Amphidromy related to genetic structure of a hybrid temperate bass population in Ariake Sea

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It has been demonstrated that there is a very unique hybrid population between Japanese temperature bass Lateolabrax japonicus and Chinese temperate bass Lateolabrax sp. exists in Ariake Sea. The Ariake temperate bass has been known to immigrate into freshwater area during the early juvenile period. The present research was conducted to confirm existence of amphidromous migration and genetic intra-population structure of the Ariake temperate bass.1) Otolith microstructure analyses revealed the spawning occurred between late November and mid. February with a peak of late December. The larvae appeared around the Chikugo river estuary at about 16mm SL and 80 days of age, then shifted their habitat to freshwater area at about 18mm and 100 days. Fish size collected at the upper reach of the river was larger than that at the lower reach. Hatch-date and length distributions were not different between river-ascended juveniles and non-ascended juveniles collected at Omuta tidal flat area.2) Oto … More lith Sr/Ca trajectories of 0-group fish were roughly divided into early freshwater ingress and non-freshwater ingress groups. Proportion of freshwater ingress group fluctuated annually between 50% and 20%. Otolith microchemistry for spawing adult fish suggested 2 different groups, Shimabara group included fish with freshwater experience during the early life stage and Misumi group didn't show freshwater experience.3) Mitochondria-DNA analyses showed quite different genetic features between the different spawning grounds ; the adult fish collected from Shimabara had a very high percentage of Chinese haplotype (85%), whereas a very low percentage (26%) in the adult fishes from Misumi. These findings suggest that there are at least two subpopulations of temperate bass in Ariake Sea ; the one more highly influenced by Chinese temperate bass gene has a potential of amrphidromous freshwater ingress and the other one with less influenced by Chinese gene doesn't have amphidromous potential. Migration to freshwater during the early life stage was speculated to be determined by 2 main factors, that is genetic background of subpopulation and biotic/abiotic environmental conditions after spawning.4) All of the continental relict fishes endemic to Ariake Sea were found primarily dependent on a brakish water copepod Sinocalanus sinensis during the late larvae and early juvenile periods. This endemic and continental relict copepod was speculated to largely contribute to freshwater ingress of the Ariake temperate bass as an essential food resource prior to the habitat shift into freshwater. Distribution of this species in Ariake Sea was restricted to the bottom of the bay and to estuaries with highly turbid water. Less