Unfortunately for the slug, it has no protective barrier between it’s cell walls and the outside world. This probably seems like a fatal flaw, but surely no slug expects to have pure salt dropped on it. But when such a thing does happen, the high concentration of the salt on the outside of the slug causes the cells to start trying to balance concentrations. In non-scientific terms, the water is “sucked” right out of the slug’s body. The slug will dry up and die if enough salt is applied.