Establishment of a Simplified System to Evaluate Salinity Preference and Validation of Behavioral Salinity Selection in the Japanese Medaka, Oryzias latipes

In fishes, it is necessary to select a salinity environment suitable for survival. However, little is known about the mechanisms regarding detection and selection of salinity environments in fish. This study involved the establishment of a simple aquarium system in which fish can swim between freshwater (FW) and seawater (SW) in a single tank. In this tank, the lower level contained SW, the upper level contained FW, and the FW and SW levels were clearly separated as different salinity areas. Behavioral experiments of salinity environment selection using this simplified system to evaluate salinity preference showed that FW-acclimated medakas preferred FW to SW. In contrast, SW-acclimated medakas preferred SW to FW. These results indicate that euryhaline medakas prefer the saline habitats to which they are acclimated, when able to select the salinity environment. We identified the taste receptor type-2 and polycystic kidney disease 2-like 1 genes as possibly related to high-salinity taste in medaka. The expression of these genes increased at certain time points after SW challenges. In this study, we established an aquarium system to facilitate a simple experiment for salinity preference. Our results suggest that the medaka is good model for research related to seawater environment selection in fish.

Habitat segregation of Gerres japonicus and G. limbatus in early stages in the Tien Yen estuary, Northern Vietnam

Silver-biddy (Gerreidae: Gerres) are commercially important euryhaline fishes in coastal waters; however, information about their distribution during early stages in estuarine environments is poorly known. The present study aims to elucidate the habitat utilization of two species of the family, Gerres japonicus and G. limbatus as larvae and juveniles based on monthly collections from 2013 to 2015 in the Tien Yen estuary, northern Vietnam. They occurred from April to June for G. japonicus and in April and May for G. limbatus with a peak in May for both the species. Both G. japonicus and G. limbatus used the Tien Yen estuary as a nursery ground; however, the former species that colonized the shallows along the banks of the estuary extended to near freshwater habitat, whereas the latter was found at the end of the upper reach and the middle reach. The difference habitat use between the two species in the early life history may be related to their specific salinity preference as G. japonicus prefer lower salinities than G. limbatus.