Authentic FMRFamide is present in the polychaete Nereis virens.

The tetrapeptide FMRFamide is but one member of a large family of invertebrate neuropeptides which includes another tetrapeptide, FLRFamide, and several longer peptides terminating in one or the other of these tetrapeptide sequences. These peptides have been isolated from both molluscs and arthropods, but so far not one has been isolated from an annelid. Since the annelid worms are believed to share a common ancestor with molluscs and arthropods, they should contain FMRFamide-like peptides. We found two immunoreactive peaks in Nereis virens, but microsequencing and fast atom bombardment mass spectrometry revealed that they represent only one native peptide, FMRFamide. (The other peak is its methionyl sulfoxide derivative.) Each worm contained only 100 to 600 fmols of peptide, which is at least 10-100 times less than the levels in molluscs. Our identification of a tetrapeptide, and only a tetrapeptide, in this worm suggests that the tetrapeptides are the more ancient members of the family, and were probably present in the common ancestors of the annelids, arthropods, and molluscs.