. Thymus volume was measured by computed tomography and thymus development of experimental fish was evaluated as the ratio of thymus volume to body length. The thymus volume ratios of juveniles immediately after rearing at lower water temperatures (10 ° C, 12 ° C) were higher than in those reared at the higher temperatures (18 ° C, 22 ° C). These levels of thymus volume ratio were preserved in grown ayu continuously reared at 15 ° C. After the summer solstice, the thymus began to atrophy and no longer exhibited differences among the experimental groups. Thus the thymus formed at the juvenile stage retains its morphological properties until the summer solstice.