King crabs belong to the infraorder Anomura, a group sometimes called “false crabs,” rather than to Brachyura, the group that contains all true crabs. Despite looking almost indistinguishable from a true crab on a dinner plate, king crabs are more closely related to hermit crabs than to any Dungeness or blue crab you have ever seen. The story of how they ended up looking so crab-like anyway is one of the more fascinating examples of convergent evolution in the animal kingdom.
What Makes a “True” Crab True
True crabs are members of the infraorder Brachyura, a massive group of decapod crustaceans. They share a set of defining features: a broad, flattened carapace, a short abdomen that is folded tightly underneath the body and essentially invisible from above, and five pairs of walking legs, the first of which are modified into claws. That gives a true crab four pairs of visible walking legs plus its claws. The abdomen, while hidden, is roughly symmetrical.
King crabs look a lot like that description at first glance. They have a broad body, a tucked abdomen, and big claws. But count the walking legs, and you will notice something off: king crabs appear to have only three pairs of walking legs rather than four. The fifth pair is reduced and tucked under the carapace, often used for cleaning the gills. That is a telltale anomuran trait. Their abdomen, though folded under the body, is also asymmetrical and somewhat soft compared to the heavily calcified plate of a true crab. These differences might seem minor, but they reflect a deep evolutionary split. Brachyura and Anomura together form a larger group called Meiura, but within that group they sit on separate branches that have been evolving independently for a very long time.
Descended from Hermit Crabs
The evolutionary origin of king crabs has been debated for more than a century. The leading hypothesis, now strongly supported by molecular evidence, is that king crabs evolved directly from hermit crab ancestors. A landmark molecular study published in Nature presented evidence that king crabs are not merely descended from hermit crabs but are actually nested within the hermit crab genus Pagurus, meaning they are essentially a highly modified lineage of hermit crabs that abandoned the shell-dwelling lifestyle.1Nature. Evolution of King Crabs from Hermit Crab ancestors
Subsequent phylogenetic work using five nuclear gene sequences confirmed that hermit crabs have a single evolutionary origin and, more surprisingly, that nearly all other major body forms within Anomura are derived from within the hermit crabs.2PubMed. Hermit to king, or hermit to all: multiple transitions to crab-like forms from hermit crab ancestors King crabs specifically descend from a lineage of asymmetrical hermit crabs that had adapted to living in right-handed (dextral) snail shells. At some point, their ancestors gave up the shell entirely, and their bodies broadened, flattened, and calcified into the armored, crab-shaped form we see today. While the phylogenetic position of the king crab family Lithodidae within the hermit crab family Paguridae has been strengthened in recent years, identifying which specific living hermit crabs are the closest relatives of king crabs remains an open question.3Oxford Academic. The origin of king crabs: hermit crab ancestry under the magnifying glass
If you have ever held a hermit crab out of its shell, you may have noticed its soft, curved, asymmetrical abdomen. King crabs retain traces of that asymmetry. Their abdomen is not the neat, symmetrical plate you find on a blue crab or a Jonah crab. It is slightly lopsided, a ghostly reminder of the coiled body plan their ancestors needed to fit inside a snail shell.
Carcinization, or Why Everything Keeps Evolving into a Crab
The process by which non-crab crustaceans evolve a crab-like body shape has a name: carcinization. The term was coined by the English zoologist Lancelot Alexander Borradaile in 1916, and it describes one of biology’s most striking cases of convergent evolution.4Biological Journal of the Linnean Society. One hundred years of carcinization – the evolution of the crab-like habitus in Anomura (Arthropoda: Crustacea) At least five separate lineages of decapod crustaceans have independently evolved the wide, flattened body with a tucked abdomen that we recognize as “crab-shaped.”5PubMed. How to become a crab: Phenotypic constraints on a recurring body plan
King crabs are just one example. Porcelain crabs, which look like small, delicate true crabs, are another anomuran group that independently evolved a crab-like body. They did so on a completely separate branch from king crabs.6PubMed. Evolutionary morphology of the organ systems in squat lobsters and porcelain crabs (crustacea: Decapoda: Anomala): an insight into carcinization The family Lomisidae, a small and obscure group sometimes called hairy stone crabs, represents yet another independent instance. Within Anomura alone, the crab-like body plan evolved at least three times independently, each time from ancestors that looked nothing like crabs.4Biological Journal of the Linnean Society. One hundred years of carcinization – the evolution of the crab-like habitus in Anomura (Arthropoda: Crustacea) And of course, true crabs (Brachyura) evolved that same shape on their own branch.
Why does evolution keep arriving at the same destination? The honest answer is that nobody is entirely certain. One hypothesis centers on phenotypic integration, the idea that the wide, flattened carapace shape and the tucked abdomen evolve together as a correlated package because of underlying developmental or biomechanical constraints.7Wiley Online Library. How to become a crab: Phenotypic constraints on a recurring body plan A broad, flat body with a hidden abdomen is strong, compact, and allows the animal to wedge itself into rocky crevices for protection. It may also improve stability on the seafloor and provide a more efficient platform for the powerful claws used in feeding and defense. But these are educated guesses about selective pressure. The remarkable repeatability of carcinization suggests that something about crustacean development makes the crab shape an unusually accessible evolutionary outcome, almost as if the body plan is a path of least resistance that natural selection keeps stumbling onto.
