Origins and Mechanisms of Diadromy in Fishes

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Ecological Approach : Ecological studies were performed for several diadromous species, such as white-spotted charr, masu salmon, coho salmon, dolly verden, Japanese eel, freshwater sculpins, freshwater gobies and threespine stickleback. The life-history parameters concerning their growth, movement, reproduction and metamorphosis were solved.Physiological Approach : Administration of testosterone facilitated the upstream migratory behavior and suppressed the downward migration in the masu salmon smolts and immature kokanee, suggesting an important role of testosterone in determination of the seaward migration or residence in a river. High administration of testosterone induced a quivering in male and a digging behavior in female in the kokanee.Parasitological Approach : Diadromous freshwater eels Anguilla are parasitized with the monogenean Pseudodactylogyrus spp., which have adapted from marine to freshwater life together with the host fish. A lower homology in the base sequence of the ITS region of sRNA between two species of Pseudodactylogyrus on the Japanese eel and Australian eel suggested that the eel Pseudodactylogyrus had co-evolved independently in Japan and Australia, respectively.Population genetic Approach : Estimated molecular phylogeny of the osmerid fishes suggested that the unique amphidromy of ayu might derive first in the osmerid lineage. Genetic variability of 42 isozyme loci between landlocked kokanee and sea-run sockeye salmon revealed that no genetic polymorphism was observed in kokanee, while high polymorphism was detected at two loci in sockeye salmon. Decrease of genetic variability could be caused by land-lock event during evolutional process.Modeling Approach : Evolution of migration strategies of diadromous fish and its coevolution with other traits were studied by mathematical modeling. Coevolution of egg size and timing of migration was considered, and its relevance to speciation of diadromous fish was discussed.