Incorporated nematocysts in Aeolidiella stephanieae (Gastropoda, Opisthobranchia, Aeolidoidea) mature by acidification shown by the pH sensitive fluorescing alkaloid Ageladine A

► The enigmatic question of how incorporated nematocysts in gastropods mature is addressed. ► Acidification contributes to mature nematocysts within 3 days. ► We apply a new fluorescence dye to detect acidification in living tissue.