Temperature-dependent differences in seawater tolerance between amphidromous and landlocked ayu Plecoglossus...

Ayu Plecoglossus altivelis, one of the most important inland fishery species in Japan, exhibits a semelparous annual life cycle. Although native populations are primarily amphidromous, landlocked ayu from Lake Biwa have been widely released into rivers nationwide. This landlocked population diverged from amphidromous ancestor approximately 100,000 years ago and exhibits distinctive morphological and physiological traits. In order to assess how translocation of landlocked ayu may affect native populations, we examined the mechanisms underlying the differences in seawater acclimation capacity between the amphidromous and landlocked forms, with particular focus on the effects of water temperature. Juvenile and larval ayu from both populations were reared in freshwater at 18 °C or 24 °C and subsequently transferred to seawater. In the juvenile stage, the landlocked form exhibited a markedly lower capacity for seawater acclimation (lower survival rate and higher plasma osmolality after seawater transfer) than the amphidromous form, primarily due to the delayed appearance of seawater-type ionocytes in the gills. While temperature had little effect on the amphidromous population, landlocked fish acclimated to seawater at 24 °C exhibited a tendency toward increased plasma osmolality and mortality. In embryos and larvae, both populations possessed seawater-type ionocytes prior to hatching, indicating inherent preparatory adaptation for seawater migration. However, following seawater transfer, landlocked larvae showed significantly reduced survival at high temperature, whereas amphidromous larvae were unaffected by temperature. These findings demonstrate that landlocked ayu retain fundamental mechanisms of seawater acclimation, but their salinity tolerance considerably decreases under elevated temperatures during the larval stage.