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How To Measure Shells For Your Hermit Crabs

Not many people know just how important it is to supply the right size shells for your Hermit Crab. Many owners will purchase a variety of shells, place them in the tank and hope for the best.


It doesn't really work that way with captive crabs. Some can be extremely picky about the shell that they will choose to live in.


It is very important to supply the right sized shell for each of your crabs. If the shell is too big, they will have a hard time moving around the tank, if it is too small, they cannot fully retract into their shell to protect themselves from predators (in this case, other crabs)


So how do you know when a shell fits perfectly? If you pick up your crab, watch as they retract back into their shell. The large claw should comfortably cover the shell opening, like a door. This protects the crabs body from predators trying to harm them.


If they are hanging out of their shell when they are in full retract, they are too big for their current shell and will need to upsize.


If they can retract fully back into the shell, and there is a large amount of space at the front of the opening that the crab is not covering, they are too small for their shell.


How to measure your Hermit Crabs shell opening


Using either a digital Caliper, ruler or tape measure, pick up your crab and turn the shell opening to face you. Place your measuring device over the longest part of the shell opening. You ONLY want to measure the inside of the shell, do not include the edges of the shell in your measurement. Depending on the shell opening type (D, O, Oval), measurements can read quite differently than what you may think.


When handling your crab for a shell measure, only handle their shell, you do not want to hold them by their legs, face, antennae or body. Using your fingers, hold the very back of their shell to prevent being pinched.


Write down the measurement so you don't forget it. The most common measurement type for shells used in Australia is centimetres.



How often should I measure my crabs shells?


Measuring should be done each time you add a new crab to your tank. This is to ensure that the new crab has an adequate amount of appropraitly sized shells available.


We also recommend measuring your current Hermit Crabs shell openings atleast 3 times a year minimum. Each time your crab moults, they have grown. If you start to notice that they are looking abit to big for their pants, it's time to pull them out for a measure.


Yes, measuring their shell can be stressful for them, however, it only takes a minute and can be safely done inside/on top of the tank and then you can safely place them back into their home. Of course we want to minimise stress as much as possible, however, measuring their shells is an important and crucial process that should take place, regardless of if the crab likes it or not.


My crab won't retract, how do I safely measure their shell without being pinched!?


Ahh yes, some crabbies are very stubborn and choose to come out and say hello during shell measuring. This is especially common for crabs that cannot fully retract into their shell as they have nowhere to go other than to come out to play while you just want to measure their shell.


Because their body will be in the way of measuring their shell opening, this is the time to use a digital caliper, although we always recommend using a digital caliper as this is the most accurate way to measure a shell opening, you can simply place the measuring devices around the crabs body, coming down from the top of the crab to measure the opening. This will not harm your crab and will give you an easier, more accurate reading.


If you've got a cheeky pincher (let's face it, we all have one of those types in our tank)

You can use gloves, oven mitts or a towel to hold your crab to prevent being pinched.


If you choose to measure your crabs shell out side of the tank, ensure you have a soft surface under them incase you accidently drop them. This can be a couch, bed, blanket or towel. Never measure over tiles, concrete, marble benches, or tables. A drop from such a height can seriously injure your crab if they were to land on such a hard surface.


Sometimes, our crabs fit perfectly in their shells, but they still refuse to retract for us to get an accurate shell measure. Another tip if your cheeky crab just wants to say hi, is gently stroking or taping their claw/leg. Often this will cause them to go back into their shell. Although this may not work if you have a super outgoing crab who couldn't care less. Yes, those personality plus crabs do exist!


Below are some photos showing how to correctly measure shells, including different techniques, as well as how to identify a crab that is too big or too small for their shell.

If you would like to read more about Hermit Crab shells you can view our shell blog here https://www.happyhermie.com.au/post/hermit-crab-shell-guide


This Hermit Crab is too big to fit securely inside their shell and will need to be offered a larger size.


This Hermit Crab is far too small for their shell, they can hide all the way back until you can no longer see them. They need to be offered a smaller sized shell.


This crab fits nicely into their shell, they have enough room to fully retract and protect themselves.


Using a digital Caliper to measure shell openings is the most accurate way.


You can also use a tape measure


Or a ruler, stiff ruler works better.



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