With their hard exoskeletons, crablike appearance, and tendency to occupy abandoned seashells, hermit crabs are some of the most unusual creatures found in tidepools and aquatic habitats around the world. But what exactly are these eccentric animals?
When examining hermit crab taxonomy, their classification may come as a surprise to some.
If you’re short on time, here’s a quick answer: Hermit crabs are not reptiles. They are crustaceans more closely related to shrimp, lobsters and crabs than reptiles like snakes, turtles or alligators.
In this nearly 3,000 word guide, we will analyze the biology and taxonomy of hermit crabs to definitively determine whether they are reptiles. We will examine the characteristics that qualify an organism as a reptile versus a crustacean.
An overview of unique hermit crab adaptations and behaviors will illustrate precisely why hermit crabs are fascinating creatures in their own right, related to but quite distinct from reptiles.
Defining Reptiles
Cold Blooded Vertebrates
One of the defining characteristics of reptiles is that they are cold blooded, meaning they lack the ability to regulate their own internal body temperature. Instead, their body temperature is dependent on external sources like the ambient temperature in their environment.
This is different from mammals and birds which are warm blooded and can maintain a constant internal body temperature.
Being cold blooded has advantages and disadvantages for reptiles. On the plus side, not having to maintain a high temperature allows reptiles to survive on much less food than warm blooded animals. The downside is that it makes them dependent on the environment for heat and unable to be as active in cold conditions.
Scaled Skin
Another key feature of reptiles is their scaly skin, composed of the protein keratin. Scales protect the reptile’s body from cuts, abrasions, heat, and water loss. They also come in a variety of shapes and sizes, from the large plated scales of crocodiles to the small granular scales of snakes.
Reptile scales are produced by the deeper layer of skin called the dermis. New scales are constantly being formed to replace old ones that are shed through a process called molting or ecdysis. The frequency of molting depends on factors like the age and growth rate of the reptile.
Specialized Eggs and Terrestrial Young
With few exceptions, reptiles lay specialized shelled eggs called amniotic eggs. Inside these eggs, the embryos are surrounded by protective membranes and fluid which allow them to develop on land without drying out.
Young reptiles hatch out fully formed and ready to survive in their terrestrial world. They do not go through a larval stage like amphibians. Most reptile parents provide no care for their offspring after laying eggs.
This advanced egg structure and self-sufficient hatchlings enabled the reptile conquest of land and are key reasons they became the dominant terrestrial vertebrates for millions of years. It allowed them to reproduce successfully away from water sources that amphibians were still tied to.
Defining Crustaceans
Invertebrates
Hermit crabs are invertebrates, meaning they do not have a backbone. As arthropods, hermit crabs have an exoskeleton made up of chitin that protects their internal organs. They belong to the subphylum Crustacea, a diverse group of aquatic invertebrates including crabs, lobsters, crayfish, shrimp, krill and barnacles.
There are over 67,000 described species of crustaceans. What unites them all is the lack of an internal skeleton and their hard exoskeleton.
Hard Exoskeletons
The exoskeleton of a hermit crab serves many important functions. First, it provides protection for the crab’s soft abdomen and internal organs. The shell also prevents water loss in an otherwise porous crab.
Additionally, the rigid exoskeleton provides points of attachment for the crab’s muscles, allowing for movement. As the hermit crab grows, it must periodically shed its exoskeleton in a process called molting.
After molting, the soft crab will seek out a larger shell for protection while its new exoskeleton hardens.
Specialized Larval Forms
Like other crustaceans, hermit crabs undergo a complex larval development after hatching from their eggs. The earliest larval stage, known as the zoea, is followed by the megalopa stage. These larvae are specialized for planktonic dispersal and look very different from the adult crab form.
After several molts and transitional stages, the larvae settle down in a snail shell and take on the recognizable shape of a hermit crab. This complex life cycle allows hermit crab populations to disperse to new habitats.
Hermit Crab Characteristics and Behaviors
Physical Attributes
Hermit crabs have a soft abdomen that is unprotected, unlike other crabs that have hard protective exoskeletons over their whole bodies. To protect their vulnerable hind region, hermit crabs utilize empty seashells or other hollow objects as mobile shelters, carrying the shells on their backs as they crawl around.
