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Home › Sea Life › Animals › Invertebrates › Crustaceans › Crabs › Hermit Crab

Information and Facts about Hermit Crabs

[Hermit Crab Phylum: Arthropoda] [Class: Malacostraca] [Order: Decapoda] [Superfamily: Paguroidea]

There are more than one thousand recognised hermit crab species belonging to the superfamily Paguroidea, which contains marine hermit crabs (Paguridae) and land hermit crabs (Coenobitidae).

Check out these fascinating facts about hermit crabs, including where they live, what they eat, and how anomuran decapod crustaceans reproduce.

Hermit Crab Geographic Range and Habitat

Around 95% of the hermit crab diversity is represented by the marine species that live in seas and oceans.

They are spread across several divergent families, but in particular:

• Diogenidae (left-handed hermit crabs)
• Paguridae (true hermit crabs)
• Parapaguridae (anemone hermit crabs)
• Pylochelidae (symmetrical hermit crabs)

Yet, the global distribution is immense. You can find them in almost all benthic habitats, from the tropics to the polar regions, and even down to abyssal depths greater than 5000 metres (1,600 feet) near hydrothermal vents and cold water seeps.

Hermit crabs are most prevalent in small intertidal rock pools, seagrass beds, mangroves, and sandy shallow coral reef formations, where empty gastropod shells (or deep-sea substitute shells) are readily available.

But, being able to tolerate a wide range of water salinity (generalistic) means this crab species has a wide variety of options for suitable habitats.

For example:

Some species prefer to live in shallow coastal zones and rocky shorelines where there's an abundance of tidepools and sand patches.

Others thrive better around coral reefs where they can hide amongst coral rubble and forage in the reef crevices.

Plus, some hermit crabs have symbiotic associations with sea anemones, sea sponges, and hydroids, benefitting from enhanced protection.

Hermit Crab Characteristics and Behaviour

Like some Xenophora snails, marine hermit crabs are soft-bodied crustaceans that rely on empty, discarded shells for defence against their natural predators.

These opportunistic scavengers are known to exhibit complex shell-selection, often forming symbiotic partnerships (symbiosis) with other living organisms.

Here's the thing:

The morphological characteristics of oceanic hermit crabs combine an asymmetrical body shape with a soft, coiled abdomen, and a fused hard head and thorax (cephalothorax).

Their chelae (claws) are rarely equal in size, and the largest sometimes operate like a "doorway" - such as when it retracts into its shell.

They have well-developed antennae for sensing food and for spotting danger, and their stalked eyes provide a wide field of vision.

Interesting Fact: Even though most species of marine hermit crabs use gills to extract oxygen from water, some of the intertidal species can tolerate some limited exposure to air, such as the striped hermit crab (Clibanarius vittatus).

What Do Hermit Crabs Eat?

Some aquatic crustaceans tend to specialise in a narrow range of foods. Whereas, marine hermit crabs are more opportunistic in their eating habits and they will consume almost anything organic, including dead carcasses and decaying leftover scraps.

As a consequence of that, they fulfill an important clean-up role in healthy ecosystems.

By and large, the omnivorous diet of hermit crabs includes:

• Algae
• Detritus (including carrion)
• Marine snow (deep-sea species)
• Mollusks
• Plankton
• Worms (polychaetes)

Pro Tip: The crucial feeding roles of marine and terrestrial crabs help to clean up organic debris. It reduces algal overgrowth in coral reefs and tidepools, and it provides an important link between primary production (algae) and large predators (birds, fish).

Types of Hermit Crab Species

How Do Hermit Crabs Reproduce?

Marine hermit crab reproductive strategies are fascinating because they need to combine the normal mating rituals with a unique shell-bound lifestyle.

The mating process occurs between separate crab sexes (males and females). In most cases, the female needs to molt and expose her soft shell before she can receive male sperm.

In more detail...

Chemical cues (called pheromones) help males detect a receptive female. Sometimes, males resort to tapping or rocking her shell as a test of receptiveness for breeding.

If he gets an agreement, he needs to position himself partly outside of his shell. The male aligns his abdomen and special appendages to facilitate the transfer of sperm (via gonopods).

Fertilisation and Egg Brooding

A female carries the brightly coloured eggs (often red or orange) on special appendages called pleopods. The eggs turn darker when the embryos start to develop.

Depending on the fecundity of the species, and the size of the female, a single egg clutch can be a few hundred or it can be tens of thousands.

Key points:

After the larva is released into the water column, it goes through two specific stages. Several molts occur during the initial planktonic larval stage (zoea).

They are free-swimming and feed on plankton as they go through the transitional stage (megalopa). This is when the young start to look more crab-like.

Once megalopods settle somewhere suitable on the seafloor, they need to find an empty tiny gastropod shell to live in. Following that, they can begin metamorphosing into juvenile hermit crabs.

Even so, they need to continue molting as they grow bigger and periodically upgrade to larger shells. It's common for smaller hermit crabs to "move in" to any discarded shells, and it often involves physical fighting for superiority.

Hermit Crab Predators and Threats

In the wild, the long term survival rate for marine and terrestrial hermit crabs mostly relies on their ability to find suitable protective shells.

Nonetheless, the soft abdomen of the marine hermit crab is a nutrient-rich food source for a number of predators, including:

• Crabs (especially box crabs)
• Fish (e.g., flounder, pufferfish, triggerfish, wrasses)
• Lobsters
• Marine mammals (otters, seals)
• Octopuses
• Seabirds (crows, gulls, herons)

Even though the hermit crabs that live in the ocean are not a primary food source for humans, they are collected and used for fishing bait in some coastal regions.

Pro Tip: An assessment released by the IUCN Red List of Threatened Species shows that the conservation status of most hermit crabs worldwide is of "Least Concern" (LC). However, data gathered in 2018 shows the coconut crab (Birgus latro) is "Vulnerable" (VU) with a "Decreasing" population trend.

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