Hermit crabs are decapod crustaceans of the superfamily Paguroidea. The fossil record of in situ hermit crabs using gastropod shells extends back to the Late Cretaceous period.
Hermit Crabs are found worldwide. Long-clawed hermit crabs live mostly in shallow water and are common in tide pools and salt marshes. Broad-clawed hermit crabs live in deeper waters. Hermit crabs are active animals, moving about in search of food.
Hermit crabs are omnivorous animals that eat pretty much anything they can find in the surrounding water.
Small fish and invertebrates including worms are the most common prey for the hermit crab along with plankton and other food particles in the water.
Land Hermit Crabs eat all kinds of plants, and can often be found climbing high up in coastal trees.
The hermit crab finds food with its extremely well-developed sense of smell. Like an insect, it uses antennae to zero in on its meal.
Most hermit crabs are nocturnal.
Most of the approximately 1,110 species possess an asymmetrical abdomen that is concealed in a scavenged mollusk shell carried around by the hermit crab.
Despite its snail-like appearance, the hermit crab is related to crabs, although they are not that closely related as the hermit crab is not a true crab.
Most frequently, hermit crabs use the shells of sea snails (although the shells of bivalves and scaphopods and even hollow pieces of wood and stone are used by some species),
The tip of the hermit crab’s abdomen is adapted to clasp strongly onto the columella of the snail shell.
There are two groups of hermit crabs.
- Marine hermit crabs spend most of their lives underwater as aquatic animals, live in varying depths of saltwater from shallow reefs and shorelines to deep sea bottoms and rarely leave for land.
- Land hermit crabs, spend most of their life on land as terrestrial species in tropical areas, though even they require access to both freshwater and saltwater to keep their gills damp or wet to survive and to reproduce.
The Molting Crab…
Hermit crabs earn their name because they live in mollusk shells, and they have a tendency to withdraw into their shells when they are threatened. These shells protect the hermit crab’s soft, curved abdomen.
The crab can grow and molt every year like this for more a century. The hermit crab wears its skeleton on the outside and must shed it as it grows, so once a year it crawls into the safety of a burrow and molts.
It’s highly vulnerable once it steps out of this rigid shell, so to hasten the development of new armor it … consumes its old exoskeleton.
It is, in effect, recycling the materials, which are in short supply in its terrestrial environment. Coconut crabs that are disturbed before they have consumed the entire shell often have soft exoskeletons until they have time to reaccumulate the necessary calcium and other minerals,
As hermit crabs grow, they require larger shells. Since suitable intact gastropod shells are sometimes a limited resource, vigorous competition often occurs among hermit crabs for shells.
The availability of empty shells at any given place depends on the relative abundance of gastropods and hermit crabs, matched for size. An equally important issue is the population of organisms that prey upon gastropods and leave the shells intact
When an individual crab finds a new empty shell it will leave its own shell and inspect the vacant shell for size.
If the shell is found to be too large, the crab goes back to its own shell and then waits by the vacant shell for anything up to 8 hours.
As new crabs arrive they also inspect the shell and, if it is too big, wait with the others, forming a group of up to 20 individuals, holding onto each other in a line from the largest to the smallest crab.
As soon as a crab arrives that is the right size for the vacant shell and claims it, leaving its old shell vacant, then all the crabs in the queue swiftly exchange shells in sequence, each one moving up to the next size. Hermit crabs often “gang up” on one of their species with what they perceive to be a better shell, and pry its shell away from it before competing for it until one takes it over.
For some larger marine species, supporting one or more sea anemones on the shell can scare away predators. The sea anemone benefits, because it is in a position to consume fragments of the hermit crab’s meals. Other very close symbiotic relationships are known from encrusting bryozoans and hermit crabs forming bryoliths.
Life Span: Up to 30 years
Hermit crabs don’t live in the water, but they live relatively close. One of the reasons for this is because that’s where they mate. In some areas, hermit crabs travel to the water in mass for a mating ritual in which all find mates. The males and females come out of their shells a little and the male transfers a spermatophore to the female. When it implants, it can deposit thousands of eggs along the female’s abdomen.
