Hermit Crabs comprise over 1100 species of crustaceans belonging to the Paguroidea superfamily. They are famous for using discarded mollusc shells to protect their soft abdominal exoskeletons. Although competition for limited shells is fierce, these crabs thrive in huge colonies along tropical coasts, but suffer in home aquariums worldwide.
1. All Hermit Crab Species are Divided into Marine or Land Hermit Crabs
Of the 1100 hermit crab species, most are marine hermit crabs. Marine hermit crabs live in the ocean. They can live among coral reefs in shallow waters, or on the ocean floor 300 meters below the surface. Marine hermit crabs can survive short trips on land as long as their gills are damp.
Land Hermit Crabs Can Drown, Despite Having Gills
Land hermit crabs are hermit crabs that live along shores, but spend their lives on land. Even though they live on beaches or rocky shores, they still have gills. These hermit crabs drink freshwater, but use saltwater to bathe and keep their gills damp. Land hermit crabs can only stay submerged for 15-30 minutes at a time. A land hermit crab submerged in a tank will drown.
There is One Species of Hermit Crab that Lives in Freshwater
The hermit crab species Clibanarius fonticola is unique because it is the only species that lives in freshwater. The species was discovered on the island Vanuatu in the south Pacific Ocean. All fonticola live in a small pool fed by natural springs near an abandoned airstrip. These hermit crabs only grow as big as pencil erasers. All the adult crabs occupy the shells of the freshwater snail species Clithon corona.
2. Hermit Crabs Have Hard Front Exoskeletons and Soft Abdomens
Hermit Crabs are more closely related to some lobster species than crabs. Actual crabs have hard exoskeletons covering their entire bodies. Hermit crabs have hard exoskeletons covering the upper half of their body. However, the lower half, which includes the abdomen, is unprotected and very fragile. Without protection, exposed hermit crabs are vulnerable to predators.
3. Hermit Crabs Use Discarded Shells to Protect their Lower Bodies
Unlike most crab species, hermit crabs do not grow their own shells. Instead, they have to fit into shells they find in the ocean or along the beach. Hermit crabs like to occupy seashells or discarded snail shells. Hermit crabs do not fight vigorous snails for their shell, but they will oust a dead or dying snail. A hermit crab without shell is a hermit crab not long for this world. Even still, hermit crabs carefully inspect and try on shells before committing to one.
Hermit Crab Tails are Shaped Like Hooks That Grasp Small Columns in Shells
While the lower half of a hermit crab’s body is soft, it is adapted to locking a hermit crab into a shell. A hermit crab abdomen is asymmetrical and in the shape of a spiral. Its tail is shaped like a hook, adapted to lock the crab into a shell. The tail wraps around a small natural column in the shell called a columella. This makes it almost impossible to pull a hermit crab out of its shell.
4. Hermit Crabs Molt Every 18 Months
Every 18 months, hermit crabs shed their exoskeletons and regrow a new one that fits their larger body. When a hermit crab molts, it builds up enough water pressure in its body to split its current shell. Many will bury themselves in the sand until molting is finished, while others stay in their shell. Sometimes crabs eat their molted shell, as it is rich in vitamins, minerals, and calcium.
Hermit Crab Molting Takes Between 45-120 Days
Molting isn’t just a frequent practice for hermit crabs, it’s also time-consuming. It takes a hermit crab 45-120 days to complete molting. Going one or two months without protection leaves hermit crabs very vulnerable to predators, so it makes sense that some just bury themselves until it's over!
5. Discarded Hermit Crab Shells Get Passed Along to Smaller Hermit Crabs
Molting Hermit Crabs sometimes form vacancy chains. First, a large molting hermit crab will find a suitable new shell. Smaller hermit crabs will form a line behind it in order of size. When the largest hermit crab climbs into its new shell, the second-largest crab takes its old shell. Then the third-largest crab takes that crab’s old shell, and so on. However, if an extra crab comes to the line, then two crabs of the same size will have to fight for a shell.
