1969 Volume 35 Issue 3 Pages 273-283
1) In order to get a morphological basis for the development of osmoregulatory adaptation mechanism during the seaward migration in salmonid fry, histological observations were made on various organs of the chum salmon fry, Oncorhynchus keta, captured at some points on the route of their seaward migration. The same examinations were also done using some groups of the fry of chum salmon and of rainbow trout, Salmo gairdnerii irideus, which had been for kept in diluted sea water one or three weeks after in fresh water.
2) In the trout, peculiar enlargement of the anterior region of the adenohypophysis was noted after the transference into the diluted sea water from fresh water. On the other hand, in the salmon fry which showed no distinct changes such as that above-described.
3) Chloride secretory cells had been clearly differentiated in the salmon fry immediately after hatching. In contrast, in the trout fry, the cells were in no case detectable in their fresh water stage, and made their obvious appearance after having been transferred to the diluted sea water.
4) Histological informations about the osmoregulatory role were hardly obtained from examinations on the gonad, the thyroid, the interrenal gland, the caudal neurosecretory system and the liver.
5) Discussions were made on above characteristics of chum salmon in comparison with the poor salinity tolerance of rainbow trout fry.