The young develop in stages, with the first two (the nauplius and protozoa) occurring inside the egg. Most hermit crab larvae hatch at the third stage, the zoea. In this larval stage, the crab has several long spines, a long, narrow abdomen, and large fringed antennae. Several zoeal molts are followed by the final larval stage, the megalopa.
Carrying the Eggs
The female carries eggs for about a month, during which the eggs go through their supply of yolk. The female spends this time out of the ocean, staying away from saltwater until it’s time to hatch the eggs. You can tell when the eggs are about ready to hatch because they change color as a result of the yolk depletion — they go from being brick red to dull dark gray.
Dumping the Kids
When the eggs are gray, the female travels back to the ocean and submerges herself in the water, simultaneously hatching the eggs. When saltwater surrounds the eggs they hatch, releasing undeveloped hermit crabs known as zoeae into the water. Because hermit crabs rely on saltwater to hatch their babies, hermit crabs kept in freshwater captivity won’t mate.
The reproduction process isn’t over once the eggs are hatched — zoeae have to grow up. For a period that lasts up to two months, the zoeae go through various stages of metamorphosis until they become crustaceans called megalopae.
After another month or so, each megalopa finds its first shell and starts spending time outside of the water. After it buries itself and molts for the first time, it is a genuine hermit crab that lives on land instead of in the ocean
When properly sized gastropod shells are not available, hermit crabs have been known to resort to wearing other objects such as tree nut hulls or pieces of beach litter rather than go unprotected. This has become very common in some areas, due not only to the increased prevalence of marine debris but also due to depletion of seashells from beaches due to humans collecting them.
Create a “Crabitat”…
Setting up a hermit crab habitat, or “crabitat,” correctly is very important for the overall health of your hermit crabs. Many potential hermit crab owners think that the small plastic cages they see in pet stores are what hermit crabs require, but that couldn’t be further from the truth! Hermit crabs are not easy, throw away pets that many people believe them to be, and setting up a proper habitat takes time and effort.
Never use a small plastic container to house your hermit crabs. They need plenty of room to climb and move around. The smallest enclosure you should use for two small to medium sized hermit crabs is a 10-gallon aquarium. Housing multiple medium sized hermit crabs or two large hermit crabs can require an aquarium as large as 30 or 40 gallons.
Hermit crabs are good escape artists, so the enclosure will need a secure lid. If your house is humid and warm, you can use a wire mesh lid with latches.
However, if there is a chance that the dry air in your home will affect the humidity levels of the habitat, we recommend the use of a glass or Plexiglas lid. You could also use a wire mesh lid with most of the lid covered by plastic wrap.
Though it is important to keep the enclosure warm, never place it on a windowsill or anywhere else that it will be in direct sunlight.
This will raise the temperature dangerously high, and you run the risk of actually baking your hermit crabs.
Substrate You should choose a substrate that is easy to clean and that will allow your hermit crabs to tunnel and burrow in it. The two preferred substrates are sand and coconut fiber substrate.
You can purchase special terrarium sand or play sand. Coconut fiber substrate works well also and is especially good for tunneling and burrowing.
Many hermit crab owners elect to use a combination of the two. Crushed coral is also an option for a substrate. Avoid gravel, wood chips, or other substrates that are unsafe or do not allow them to dig.
Decorations and Accessories Hermit crabs love to climb, so they will need plenty of accessories in their cage for entertainment. The following items work well as decorations in a hermit crab enclosure:
- Sanitized shells
- Fake plants and vines
- Ceramic pots
- Driftwood and branches
- Rough rocks
- Climbing toys
- Wooden or plastic shelters
Water In addition to the above accessories, you will also need to place two water dishes in the habitat. One dish is for fresh water and one for saltwater.
They should be big enough that your hermit crabs can submerge themselves but set up in such a way that it is easy for your crab to get in and out of them.
The fresh water container should hold spring water or de-chlorinated water. The saltwater container should be a mixture of de-chlorinated or spring water and a special aquarium salt mix (regular table salt is not acceptable).
You should also place extra shells around the habitat for the hermit crabs to change into. There should be at least three shells per hermit crab, and the shells should be at least as big as or slightly larger than the crab’s current shell.
