Hermit Crabs have lots of adaptions to make life in a salvaged shell much easier. They have 10 legs in total, but only 6 are visible - these are called the walking legs. The remaining 4 are small and hidden in the shell. These legs are so small that they almost appear vestigial, but they're actually used to cling to the inside of the shell, making sure that the crab doesn't slip out. When they sense a potential predator they retreat deep into their shell and use their hard claws to block the entrance.
This man-made glass shell allows us to see the long, soft back end of the crab and its small, usually hidden back legs.
Though most are fully aquatic, some are terrestrial - but their larvae still need to develop in the water. One of these terrestrial species is the enormous Coconut Crab, which despite being a Hermit Crab, does not use a salvaged shell. It is mainly found on islands in the Indian Ocean. This remarkable animal can grow up to a metre long and weigh a whopping 4.1 kg - earning it the title of the world's largest terrestrial invertebrate. These crustaceans get their name from reports of them climbing trees and cracking open coconuts to feed on the flesh inside. Although they can and do climb trees, and they are strong enough to crack open the hard husk, coconuts do not form a major part of their diet. They mainly feed on seeds, fruit, nuts, and even carrion.
A Coconut Crab inside its burrow.
Some Hermit Crabs have developed a symbiotic relationship with sea anemones. The crab attaches the anemone to its shell and allows it to feed upon leftovers from the crab's meal. In return the anemone protects the crab using its stinging tentacles.
A Hermit Crab with an Anemone on its shell.
An interesting experiment has been done with Common Hermit Crabs (Pagurus bernhardus), which is native to the UK. This experiment includes 2 tanks full of sea water, one of which has black sand, and the other golden sand. In the black sand tank several black shells are placed, and in the golden sand tank several golden shells are added. Now some Hermit Crabs with golden shells are added to the tank with black sand and shells. Some Hermit Crabs with black shells are then added to the tank with golden sand and shells. At this point most Hermit Crabs swap their shell for one already in the tank. This shows that the crabs are capable of noticing colours and behaving accordingly so that they can remain camouflaged.
A Common Hermit Crab.
Hermit Crabs have been around for many millions of years. Interestingly, a fossil Hermit Crab from the Early Cretaceous period (146 million years ago) has been discovered using an abandoned ammonite shell as a home!
As I have already mentioned, I used to keep several tropical Hermit Crab species in a marine tank. They were fascinating to watch, and several times I observed them swapping shells. I had some particularly large ones with blue and black legs called Calcinus elegans. Sometimes one of these crabs would see a shell that it liked being used by another. It would then tap unrelentingly at the shell until the owner was eventually forced to leave it. The tapping was pretty loud and could easily be heard through the glass! My favorite species that I had was Ciliopagurus strigatus, also known as the Halloween Hermit Crab. It had bright orange and red bands on its legs, and was one of my larger species. I also had some Scarlet Reef Hermit Crabs (Paguristes cadenati) and Dwarf Zebra Hermit Crabs (Calcinus laevimanus).
A Halloween Hermit Crab.
Probably the strangest Hermit Crab species that I've ever seen is the Hairy Yellow Hermit Crab (Aniculus maximus), a large marine species from the Indo-Pacific. They are opportunistic feeders, eating pretty much anything that they find. They are well known as a bad aquarium inhabitant because they usually eat everything else in the tank and destroy the rock work.
A Hairy Yellow Hermit Crab
I hope that you've enjoyed today's post as much as I've enjoyed writing it. Hermit Crabs really are amazing creatures.
Until next time, keep on the wild side!