What kind of animal / organism needs calcium?
Many kinds of animals need calcium to help their bones and skeletons grow and remain strong. Invertebrates are animals with their skeletons on the outside of their bodies. In the wild animals can find the calcium that they need. Invertebrates that are kept as pets rely on their keepers to provide calcium for them. These pets can include isopods, snails, shrimp, beetles, arachnids, millipedes, mollusks, crabs, lobsters and other crustaceans. 
Why is Calcium Important?
Invertebrates have exoskeletons. This is a hard, shell-like structure that grows on the outside of their body. The exoskeleton protects their organs and body. During their growth cycles, invertebrates will grow new exoskeletons after the molt their old ones. When invertebrates undergo this process, they will need extra calcium.
What Kinds of Calcium Are Available?
Since many invertebrates and mollusks have exoskeletons made from chitin (a kind of protein) and calcium carbonate, it is important to use those same materials when offering calcium to our pets. Some examples include cuttlebone, calcium powder (without vitamin D), Oolite sand, limestone pellets or powder, limestone rock, oyster shell in various forms, and egg shells.
Where Can I Get Calcium Sources?
Pet stores in your area may have cuttlebone in the bird section of the store. The pet supply shop may also carry calcium powder in the reptile section. Make sure to get the calcium without vitamin D as this addition, while great for reptiles, may harm your invertebrate. Some pet shops that carry supplies for hermit crabs may also have oolitic sand in that department.
Garden centers may carry dolomite limestone pellets or powder as they are used in gardens and for plants. You may also find limestone rock there as well, but it may be very large or sold by volume and give you more than you will likely need.
Some garden centers and pet stores might also carry flaked or crushed oyster shell, as chickens and poultry birds use this form of calcium often.
Limestone rock can be found in fish shops as base rock, in garden centers as filler rocks for landscaping or even in your local area if you know where to look. Remember, that before putting found sources of limestone into your pet invertebrates’ home, make sure to clean or remove dirt from the rock and ensure that you have sourced it from a place that does not use pesticides. Pesticides will harm any captive invertebrate.
Eggshells are easy to obtain, since many people use eggs in their home kitchens. Once you crack your eggs, place the shells in a bowl or container to rinse them. Then you can bake the eggshells in the oven at 220F / 104C for 15 minutes or so, to dry them out. Then you can crush them into smaller pieces or leave them whole for your invertebrates to use. I have found that any bits of egg, is a welcome protein bonus for my pet land snails or isopods.
How Do I give the Calcium to My Isopod / Invertebrate?
Most of the time, you only need to sprinkle a little bit of your chosen calcium source in your pet’s enclosure at a time. With isopods, snails I usually add 1 tsp / 4.5 gram of calcium powder or pellets. I will make a small pile of eggshells available near the food bowl. Limestone rock can be placed in the enclosure as a calcium source, enrichment item or natural décor.
Invertebrates such as spiders, scorpions and crabs will usually accept calcium powder that has been dusted on their prey item. You can do this every other feed to ensure your pet does not suffer from a deficiency in calcium.