Humidity levels are very important to the survival of hermit crabs. Hermit crabs have modified gills that allow them to breathe, but to be able to breathe, they need to breathe moistened air. Air that is too dry will actually dry out their gills, making it increasingly difficult to breathe until they finally end up suffocating.
This is why their habitat needs to have a moist, tropical feel and a relative humidity that is no lower than 70%.
To help keep humidity levels where they need to be, keep the substrate damp, and avoid using substrates that do not retain moisture well.
However, the substrate should not be soaking wet, as humidity that is too high can cause health issues as well.
We recommend moistening the low and middle areas of the substrate while leaving the higher hills dry. You can also mist the entire enclosure daily to provide plenty of moisture in the air.
Another way to maintain humidity levels is to use bubble bowls made from airstones, airline tubing, and air pumps typically used for aquariums. Simply put an airstone in the bowl of water and use the tubing to connect the airstone to the pump. This will cause the water in the bowl to bubble and circulate, increasing humidity as well as helping to keep the water fresh.
Always have a hygrometer in the habitat to monitor humidity levels. Guessing or approximating humidity levels can result in the death of your hermit crab. Be sure to check whether your hygrometer measures relative or actual humidity. Relative humidity levels should be 75% to 90%. Actual humidity levels will be approximately 50% to 60%.
Temperature Keeping within your hermit crab’s temperature range is equally as important. Temperatures that are too high will causeirreversible heat damage, and improperly low temperatures will alter a hermit crab’s metabolism.
You may need to use outside heat sources to maintain temperatures within the habitat, especially during colder winter months. Primary heat sources can include under tank heaters and 15-watt reptile heat lamps. The temperature of the habitat should always be monitored with a thermometer.
Lighting can be very beneficial, both for providing healthy daylight and for maintaining proper temperature. When choosing lights, pay close attention to wattage. Lights with a high wattage over a small tank will cause the humidity to drop below proper levels as well as raise the temperatures into a dangerous range for your crabs. We recommend the use of a 15-watt bulb for a 10-gallon tank. The bigger the tank, the higher the wattage can be, such as a 60-watt bulb for a 55-gallon tank.
Lights that you can use include fluorescent bulbs that provide UVB and UVA light, incandescent bulbs, nighttime heating lamps, or a combination of these. You should use a timer to make sure that the light cycles are maintained appropriately.
Cleaning regularly is necessary to keep your hermit crabs healthy and their environment safe. The following tasks should be part of your cleaning routine:
- Spot clean substrate by straining the sand to remove waste and debris.
- Change food and water as needed (when soiled, or when a film starts to build up on the bottom of the dish).
- Wash food and water dishes as needed.
We also recommend that you change the substrate completely at least once a month or more often if you see flies, gnats, or any other insects inside the enclosure. At this time, you can remove all of the accessories, wash the enclosure with vinegar and water (rinsing thoroughly), and boil toys and empty shells.
Always set up the enclosure before bringing home your hermit crabs. It is crucial that you stabilize the temperature and humidity levels before putting any crabs in the habitat. The hermit crabs you bring home will already be under stress, and putting them into an enclosure with improper or fluctuating temperatures and humidity levels can cause serious adverse side effects including lethargy, loss of limbs, and even death.
Hermit crabs kept together may fight or kill a competitor to gain access to the shell they favor. However, if the crabs vary significantly in size, the occurrence of fights over empty shells will decrease or remain nonexistent. Hermit crabs with too-small shells cannot grow as fast as those with well-fitting shells and are more likely to be eaten if they cannot retract completely into the shell.
Hermit crabs play an important role in the benthic, or bottom-dwelling, community. They are scavengers, helping to recycle energy back into the ecosystem.
The tails of larger hermit crabs are often used as bait in recreational fishing.
Hermit crabs do not breed in human care; they return to the ocean to breed and live out the first part of their lives. As a result, all hermit crabs purchased through pet shops come from the wild.
Land hermit crabs are also at risk of habitat loss, as the mangroves and coastal areas where they live are taken over and developed by humans.
Hermit crabs are a part of nature’s clean up and recycling service, as they scavenge rotting animals and dead fish.