Influence of seasonal changes and high salinity conditions on free <scp>d</scp> -glutamate levels in the reproductive tissues of male kuruma prawn <i>Marsupenaeus japonicus</i>

Aquatic invertebrates contain high levels of D-alanine in their tissues. D-Glutamate, a novel D-amino acid found in animal tissues, is exclusively found in the male reproductive tissues of the kuruma prawn Marsupenaeus japonicus. In this study, we investigated the effects of seasonal changes and high salinity seawater exposure on D- and L-glutamate and D- and L-alanine contents in male reproductive tissues of M. japonicus. We found that D-glutamate content was high during the breeding season specifically, D-glutamate content in the seminal receptacles was particularly high, suggesting that it is actively biosynthesized and supplied to the seminal receptacles for the regeneration of the spermatophores released during mating. The D- and L-alanine contents in the testes increased significantly after exposure to high-salinity seawater, whereas L-glutamate content did not change, and D-glutamate increased in some case, suggesting that the increase was due to factors related to reproductive function rather than their response to hyper-osmotic stimulation. This indicates that D-glutamate is crucial for the reproductive function of M. japonicus.