6. Hermit Crabs Live in Colonies of 100 or More
Hermit crabs, despite their name, are very social creatures. They live in large colonies with hundreds of other hermit crabs. Such large populations often mean stiff competition for shells. Hermit crabs will fight among themselves for an available shell. However, they are also collaborative. Hermit crabs often go scavenging for food in groups.
7. Land Hermit Crab Eggs Need Saltwater to Hatch
To mate, a male hermit crab will grab a female with his claw. The two will partially emerge from their shells and press their stomachs together. The male will fertilize the female’s eggs. Pregnant land hermit crabs go to the edge of the ocean, and contact with saltwater causes the eggs to hatch. The newborn crabs will spend some time in the water before emerging to live on land.
Hermit Crabs do Not Breed Well in Captivity
Hermit crabs rarely breed in captivity. So nearly every hermit crab sold in pet stores has been taken from its native habitat. Workers who collect hermit crabs to be sold in stores don’t just take the crabs. They also take seashells to sell alongside the crabs, depriving wild hermit crabs of much-needed shelters.
8. Hermit Crabs are Scavengers, Not Predators
Hermit crab food is whatever hungry hermit crabs find. Hermit crabs are omnivorous scavengers, but not predators. They eat algae, plankton, and bits of small dead fish. They’ll also eat worms and microscopic clams. However, when a hermit crab moves into a dead creature’s shell, it does not eat the remains of the previous occupant.
The Giant Hermit Crab Will Attack and Eat Large Snails
Generally, hermit crabs do not attack healthy snails for their shells. The giant hermit crab, however, is an exception. Petrochirus diogenes, as they are called, will attack and devour Aliger gigas, aka Queen Conch. After eating the conch, the giant hermit crab will occupy its shell. In just one fight, this efficient hermit crab gets dinner and a new home.
Most Commercial Hermit Crab Food Contains Poisonous Ethoxyquin
Ethoxyquin is a preservative that is very common in pet food. Commercial hermit crab food is vegetables, fruit, and meat bits combined and sold as tiny flakes or pebbles. Ethoxyquin is added to keep fatty acids in the food from going rancid. However, ethoxyquin has insecticidal properties that are harmful to crabs. The compound can even hurt a hermit crab’s ability to molt.
9. Hermit Crabs Live Up to 40 Years in the Wild
Hermit crabs live very long lives for how small they are. Hermit crabs in the wild usually live around 30 years, but can live to be over 40. One species, Coenobita brevimanus, can live up to 70 years old. A 70-year-old crab will have molted around 46 times.
Hermit Crabs Only Live up to 10 Years in Captivity
Hermit crabs kept as pets are lucky to live ten years. But many hermit crabs only survive a few months to one year as pets. This is because many owners do not maintain the humidity their pet crabs need to survive. Without high humidity, the pet crabs suffocate. The blue leg hermit crab, Clibanarius tricolor, is very popular because of its electric blue coloring and tendency to clean sand. But this striking crab only lives two years in captivity.
As Cute as Hermit Crabs Are, It’s Best to Leave Them in the Wild!
Hermit crabs, while adorable, are not disposable. Sadly, many people dispose of dead hermit crabs and then go to the pet store to get a new one. Hermit crabs are charming creatures, but they deserve to have long, happy lives in the wild. Besides, who would want to hurt this cute little face?
How endangered is this animal?
- Almost 570,000 hermit crabs die every year from getting trapped in plastic debris
Large mounds of plastic debris that wash up on shores kill over half a million hermit crabs every year. Small hermit crabs searching for shells along plastic debris can easily fall into open bottles and die trapped. As hermit crabs die, they release pheromones that signal to other hermit crabs that their shells are available. Thus, the entrapment and death of hermit crabs in plastic is a vicious cycle.
- Noise Pollution Distracts Hermit Crabs
Recent studies demonstrate how noise pollution from boats hurt hermit crab populations. Hermit crabs subjected to ship noise did not demonstrate the same awareness of predators as hermit crabs not subjected. Even worse, ship noises interfered with a hermit crab’s ability to assess a potential new shell.
Also Known As
10 to 12 cm (4 to 5 inches)
Microscopic mussels and clams, bits of dead animals, and macroalgae
Up to 40 years