Hermit Crab

Hermit Crab Summary

Hermit crabs are small crustaceans that live in shallow waters around the world. Contrary to their common name, hermit crabs are not solitary creatures but frequently live in communities of 100 or more. These crustaceans get their name from the mollusk shells that they carry on their backs and must periodically swap out as they grow. Hermit crabs need these appropriated shells as unlike other crustaceans, they have soft bodies and only have hard exoskeletons for the front part of their bodies.
 

Five Hermit Crab Facts

• There are more than 1,100 hermit crab species worldwide
• Hermit crabs do not breed in captivity; thus all that are sold as pets have been harvested from the wild
• Some crabs eat their soft molted shells to get vitamins, minerals and calcium
• When selecting a new shell, a hermit crab will inspect it visually and physically before deciding to move in
• Competition for shells can be fierce and often results in fights between two crabs to secure a prime shell

Hermit Crab Scientific Name

The scientific name for hermit crabs is Paguroidea, representing a superfamily of decapod crustaceans that have a soft asymmetrical abdomen and occupy empty shells of other mollusks. The taxonomy of Paguroidea is further organized into seven subfamilies that represent both land and marine species.
 

Hermit Crab Appearance and Behavior

Because hermit crabs encompass so many different species, they generally range in size from about a half-inch to over four inches in length. A few exotic species grow to 11 inches. You can also find them in many different colors, including green, red, blue, yellow, orange, brown, pink and white.

A hard exoskeleton covers the front half of the hermit crab's body, like that of other crabs. Where hermit crabs differ is that they have long, sometimes twisted, abdomens that are soft and can fit into a discarded shell. As hermit crabs grow, they need to find larger shells to accommodate growth.

Hermit crabs molt when they grow, building up water inside their bodies to split the old shells. Some species will leave their shell and bury themselves in the sand to molt, while others remain in their shell and only emerge right before molting. The process takes 45 to 120 days. Newly molted crabs are blue. To fit inside a shell, a hermit crab presses its abdomen, its fourth and fifth pairs of legs, and its uropods against the shell's inner wall.

Land and marine hermit crabs have gills with highly vascular areas for oxygen exchange. Land crabs keep their gills moist by storing water in their bodies. Their eyes are atop stalks, and their heads have two pairs of antennae. They use the longer ones for feeling and the shorter pair for tasting and smelling. The antennae are also vibration sensors. The first pair of legs is a set of pincers, with one side larger than the other. Hermit crabs walk on their second and third set of legs.
 

Hermit Crab Habitat

Whether a land or marine species, hermit crabs are usually found near the shoreline because of abundant food and places to hide. Land crabs use pools of seawater to wet the interior of their shells and their gills. They also use these pools for reproduction. Semi-terrestrial specials live in tubes or plant stems, sections of bamboo and broken coconut shells in addition to seashells. Habitats can include coastal forests and salt marshes. You'll often find them hiding under vegetation, under rock ledges, and in holes in trees where predators can't find them.

Aquatic species live in sandy- or muddy-bottomed environments and occasionally venture out into deeper water. Pyloches, a species that lives in the Indian Ocean can be found at depths of 600 to 1,200 feet where it lives in hollow wood. Other species live inside coral or sponges. Some species, such as Pagurus bernhardus, a red crab found in North American and European waters, often live with anemones on its shell.
 

Hermit Crab Diet

All species of hermit crabs are active in their quest for food, usually moving about at night. They are foragers, meaning that they are omnivores and eat a variety of detritus that other creatures won't eat. They play an essential role in the benthic, or bottom-dwelling marine community by cleansing the environment. Their favorite food consists of small fish and invertebrates such as worms, along with plankton and other similar particles in the water. They will even consume dead hermit crabs if given a chance.
 

Hermit Crab Predators and Threats

The small size of most hermit crabs makes them vulnerable to many different predators, including sharks, many different fish species, cuttlefish, squid and octopus. Although fisheries don't target these crabs for food, they often get caught up when fisheries try to trap other types of seafood.
 

Hermit Crab Reproduction, Babies, and Lifespan

Hermit crabs need seawater to reproduce, which is why land crabs head to shallow waters to mate. Both males and females need to emerge partially from their borrowed shells to reproduce. Mating occurs once a year. The male holds the female with one claw, pulling her back and forth while stroking her to fertilize her. Each female has abdominal appendages that allow her to carry the eggs until they are ready to hatch. Incubation is approximately one month. The females must be in the water to release the eggs, which turn into swimming larvae called zoea upon release. These larvae live like plankton for a time until finally descending to the seafloor. Zoea grow and molt several times to become megalops and then grow and molt into juveniles, eventually reaching the stage where they must find their shells. Terrestrial species of hermit crabs only return to land when they are adults. Younger hermit crabs molt every few months while older ones may not molt for as long as 18 months.

Most hermit crabs have an average lifespan of one to 10 years. Some species, however, can live for as long as 30 years. One land species, the Coenobita brevimanus can live as long as 70 years.
 

Hermit Crab Population

Hermit crabs are found all over the world in tropical and sub-tropical waters as well as in many temperate zones in the northern hemisphere. They are not considered endangered, although the condition of many of their habitats threatens their survival. In some places, hermit crabs are beginning to mistake plastic containers for shells, which is further threatening their survival. The number of these animals worldwide is unknown.

Even though hermit crabs make interesting pets, they are not recommended for beginners. Furthermore, many animal rights groups recommend not buying them as all hermit crabs sold in pet stores have been harvested from the wild. The painted shells that the crabs are often sold with slowly poison the animal. This practice of keeping them as pets is unsustainable as hermit crabs do not breed in captivity. Therefore, keeping these animals as pets unadvisable.
 

Hermit Crab FAQ

Why is one pincer larger than the other?

The larger pincer serves as a protective door when the crab retreats inside its shell.

 

Are hermit crabs true crabs?

Although they are crustaceans, hermit crabs are more closely related to lobsters.
 

Can you pull a hermit crab out of its shell?

It's nearly impossible to do so because the crab wraps its abdomen tightly around the shell's interior while also holding on tightly with one of its uropods.
 

Why do hermit crabs fight over shells?

Good shells are at a premium for these crabs. As they grow, hermit crabs need bigger shells, so they often steal shells when someone in their community is molting.
 

Can hermit crabs grow smaller?

If they find themselves in a shell that is too small, hermit crabs have the rare ability to grow smaller.
 

What is a "vacancy chain?"

To avoid fights over shells, hermit crabs looking for new ones will sometimes line up in order of size to enter a new home as larger crabs vacate their old ones.

 

Sources:
1. David Burnie, Dorling Kindersley (2011) Animal, The Definitive Visual Guide To The World's Wildlife
2. Tom Jackson, Lorenz Books (2007) The World Encyclopedia Of Animals
3. David Burnie, Kingfisher (2011) The Kingfisher Animal Encyclopedia
4. Richard Mackay, University of California Press (2009) The Atlas Of Endangered Species
5. David Burnie, Dorling Kindersley (2008) Illustrated Encyclopedia Of Animals
6. Dorling Kindersley (2006) Dorling Kindersley Encyclopedia Of Animals