Snails require calcium to build and repair their shell so obviously it's extremely important they get enough. Shells consist mainly of calcium carbonate and a small amount of organic compounds like protein. (More on shells)
Without enough calcium their shells become thin and rough instead of being thick, smooth and glossy.
There is calcium in vegetables and fruits but they don't get enough from just their food. Snails in the wild often eat dirt and other things to get enough calcium. They also use some organic matter including protein to build their shell so it's important to provide a varied diet with some meat protein like freshwater, sun-dried baby shrimp and because they are shell and all they will get protein and calcium in one. Simply soak in cool water a few minutes. There is also protein in soybeans but don't feed them tofu because it has salt in it and beans are highly starchy which they seem to have a hard time digesting.
Always keep a source of calcium in your snail's house for them to munch on at their leisure. The easiest way to supply calcium is with a piece of washed cuttlebone. They are inexpensive costing around a dollar and can be found in most pet store in the bird section. An added benefit is they get a small amount of trace minerals from them.
They only need a piece that's about four inches long so if it's a large cuttlebone you can break it in half or just put a small cuttlebone in whole. Be sure to wash thoroughly. Make sure the pieces are big enough that all your snails can get some. Simply place in their house by their food. You'll need to take it out everyday to wash it so it stays clean and isn't so dry. Replace it every other week or whenever needed.
Other acceptable sources of calcium include: cleaned egg shells, natural chalk, natural limestone, ground oyster shells and reptile calcium supplements from pet stores. A good product to look for is Rep Cal Calcium. It's made of ground oyster shells and has no phosphorus or vitamin d3 added. There's three different products that look the same so just make sure it says on the front no phosphorus or d3 added! Another great product is JurrassiCal from JurassiPet. It's made of natural limestone and has no phosorous or Vitamin D3. Look for them in the reptile section of any major pet store or reptile specialty store.
Never give chalkboard or sidewalk chalk to your snail because it's not natural chalk which is a kind of soft gray limestone but rather synthetic stuff.
Can you feed them too much cuttlebone or calcium? Yes, if you feed them a mix with calcium in also, like chicken mash or homemade mash. The idea amount of calcium is around 12% for optimal growth and health. Keep in mind it would be far worst for them to not get enough then if they ate too much one day.
Also, baby snails obviously require more calcium than adults because their growing. Depriving a baby snail of calcium is a death sentence! So if they won't eat the source of calcium your giving them try something else. Keep in mind they may be eating it when your not looking. If they have a calcium deficiency their shell will be thin and rough. It should be thicker, smooth and glossy. If you think your snail has a calcium deficancy set them on the cuttlebone, they may not realize it's good to eat and make sure you give them a varied diet because their shell is made up of things other than calcium. What they eat also helps determine how dark there shell is too.
If they don't eat the cuttlebone whole you could try powdering it and placing it on a leaf of something then get it slightly damp. Just remember to replace daily. You might also try soaking it to make it softer.
Some people have also tried liquid calcium. They put 2 drops in a spray bottle and then mist the snail with it because snails can absorb calcium through their skin. Seems to work really well and makes their shell grow better. Just make sure you don't add it to their water dish or add more than a drop or two.
The three best sources of calcium are ground oyster shell(without additives) like RepCal, JurassiCal which is natural limestone and cuttlebone. If you can, always have a fresh supply of them all. Otherwise make sure you always have a clean cuttlebone in their house.
Important: You MUST wash all food and forms of calcium before they are offered to snails.
One of the best and most readily available calcium source for snails. Try to always have some clean cuttlebone in with your snails, either whole or powdered. An added benefit is they get a small amount of trace minerals from them. You can find them in the bird section of most pet stores.
You have to remove the inside skin and wash thoroughly. Baking in the oven at 250 degrees Fahrenheit(121 Celsius?) for around 10 minutes helps kill any bacteria like salmonella and makes them more brittle. Not the best source and not readily accepted but better than nothing.
There's several different calcium powders and sprays for reptiles. I only recommend two products: 'Rep Cal Calcium without d3 or phosphorus' which is only ground oyster shell. Their are a few similar products but they have added D3 and phosphorus they don't need and it could possibly poison them. Also 'JurassiCal' from JurassiPet. It's made of natural limestone which is a great source of calcium carbonate and has no phosorus or vitamin d3. JurassiCal is also usually easier to find than RepCal and seems to be less expensive.
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Has to be the powdered kind not the ground oyster shells they sell for bird grit. Some snails like it better than cuttlebone. Can be hard to find. Usually found for reptiles as calcium supplements. Only give them the kind without vitamin d or phosphorus like 'Rep Cal Calcium without d3 or phosphorus'.
Don't confuse this with chalkboard chalk! Natural chalk is a type of pure limestone which is soft, white or grayish and made from the shells of millions of tiny ancient marine organisms. Good source of calcium if you can find it. I don't know where you get it though.
Natural limestone is what your looking for. Jurassi Cal from JurassiPet is made from natural limestone and has no phosphorus or Vitamin D3 added. I highly recommend it. Natural limestone is mostly calcium carbonate. More information on limestone. I've heard you can get powdered limestone at very low cost in bulk from agricultural feed stores too. Also, be aware there is more than one kind.
Ground limestone and calcic limestone: Calcium Carbonate (CaCO3) - almost pure calcium carbonate with varying amount of magnesium carbonate. Sold for lawn care. You could give this to snails if it has no added ingredients but it's better to only give them JurassiCal.
Dolomite limestone: Contains about equal parts of magnesium and calcium carbonate. Never give to snails because it's half magnesium carbonate.
Burnt lime: (CaO) or quick lime, caustic lime. It's caustic so never give to snails.
Hydrated lime: [Ca(OH)2] or slaked lime - Also caustic so never give to snails.
I wouldn't give my snails anything but the JurassiCal reptile calcium powder from JurassiPet.
Chalk board/sidewalk chalk:
Is not made of natural chalk which is a soft, gray limestone but rather synthetic stuff.
Used as a plant fertilizer. Shouldn't be give to snails because it my contain contaminants.
Reptile calcium supplements or sprays with added vitamin d or phosphorus:
Should never be given to snails because of the possibility of accidental poisoning by overdose.
A type of limestone that contains about equal parts of magnesium and calcium carbonate. Should not be given because they need mostly calcium carbonate and it contains magnesium which is believed to retard calcium absorption.
I would be very careful if you decide to give your snails milk. Powdered baby milk is thought to be good for them because it has calcium but I have never fed it to my snails so i'm not sure if it's detrimental or not. Whatever you decide, don't give them milk as their main calcium source.
I would never give a deceased snail shell to another snail because I find it morally wrong considering it is the remains of the snail. I guess it could be a good source of calcium but with all the other calcium sources (like ground oyster shells) out there why give them a deceased friend. If you do give your snails another deceased snails shell you must boil it first.