Cuttlebone

A woodlouse does not moult in one go. It sheds the back half of its shell first and the front half a short while later, and it gets through that gap on calcium it banked in advance. In the run-up, chalky white deposits build under the first four plates of the underside, then dissolve away again so the mineral can be pulled back out and used to harden the new shell. We can usually pick them out as pale patches on an animal that is about to turn. All of it depends on there having been enough calcium in the enclosure to build the reserve from in the first place.

A block of cuttlebone is the easiest way to make sure there was. Cuttlebone is the internal shell of the cuttlefish, Sepia officinalis, and runs to about 95 per cent calcium carbonate, the remainder being the chitin and protein that hold the architecture together. The mineral is aragonite, stacked into chambers separated by thin corrugated pillars, which is why a piece weighs almost nothing and why it gives way under a mandible instead of resisting one. Limestone and oyster shell are the other crystal form, calcite, and both are harder going. Cuttlebone is the softest calcium we stock and the animals treat it accordingly. Sodium and magnesium come along with it as trace elements.

Cuttlebones strand along the southern and western coasts of Britain, which is where these are gathered before being rinsed in reverse osmosis and deionised water and packed. They arrive clean. We would still drop a piece into just-boiled water for twenty minutes and let it cool and dry before it goes in, because anything off a beach has sat in salt water, and twenty minutes is a small price for not wondering about it. Expect a faint fishy smell on opening. That is normal for a marine product, it fades, and the animals are drawn to it rather than put off.

Using it

Put a piece in and leave it. The drier end of the enclosure keeps it in better condition for longer, hard side down if you are laying it flat. Break it up and mix it through the substrate when you are setting a tub up, or half-bury a chunk so there is something to work at. You will know it is being used because the surface goes pitted and grooved where mandibles have been scraping. Armadillidium and the big Spanish Porcellio strip a block fastest, millipedes work through one steadily, and land snails or hermit crabs sharing an enclosure will use it too. One claim we are not going to repeat is that powdered calcium sticks to isopods and dries them out; we went looking for the basis of it and found nothing behind the repetition. The case for a solid block stands on its own anyway, because it stays where you put it, a misting will not wash it away, and you can see at a glance how much is left.

Supplied in pieces. Keep one available permanently instead of offering calcium now and then, store the rest somewhere cool and dry, and swap the piece out once it has been scraped hollow. Worth reading alongside our isopod calcium guide and our guide to moulting, and pairing with crushed oyster shell if you want something slower to disappear.