These anodes protect your aluminum housing from corrosion caused by an electrolytic reaction. Loose electrical currents created by different metallic accessories on your housing will attack the softer zinc anodes instead of its aluminum shell. Over time these anodes will wear out (as they should). They are easily field replaced by simply screwing new ones into the housings base.
19219: Replacement anodes kit for Aquatica D2x housing only (two complete sets)
The Aquatica housings for the Nikon D2x are equipped with sacrificial zinc anodes.