Salinity preference of silvering Japanese eel Anguilla japonica：evidence from pituitary prolactin mRNA levels and otolith Sr：Ca ratios

Abstract

The salinity preference of Japanese eel Anguilla japonica during silvering was validated from the viewpoints of physiological ecology by examining the pituitary prolactin (PRL) mRNA expression and the otolith strontium: calcium (Sr:Ca) ratios of the eels of different sexual maturity stages collected in an estuary of Taiwan. The PRL possessed a putative signal peptide of 24 amino acids and a mature peptide of 185 amino acids. RT-PCR and real-time quantitative PCR analyses indicated that the transcript levels of PRL mRNA significantly decreased from yellow to silver stage. On the contrary, the Sr:Ca ratios at the otolith edge of silver eels, which recorded their latest experience of salinity history, significantly increased. The increase in otolith Sr:Ca ratios and the decrease in PRL mRNA expression are consistent with the timing of migration from freshwater to seawater during silvering of the eel.