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Physiological Role of a Calcitonin-Like Substance in the Life Cycle of the Lamprey, Lampetra japonica (Cyclostomata)

The present study elucidates the physiological role of a calcitonin (CT)-like substance in the lamprey (Lampetra japonica) captured in a river. A CT-like substance in the plasma of lampreys (n = 13, both sexes) was examined by an enzyme-linked immunosorbent assay using an anti-salmon CT serum when they were transferred from freshwater (FW) to 1/3 seawater (1/3 SW). After keeping them in 1/3 SW for one day, the CT-like substance in 1/3 SW-adapted lampreys was significantly higher than that in FW-adapted lampreys. This substance was significantly correlated with the gonad somatic index (GSI) in males; however, the GSI could not be measured in post-spawned females. The present study is in agreement with a previous study showing that the CT-like substance had a significant correlation with the GSI in both sexes. This species of lamprey is born in a river and then migrates from FW to SW after metamorphosis. Subsequently, the lamprey grows up in the sea. Thereafter, it migrates from SW to FW and then spawns in the river to complete its life cycle. The results obtained here suggest that the CT-like substance plays important roles in their migration from FW to SW and their sexual maturation after migrating from SW to FW during their life cycle.