Change in the animal kingdom comes from special adaptations that allow a species to thrive.
Chelipeds are arguably the most obvious body structure adaptation in hermit crabs. Also, the claws are of unequal size and can be used for self-defense, eating, drinking, digging, and climbing.
Other body structure adaptations in hermit crabs are antennae, which give them the sense of scent and touch and the ability to shed and regenerate lost tissue, including lost limbs and eyestalks.
Hermit crabs have also undertaken behavioral adaptations, such as living in large colonies and spending time buried under the sand to avoid extreme weather and predators.
Marine hermit crabs can form symbiotic relationships with sea anemones. These tiny invertebrates attach themselves to an aquatic hermit crab’s shell and deter predatory fish with stinging cells. In return, the hermit crab shares food with the sea anemone.
Perhaps the main behavioral adaptation of the hermit crab is to source an external shell and live within it, thus protecting the soft abdomen from extreme weather and predation.
What Special Adaptations Do Hermit Crabs Have?
Hermit crabs have populated the earth for millions of years and have undergone various evolutions. Hermit crabs remain populous due to physical and behavioral adaptations that aid survival.
Despite their diminutive size and delicate frames, the biology of hermit crabs and the habits and routines this species has developed is why their populations continue to grow and thrive.
Hermit Crab Body Structure Adaptations
The anatomy of a hermit crab contains 3 critical elements of survival, which are as follows:
1/ Chelipeds
The claws are vital to their survival. Found at the front of a hermit crabs body and considered one of the species’ 5 pairs of legs, the mismatched claws of a hermit crab serve several purposes.
Most hermit crabs would rather run and hide from a predator than fight. If necessary, hermit crabs can pinch an enemy with their large claw, even if only to gain enough time to flee.
The claws are also essential to eat. The large claw holds food and cuts it into smaller morsels before the smaller cheliped guides food to tiny mouthparts called the maxillipeds.
As hermit crabs don’t have teeth, the maxillipeds tear food apart to be swallowed and digested.
The chelipeds allow hermit crabs to dig under the substrate and climb. As per Marine Ecology Progress Series, wild hermit crabs sometimes climb trees to avoid predators or release eggs into the ocean.
2/ Antennae
The antennae are another prominent part of the hermit crab anatomy. The eyesight of hermit crabs is average at best, so they rely more on their senses of smell and hearing.
The antennae of a hermit crab are coated with aesthetascs – thin, short hairs that are invisible to the naked eye. When a hermit crab is close to saltwater, these aesthetascs are stimulated.
The hermit crab will flick its antennae and detect scent in the air. This will identify food sources and the presence of threats or conspecifics.
Hermit crabs also use their antennae to aid their limited sense of hearing.
The sensitive aesthetascs pick up on vibrations in the air. According to Animal Behavior, excessive noise can confuse and disorient hermit crabs.
3/ Shedding and Regrowing Limbs
Hermit crabs can shed their limbs, which will then be regrown when the hermit crab next molts its exoskeleton. This adaptation can be crucial to survival in the wild.
If cornered by a predator, a hermit crab may choose to shed one of its limbs. This will momentarily confuse the predator, as it’ll believe the hermit crab has been killed.
The hermit crab will use this pause to hide within its shell or flee using its remaining limbs.
As this adaptation can be used at will by hermit crabs, they also shed limbs in the event of injury or sickness. If a hermit crab has one damaged leg slowing its ability to walk, it may sever the limb as it feels it would move with greater freedom on 9 legs.
Hermit Crab Behavioral Adaptations
In addition to these physical adaptations, hermit crabs have also taken on behavioral adaptations to remain safe and survive in the wild. These are as follows:
4/ Molting
Hermit crabs grow in size as they age, which is achieved by molting an exoskeleton and growing a replacement. When the hermit crab has completed a molt, it’ll increase in mass.
Size is often used as a barometer when aging hermit crabs. Juvenile hermit crabs molt several times in their first year of life, but the process slows to once every 18 months once it reaches maturity.
5/ Using the Shell as a Microhabitat
Unlike true crabs, hermit crabs aren’t born with a shell, so they must identify a shell that becomes their home. It’ll then carry its shell everywhere, retreating inside while sleeping or hiding from threats.
Hermit crabs take their name from the fact that they live in a shell found in the wild. Hermit crabs have soft and weak abdomens, so the shells protect the hermit crab from predators or the sun’s rays.
Mollusk shells are preferred for hermit crabs, especially those washed up on the shore that once housed sea snails. These shells are sturdy enough to protect hermit crabs but not too heavy to maneuver.
Sometimes, hermit crabs need to get creative when seeking shells. According to Nature, if a traditional shell isn’t available, hermit crabs will use bottle caps, plastic washed up from the sea, or fossilized remains.
6/ Living in Colonies
A shell will only provide hermit crabs with so much protection, so they seek safety in numbers.
If you upturn a rock in the wild and find a hermit crab, others will be nearby, as wild hermit crabs live in substantial colonies of around 100.
This group living means hermit crabs can protect each other from predators and have a near-constant supply of shells for trading. Hermit crabs often exchange shells if a conspecific has a superior vessel.
Although hermit crabs can grow territorial, these colonies usually live in peace. Hermit crabs naturally arrange themselves into a social hierarchy, with larger and stronger ones taking on a dominant status.
7/ Burrowing Under Substrate
Hermit crabs prefer to hide than fight. They’re nocturnal animals that can be spotted scurrying on a beachfront, foraging for food after dark. By day, they remain hidden.
Usually, this involves digging under the sands of a beach and remaining concealed throughout the day. This ensures that hermit crabs will avoid the attention of predatory birds, who will be patrolling the sky during daylight hours.
Remaining under the sand protects a hermit crab from the sun’s rays. While they usually flourish in warm conditions, over-exposure to direct sunlight can dry out a hermit crab’s gills and lead to suffocation.
8/ Scavenging for Food
Hermit crabs are omnivorous scavengers that will eat live insects to sustain themselves if forced to do so. They prefer to hunt for food when it’s dark, feasting on what they can find.
Many hermit crabs can sustain themselves without leaving their beachfront habitats. As per American Zoologist, a low tide will frequently bring a range of food to the shore for hermit crabs to eat. This could include algae or the carcasses of fish, shrimp, or conspecifics.
Hermit crabs also travel further to find food. Fruit fallen from a tree is a popular delicacy, as is wild-growing plant matter. The Biological Bulletin confirms that hermit crabs recall foods that make them unwell, avoiding such sustenance in the future.
9/ Symbiotic Relationships
There are over 800 kinds of hermit crabs, most of which live underwater.
While many of the adaptations discussed relate primarily to terrestrial hermit crabs, symbiotic relationships are the sole reserve of aquatic species.
Marine hermit crabs frequently form symbiotic partnerships with sea anemones. These are small invertebrates that resemble plants. Sea anemones attach themselves to the shells of marine hermit crabs, sharing any food found at the bottom of the ocean.
In exchange for this feeding, sea anemones provide hermit crabs with protection. If a predator attempts to attack a hermit crab, the sea anemone will sting it using a cell known as a cnidocyte. This won’t deter a shark or whale, but it’ll fend off small fish.
The adaptations of a hermit crab have seemingly peaked, as they’re no longer believed to be evolving. Hermit crabs have reached a physical and behavioral state that enables them to flourish.
