Hermit Crab

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 appropriate shells, as, unlike other crustaceans, they have soft bodies and only have hard exoskeletons for the front part of their bodies.

Fun Facts

• There are more than 800 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
• Competition for shells can be fierce and often results in fights between two crabs to secure a prime shell

Scientific Name

Hermit crabs are classified in the family 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.

Evolution

The hermit crab is a unique species of crustacean that has evolved over millions of years. It belongs to the family Paguridae and can be found in oceans all around the world. Hermit crabs have adapted to their environment by carrying their own home with them, where they are able to hide from predators or seek shelter when needed.

Hermit crabs first appeared during the Cambrian period, approximately 540 million years ago, but it wasn’t until much later (the Jurassic period) that they began using gastropod shells as homes. This allowed them to survive better in their environment since these shells provided strong protection against predators and harsh environmental conditions, such as temperature changes.

Over time, different species of hermit crab emerged due to selective pressures caused by changing climates and habitats, which led to adaptations such as varying shell sizes and shapes depending on what type of habitat they inhabited. These variations made it possible for some hermit crabs to live in deeper waters, while others preferred shallow areas close to shorelines.

Today, there are hundreds of species of hermit crab worldwide. However, despite this diversity, most still rely on gastropod shells for protection and shelter, just like their ancient ancestors did millions of years ago! 

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.

The largest hermit crabs in the world

The largest species of hermit crab is the coconut crab, a species that can grow to 40 inches in length! The coconut crab is an outlier in terms of size, but other species can grow much larger than average hermit crabs. The giant red hermit crab is a marine crab that can reach 12 inches in length. They live off the Eastern Seaboard of the United States.

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 species 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. Pylocheles, 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 their shells. 

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.

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.

Conservation

Hermit crabs are one of the most interesting and unique creatures that inhabit our planet. Unfortunately, their conservation status is not as well-known or cared for as it should be. Hermit crabs face a variety of threats in their natural habitat, including habitat destruction due to human activities such as coastal development and pollution from plastic garbage entering the ocean. In addition, many hermit crab species are being threatened by overfishing, climate change, and disease outbreaks caused by pathogens introduced by humans.

It is unethical to keep a hermit crab as a pet because the pet trade is harvesting wild hermit crabs, which is drastically reducing their populations. It can be dangerous for wild populations and the delicate balance of the ecosystem as a whole when people collect them from the wild instead of buying captive-bred specimens from reputable breeders — this practice disrupts local ecosystems and increases pressure on already limited resources available in nature. 

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 grows and molts several times to become megalops and then grows and molts 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. 

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 in are slowly poisoning the animals. This practice of keeping them as pets is unsustainable, as hermit crabs do not breed in captivity. Therefore, keeping these animals as pets is inadvisable.