In most organisms, the concentration of free Zn is controlled by metallothioneins (MTs). In contrast, no significant proportions of Zn are bound to MTs in the slug, Arion vulgaris. Instead, this species possesses cytoplasmic low-molecular-weight Zn (LMW Zn) binding compound that divert these metal ions into pathways uncoupled from MT metabolism. Zn is accumulated in the midgut gland calcium cells of Arion vulgaris, where they associate with a low-molecular-weight ligand with an apparent molecular mass of ~ 2,000 Da. Mass spectrometry of the semi-purified LMW Zn binding compound combining an electrospray ion source with a differential mobility analyser coupled to a time-of-flight mass spectrometer revealed the presence of four Zn-containing ion signals, which arise from disintegration of one higher MW complex resulting in an ion-mobility diameter of 1.62 nm and a molecular mass of 837 Da. We expect that the novel Zn ion storage pathway may be shared by many other gastropods, and particularly species that possess Cd-selective MT isoforms or variants with only very low affinity to Zn.