| Summary: | Abstract In this study, we investigated the composition of free amino acids and lactate (Lac) in polychaetes in river estuaries and inner bays using chromatographic techniques. Both l-amino acids and d-amino acids (d-asparagine, d-alanine (d-Ala), d-serine, d-aspartic acid, and d-proline (d-Pro)) were detected, indicating that polychaetes contain some d-amino acids. Some polychaete species exhibited notable amino acid levels, such as glycine in Capitellidae sp. and Thelepus sp., d-Pro in Glycera sp., and β-Ala in Scoletoma nipponica and Scoletoma sp.. High d-Lac levels were detected in Tylorrhynchus osawai and Hediste diadroma, (691 and 797 μmol/100 g-wet, respectively), with the d-form exceeding 98%. T. osawai was dominant in the upper tidal-sensitive zone, wherein other organisms were less abundant because of low salinity (3–8 PSU). Seasonal differences in the concentrations of components in T. osawai were observed, particularly a significant increase in d-Lac in the reproductive period. Notably, the d-Lac concentrations of T. osawai were higher upstream than downstream. Thus, d-Lac might be involved in strategies underlying adaptations to low salinity and reproductive activity. These results suggest that both the d-form of Lac and amino acids may play certain physiological roles in the life of polychaetes.
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