Hermit Crab

The hermit crabs are anomuran decapod crustaceans that belong to the superfamily Paguroidea. They lack shells but have adapted themselves to “borrow” the empty scavenged mollusc shells in order to protect their fragile exoskeletons. Hermit crabs without shells are extremely vulnerable to predators and their bodies lack a hard protected carapace. There are more than 800 species of hermit crabs and almost all of them are ocean dwellers. They occur in sandy- or muddy-bottomed marine waters and sometimes on land and trees too.

The hermit crab possesses an asymmetric abdomen that is concealed by a snug-fitting shell. They have five pairs of legs and two pairs of antennae. The first pair of legs of the hermit crab is modified to form chelae or pincers which are shaped to cover the shell entrance when the crab is inside. It then walks on its second and third pairs of legs and uses its fourth and fifth pairs to grip the central column of the snail shell. A hermit crab photo is given below:

[Image will be uploaded soon]

Hermit Crab: Scientific Classification

Hermit Crab: Appearance

As the title of the article suggests, the hermit crabs lack shells and they “borrow” it from others (snail, periwinkle or oyster drill). Most of the species of the hermit crabs have spirally curved and long abdomens which are soft and fit inside their borrowed shell. These shells protect their vulnerable abdomen from predators. Most often, they use the shells of sea snails apart from the shells of bivalves and scaphopods. Some species even use hollow pieces of stones and wood. The tips of the crab’s abdomen are adapted to clasp onto the columella strongly. 

The size of the hermit crab without shell ranges from 0.5 inches to 4 inches. A few of them can grow up to 11 inches. They are found in many colours which include green, red, blue, yellow, orange, brown, pink and white. When the hermit crab grows, they moult - building up water inside their bodies to split the old shells. Some of the species bury themselves in the sand for moulting after leaving their shell while some remain in their shell to emerge right before moulting. The process of moulting the hermit crabs takes 45 to 120 days. The newly moulted crabs(hermit crab without shell) are blue in colour. To fit themselves into the shell, the hermit crabs press their abdomen, uropods and their fourth and fifth pair of legs against the shell’s inner wall. 

There are two groups of hermit crabs: Land and Marine Hermit Crabs. Both of these hermit crabs have gills with highly vascular areas which helps in oxygen exchange. The land crabs store water in their bodies to keep their gills moist. They have stalked eyes with acute vision and they have two pairs of antennae in their head. They use longer pairs of antennae for feeling and the shorter pair of antennae for smelling and tasting. Their antennae also act as vibration sensors. The first pair of legs of the hermit crabs are pincers - they have one side larger than the other. They walk on their second and third pair of legs. A hermit crab diagram is shown below:

[Image will be uploaded soon]

Hermit Crab: Habitat

As mentioned earlier, there are two groups of hermit crabs: The Marine hermit crabs and the land hermit crabs. It includes a single species of freshwater marine hermit crab - Clibanarius fonticola. They are found on the island of Espiritu Santo, Vanuatu. The marine hermit crabs spend their life as aquatic animals living underwater in the depths of saltwater that ranges from shallow reefs and shorelines to deep-sea bottoms. In the marine aquarium trade, several species of hermit crabs are common as pets. They are kept commonly in reef fish tanks. They breathe through their gills and can survive outside water as long as their gills are damp. A few species of the hermit crab immobile structures are left by polychaete worms, vermetid gastropods, corals, and sponges. 

The land hermit crabs spend their life as terrestrial species on land and are found in tropical areas. Though they live on land, they still need access to freshwater and saltwater to keep their gills moist. The land hermit crabs belong to the Coenobitidae family and there are around 15 terrestrial species of genus Coenobita in the world. Out of these 15, the Caribbean hermit crab (also known as west Atlantic crab or tree crab or soldier crab or purple pincher), the Australian land hermit crab and the Ecuadorian hermit crab (also known as the Pacific hermit crab) are very common as pets. There are other species also which are less common but have grown in popularity and availability in recent times. These are Coenobita brevimanus (found in African east coast and south-west the Pacific Ocean), Coenobita rugosus (found in Indonesia, Australia, African east coast and south-west pacific), Coenobita perlatus (also known as strawberry hermit crab because of their reddish-orange colour, they are found in the indo - pacific region) and Coenobita cavipes (they are native to the eastern parts of Africa, the Indonesia, Philippines, China, Japan, Malaysia, Taiwan, Polynesia, and Micronesia). 

