Previous studies have demonstrated that frozen preparations of the brackish-water bivalve Corbicula japonica significantly increase the content of free ornithine found in its extracts. Here we report a novel ornithine-containing tripeptide commonly found in C. japonica, which is believed to be the source of increased free ornithine. The new peptide, named acorbine, was isolated from extracts of this bivalve obtained using ultra-filtration and gel permeation chromatography. Acorbine is comprised of N(2)-[N(2)-(beta-alanyl)-L-ornithyl]-L-ornithine as determined by amino acid composition analysis, N- and C-terminal amino acid analyses, proton nuclear magnetic resonance spectrometry, and chirality analysis of the ornithine residue. The total amount of beta-alanine and ornithine in the extract remained constant regardless of the temperature at which the bivalve was processed. The amount of free beta-alanine and ornithine increased significantly when the bivalve was frozen, with a corresponding decrease in peptidic beta-alanine and ornithine. The results suggest that changing the growth conditions triggers tripeptide proteolysis within the bivalve, which ultimately manifests in increased free beta-alanine and ornithine.