Look at your crab. What is the most dominating feature of him or her? The sea shell! That cast-off snail shell your crab is wearing is very important, as I'm sure you know. Obviously the crab wouldn't wear a shell if it had one of its own! As we know from looking at our crabs, their rear part (the abdomen) is very soft. The abdomen is arguably the most important part of the crab's body, because it holds the crabs' digestive glands and reproductive system.

But why a sea shell? Common sense tells us that the sea shell affords the hermit crab complete mobility AND security from predators. The perfect hermit crab shell has no holes, and fits the crab snugly, but allows him or her to withdraw into the shell completely.

Oddly enough, when hermit crabs first crawl ashore as juveniles, the shape of the 'shell' they find will influence the growth of their abdomen. For example, if a crab comes ashore and only finds a bamboo tube in which to tuck its' rear, the crab's abdomen will not develop the traditional curl to the right, but will remain straight. One would like to think that there are enough shells available in the natural environment for the hermit crabs, but this is not always true. Hermit crabs have been found 'wearing' plastic bottle tops and airline liquor bottles.

Another very important purpose of the crabs shell is to minimize the evaporation of water from the crab's body. As you have read on the basic care page (in the critical care tips) moisture is the most important thing in a hermit crab's life. Everything about their life cycle, aside from their mating ritual, revolves around conserving moisture -- the crabs are not active during the heat of the day to conserve moisture; the crabs are out and active during rain falls so they can experience the maximum effect of the rain; crabs molt underground to conserve moisture. So you can see why a properly-fitting shell is so important to the crab. A crab in a durable shell with a snug fit and access to adequate water lives much longer than a crab in an ill-fitting shell.

How do I Know if the Shell Fits Correctly?  Your crab should be able to completely withdraw into its shell and seal the opening. Some crabs like shells a little larger or a bit smaller, but as long as the shell covers the abdomen closely and completely, there is no real reason to fret over your crab being in a shell that is too small. The same goes for a crab in a shell that you think is too large. If the crab is able to maneuver the shell while it is walking, there is no need for you to worry that the shell is too large.  Each crab is individual, and just like you cannot make a person wear clothing they do not want to wear, you cannot force a crab to move into a shell it doesn't like.

How do I Measure my Crab for a Shell? Take your crab in your hand and look at his large claw. The perfect-fitting shell will have an opening about the size of the large claw plus about 1/10" to 1/8" all around (for larger crabs it will be more). You're ultimately looking for a shell whose entrance the crab will be able to plug with his or her large claw and far left walking leg.

Measuring the Aperture (Opening) A method sea shell collectors often use to judge the size of a shell is by its aperture or opening. Take the shell and put a ruler across the mouth of it.  The distance from one side of the shell opening to the other is called the "aperture."  Small crabs require a shell with an aperture of 1/4", large crabs with 1 1/2" to 2" and up.  If you're trying to buy your crab a shell that will fit better than the one he is in, take a rough estimate of the aperture of the shell he's occupying, and try to  guess the proper size. A good sea shell shop will accept a shell if it is returned in new condition, so you have nothing to lose if you fail to order the correct size.

What is the 'Best' Shell for my Crab? The best shell for your crab is one that he chooses, no matter if you like it or not. To be blunt, it is not your abdomen in that shell so if you provide him with a beautiful but uncomfortable shell, the crab will not want it! Provide your crabs with clean shells. All shells you give to them should be boiled on the stove at a rolling boil for about five minutes, the allowed to cool. If you have an especially beautiful shell that you're excited about giving to the crab as soon as possible, you can boil it, and then after the required five minutes, gradually pour off the boiled water while you pour cold water into the pot. This will cool down the water (and thus the shell) faster so you can give it to your crab.

Also, crabs are drawn to shells with circular openings (an exception to this is the Ecuadorian crab. This will be discussed later on).  Many hermit crabs are drawn to shells lined with mother-of-pearl. It is suspected that they like these types of shells because (1) mother-of-pearl is an excellent insulation from the environment, and (2) mother-of-pearl is very smooth and 'comfortable' on a crab's abdomen. Most crabs find a shell of the correct size and a smooth mother-of-pearl interior hard to resist!

Finicky Little Buggers: The Ecuadorians! A common complaint among people who own Ecuadorian crabs is that they refuse to change shells. Compared with their Caribbean relatives, Ecuadorian crabs aren't very hasty when it comes to changing shells. In the 35+ years that I've owned hermit crabs, I have easily witnessed over 300 Caribbean crab shell changes, but I have only witnessed five Ecuadorians change shells!

A big contributor to this is that many crab owners assume that Ecuadorian and purple-claw crabs like the same kinds of shells. This is not the case! As you can tell from looking at your Ecuadorian crabs, their head and body are much flatter and wider than a Caribbean crab's. So the Ecuadorian crabs prefer a shell with a more slot-shaped opening. Note the shell at right and compare it with the shell above. The shell at right is a 'perfect' (and recently occupied) Ecuadorian shell. See the slot-shape as compared with the shell above? Note the lack of mother-of-pearl. Ecuadorians are the very essence of au natural in the pet hermit crab world!

There you have it! An exhaustive description of the how's and why's of hermit crab sea shells. To move on to the next page, where you will look at some great crab shells and some not-so-great shells, click here!

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