[Image will be uploaded soon]

Hermit Crab: Shells and Competition For Shell

We know that the hermit crab shells are borrowed from others. As they grow, their need for shells also changes - they need a bigger shell. The gastropod shells which are intact and suitable for the crabs are difficult to find and hence, there is a vigorous competition amongst the crabs for the shell. The availability of the shells depends completely on the availability of the gastropods and hermit crabs, matched for size, in a particular area. Another major issue of the availability of intact shells is the number of organisms that prey upon gastropods and leave the shells intact. The crabs may fight among themselves to captivate the shells. But, the competition may decrease if the hermit crabs acquiring the shells vary in size. The 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. 

As they grow in size, they need to abandon the current shell and search for a new bigger shell. In this situation, both the groups of hermit crabs - the land and the marine crabs form a vacancy chain to exchange the shells. Now you may wonder what is a vacancy chain and how it is formed? A vacancy chain is a structure through which the resources are distributed among the consumers or users. When a hermit crab finds a new shell, it leaves its own shell and tries the new shell to check whether the shell meets its requirement or not. If the shell is not appropriate, it goes back to its previous shell and waits for others. The other crabs come and the process continues to form a group of around 20 crabs forming a line from the largest to the smallest hermit crab. When the crab that fits in the shell arrives finally, they swiftly exchange shells in sequence, each one moving up to the next size. 

If in a locality, enough shells are not available for the hermit crabs, they start to find other alternatives. Sometimes, they try to take shelter in the debris such as empty tin cans or plastic containers. This practice is fatal for the crabs as this debris may lead to their death. If a crab dies because of the debris, the other crabs take this as a signal that a shell is available and eventually end up dying like the previous one. The below hermit crab image shows the vacancy chain.

[Image will be uploaded soon]

Hermit Crab: Diet

The hermit crabs are omnivorous scavengers. They eat a variety of detritus that other creatures won’t eat. They are very active in their quest for food and usually move around at night. The hermit crabs play an important role in the benthic, or bottom-dwelling marine community by cleaning the environment. The favourite food of the crabs includes small fish and invertebrates such as worms, along with plankton and other similar particles in the water. They even consume dead hermit crabs if required. 

Hermit Crab: Reproduction

The hermit crabs need seawater to mate. This is the reason why the land crabs head to shallow waters to mate. Both the male and female crabs need to emerge partially from their borrowed shells to reproduce. The male crab holds the female crab with one claw, pulling her back and forth while stroking her to fertilize her. The female crabs have abdominal appendages that allow them to carry the eggs until they are ready to hatch. The incubation period is almost a month. To release the eggs, the female hermit crab must be in the water. The released eggs turn into swimming larvae called zoea. It lives like plankton for a time until finally descending to the seafloor. The zoea grows and moults several times to become megalops. They then grow and moult into juveniles, eventually reaching the stage where they must find their shells. The young hermit crabs moult almost every month while the older crabs may moult at a gap of 18 months. When the crabs become adults, the terrestrial species return to the land. 

Most of the hermit crabs have a lifespan of around 1 to 10 years. However, some can live for around 30 years. The land species Coenobita brevimanus can live for around 70 years. Image of Coenobita brevimanus:

[Image will be uploaded soon]

Hermit Crab: Population

The hermit crabs are found all over the world in tropical and subtropical waters and in many temperate zones of the northern hemisphere. Many of their habitats are about to go extinct though they are not considered endangered. In a few places, the hermit crabs are trying to find a shell in the debris which threatens their survival. When they die, they release a pheromone signal to other crabs that there may be a shell available, which lures even more crabs into a death trap. Hermit crab image in debris:

[Image will be uploaded soon]

Hermit Crabs: Fossil Record

The fossil record of in situ hermit crabs without shells using gastropod shells goes back to the  Late Cretaceous. As shown by the specimen of Palaeopagurus vandenengeli from the Speeton Clay, Yorkshire, the UK from the Lower Cretaceous, and Upper Jurassic of Russia, the hermit crabs used ammonites’ shells before that time. The oldest record of the superfamily goes back to the earliest part of the Jurassic. The oldest species is the Schobertella hoelderi, from the late Hettangian of Germany. 

Hermit Crab: Fun Facts

• More than 1,100 species of hermit crabs are found worldwide.

• They come in a range of sizes - from a fraction of an inch to nearly the size of a coconut.

• Some species of the crab eat their soft moulted shells as the hermit crab shells contain vitamins, minerals and calcium. 

• Before selecting a new shell, the crabs inspect the new shell physically and visually before deciding to move in. 

• The competition for shells between the hermit crabs can be fierce which often results in a fight between two crabs to secure a prime shell. 

• It is almost impossible to pull a hermit crab out of its shell. They wrap their soft abdomen around the inside of the shell, firmly holding themselves in place.

• They are always searching for a new shell to borrow and they sometimes steal a shell from a smaller hermit crab.