These small crustaceans have ten legs, with the front pair of legs having claws or pincers for grabbing food and for defense. Their eyes are located on movable stalks, allowing for greater environmental awareness.
Locomotion and Shell Seeking
Hermit crabs move about by crawling with their legs. They are always on the hunt for a larger shell as they grow in size over their lifespan. Hermit crabs will abandon their current shell to inspect and change into a more spacious shell when the opportunity arises.
Shell competition can occur between individuals when a limited number of satisfactory shells are available. The hermits will line up from largest to smallest beside a newly vacated shell, waiting their turn to see if it is a good fit.
Sometimes hermits may fight a shell away from another with its claw.
Social Interactions
These creatures are more social than you may imagine! Groups of hermit crabs often congregate together in the wild. They have been observed forming orderly queues to swap shells and also cooperatively transporting a large shell to a safe location.
In captivity, hermit crabs readily interact with their tankmates and owners. They seem to enjoy climbing on hands and enclosure decorations. Species like the Caribbean hermit crab are especially gregarious.
Reproduction
To reproduce, most hermit crabs migrate down to the ocean to breed and release their larvae into the sea to drift and develop. The larvae look and act much differently than the adults, going through various planktonic phases before emerging from the water to take up life on land.
Some hermit crab species have internal fertilization and the females carry the eggs with them until they hatch into tiny crabs. Females may produce up to thousands of eggs at once following a mating ritual that can last several hours!
Reasons People Mistake Hermit Crabs for Reptiles
Hermit crabs are often mistaken for reptiles due to several key similarities in appearance and behavior. However, hermit crabs are actually crustaceans more closely related to crabs and lobsters than lizards or turtles.
Physical Features
Hermit crabs have several physical traits that resemble those found in reptiles:
- Hard outer shells that protect their soft abdomens, like a turtle’s shell
- Clawed limbs and legs used for walking and climbing, like a lizard
- Variations in color from dull brown to vivid red and orange, much like many snakes and lizards
These features likely cause many people to automatically associate hermit crabs with reptiles rather than crustaceans like crabs or shrimp.
Behaviors
In addition to looks, hermit crabs behave in ways reminiscent of reptiles:
- They prefer warm, tropical environments with high humidity
- They crawl and climb using their small legs and claws
- They hide when scared, retracing their steps backwards
- They shed their hard exoskeletons as they grow, much like snakes shed their skin
These behaviors add to the perception of hermit crabs as reptile-like creatures rather than aquatic crustaceans.
Habitats
Finally, where hermit crabs choose to live resembles the habitats of many reptiles:
- They thrive on land rather than water (though they still require access to moisture)
- They enjoy hiding places like hollow logs, rock crevices, leaf litter, and underground burrows
- In captivity, they do well in warm, humid terrariums with substrates and decor for hiding and climbing
Since these environments are similar to those of pet reptiles like geckos or bearded dragons, it fuels the idea that hermit crabs are also reptiles rather than crustaceans.
Despite Appearances, Hermit Crabs Are Crustaceans
While they share many traits with reptiles, hermit crabs are actually most closely related to crabs, lobsters, shrimp and other aquatic arthropods (Fishkeeping World). As members of the crustacean subphylum, they have a genetic and evolutionary lineage tying them to these aquatic creatures rather than land-dwelling reptiles.
So the next time you come across one of these charismatic creatures, remember – that’s no reptile, it’s a crustacean! 🦀
Conclusion
As we have seen, hermit crabs share very few qualities with reptiles and are definitively crustaceans. While their appearance and behavior may seem reminiscent of some scaled, terrestrial animals, analyzing the biology of these organisms makes their true ancestry clear.
Hopefully this guide has shed light on precisely how hermit crabs develop, grow and survive in their marine environments.
The next time you come across one of these captivating creatures exploring tidepools or beaches, you can confidently share facts about their crustacean classification. Their adaptations make hermit crabs fascinating oddities of the sea in their own right!
