Detection of high concentrations of D-amino acids and D-lactate in estuarine polychaetes

Summary:
D-Lactate in Tylorrhynchus osawai inhabiting upstream areas was higher than that in individuals inhabiting downstream areas. Therefore, D-lactate may be related to reproductive activities and adaptation to low-salinity conditions. D-amino acids are widely found in aquatic invertebrates, such as bivalves and crustaceans, and play important physiological roles in osmoregulation, low oxygen tolerance, reproduction, and development. Polychaetes are annelids that inhabit various environments, from the oceans to estuaries and freshwater areas, and are sometimes a dominant group. However, there are limited reports on the concentration and role of D-amino acids in polychaetes. Therefore, the current research group quantified and compared the concentrations of free amino acids and lactate, the final products of anaerobic metabolism, in the body tissues of ten species of polychaetes collected from eight locations, including estuaries and inner bays. D-amino acids were detected, in addition to L-amino acids, in polychaetes, and their composition varied depending on the species. Furthermore, high concentrations of D-lactate were detected in Tylorrhynchus osawai and Hediste diadroma, and the ratio of D-lactate to total lactate concentration (D-lactate concentration + L-lactate concentration) exceeded 98%. Particularly, T. osawai, which is predominant in the estuary of the tidal zone affected by the tide, showed a significant increase in D-lactate concentrations during its reproductive period. Additionally, D-lactate in T. osawai inhabiting upstream areas was higher in concentration than in those inhabiting downstream areas. Therefore, D-lactate may be involved in reproductive activities and adaptation to low-salinity conditions. The results of this study suggest that D-amino acids and D-lactate play important physiological roles in the life activities of polychaetes.