Clearly, however, the most fundamental and obvious feature of hermit crabs is their reliance upon a hard shelter that affords them protection from predation. This means they can dispense with the need to grow an exoskeleton—however, a “homeless” hermit will be easily eaten! You will note that I don’t immediately refer to their shelters as shells. This is because not all species use them exclusively; some have been recorded using tusk shells, pieces of bamboo, or plastic components dumped by humans. Some hermits, such as Discorsopagurus schmitti, inhabit the empty tubes of tubeworms such as feather duster worms (Sabellastarte fallax), and they may enter aquaria as hitchhikers on live rock. However, hermit crabs mainly use the shells of gastropod mollusks.
Are there any fully aquatic, aquarium safe crabs
Arrow crabs are also regularly seen for sale, most commonly Stenorhynchus seticornis, which is another of the most popular species around. At first glance they appear to be some sort of long-legged spider, but they have a distinctive snout, called a rostrum, that projects from the front of their shell. They're carnivores that will eat any sorts of meaty foods including other small invertebrates, especially bristle worms and feather duster worms, and they've been known to go after small sleeping fishes when given the chance, as well. So, they're fine in fish aquariums, if the fishes are big enough to take care of themselves, but again, I'd be wary of putting one in an aquarium with other small invertebrates.
Please note: These are aquatic saltwater hermit crabs.
There are quite a few decorator crabs, all of which are recognized by their habits of taking small bits of algae, sponge, tunicates, and/or various soft coral polyps and sticking them to their own shells in order to make an effective coat of living camouflage. Some are omnivores, but the majority seems to be carnivores, so these aren't recommended in reef tanks, either. To stay decorated, they need a supply of things to decorate themselves with, but if you put them in a reef aquarium they may also cut up and even eat from the things they need, too. Some species also have very specific diets and will likely starve in captivity, so I do not recommend these for most folks.
Apr 12, 2016 - It would be similar with aquatic hermit crabs