RPGW-amide, KPGW-amide and TPGW-amide tetrapeptides are involved in control of muscle contraction and may function as neurotransmitters. These peptides increase tension of the pedal retractor muscle and, in conjunction with FMRF-amide, increase peak tension of the anterior byssus retractor muscle (ABRM). Expressed in cerebral, pedal and visceral ganglia. TPGW-amide is found in pedal and cerebral ganglia and in shell adductor muscle (at protein level). RPGW-amide and KPGW-amide are found in pedal retractor muscle, ABRM and shell adductor muscle (at protein level). The measured mass is that of RPGW-amide. Tetrapeptides may be further processed to GW-amide, which is found in cerebral and pedal ganglia, by a dipeptidyl aminopeptidase.