How to Spot a False Crab
For most people, the easiest way to tell a king crab from a true crab is the leg count. Pick up a king crab and count: you will find three obvious pairs of walking legs plus one pair of large claws, for a total of eight visible limbs. A true crab has four pairs of walking legs plus claws, for ten visible limbs. That missing pair on the king crab is not truly missing; it is just small and hidden, used internally rather than for locomotion.
Other differences are subtler. A king crab’s exoskeleton is covered in heavy spines and feels rougher and more armored than most true crabs. The carapace shape, while broadly similar, tends to be more rounded and less perfectly flattened. And the asymmetrical abdomen, if you flip the animal over, is a dead giveaway. True crabs have a neatly symmetrical abdominal flap. King crabs do not.
Porcelain crabs are even harder to distinguish from true crabs by appearance alone, but the same leg-count trick works. They also have only three visible pairs of walking legs. If you are at the shoreline and find a small, flat, crab-looking animal with what appears to be only six walking legs plus claws, you are almost certainly looking at a porcelain crab rather than a true one.
Larval Clues to the Relationship
One of the more intriguing lines of evidence connecting king crabs to their anomuran relatives comes from their larval stages. Decapod crustaceans go through a specific transitional larval phase called the megalopa, which occurs between the free-swimming zoea larva and the settled juvenile form. Research comparing morphological diversity across these developmental stages in true and false crabs has revealed a curious pattern: megalopae of both groups show the smallest morphological diversity of any life stage, even though the adult forms are wildly different.8PubMed Central. Morphological diversity in true and false crabs reveals the plesiomorphy of the megalopa phase
In other words, while adult false crabs display enormous morphological variety (hermit crabs, squat lobsters, king crabs, and porcelain crabs all look radically different from one another), their megalopa larvae are surprisingly similar to each other and to those of true crabs. Adult false crabs actually showed the largest morphological diversity of any group studied, but at the megalopa stage, that diversity collapses. This suggests the megalopa body plan is ancient and highly conserved, a shared ancestral template that both true and false crabs pass through before diverging into their very different adult forms. The evolutionary success of both groups may be connected as much to this shared larval phase as to the adult crab shape itself.
King Crabs as Invasive Species
Whether or not king crabs are “real” crabs, their ecological impact is very real. The red king crab (Paralithodes camtschaticus) was deliberately introduced into the Barents Sea by Soviet scientists in the 1960s to establish a new fishery. The crabs thrived and spread along the northern Norwegian coast, and their effects on native ecosystems have been extensively documented. In areas heavily populated by king crabs, large mussels and echinoderms (sea stars and sea urchins) have disappeared, and the soft-bottom fauna is now dominated by smaller individuals of prey species.9ICES Journal of Marine Science. Establishment and ecosystem effects of the alien invasive red king crab (Paralithodes camtschaticus) in the Barents Sea–a review King crabs are voracious generalist predators, eating molluscs, polychaete worms, and other crustaceans. The energy return from these prey is high relative to the effort needed to capture them, which makes king crabs efficient at stripping an area of its dominant benthic fauna.
Tracking studies in Norwegian fjords have shown that red king crabs actively manage their thermal environment. As water temperatures rise through the summer months, the crabs move progressively deeper and further into fjords, effectively contracting their range to stay in cooler water. They tend to prefer areas with the second coldest water masses rather than the absolute coldest, suggesting a balance between thermal preference and other habitat factors like food availability.10Frontiers in Marine Science. Space and Habitat Utilization of the Red King Crab (Paralithodes camtschaticus) in a Newly Invaded Fjord in Northern Norway Rapid temperature changes triggered by water mass exchange events prompt quick movements, with crabs relocating to avoid warming waters within days.
Fisheries and Why Taxonomy Matters at the Dock
King crab fisheries are some of the most economically valuable in the world, and the biological quirks of these animals create real management challenges. One example involves golden king crabs (Lithodes aequispinus) in Southeast Alaska. Fisheries managers set a single legal harvest size across broad regions, but research has found that male crabs in different management areas reach sexual maturity at different sizes. In at least two of seven management areas studied (Icy Strait and Lynn Canal), the current legal harvest size may not allow crabs to reproduce even once before being caught.11PubMed Central. Spatial variability in size at maturity of golden king crab (Lithodes aequispinus) and implications for fisheries management
The ideal solution would be area-specific legal sizes tailored to local maturity patterns, but this is impractical because fishing vessels harvest from multiple management areas on the same trip, making enforcement nearly impossible. This spatial variability in growth and maturity is partly a consequence of the king crab’s evolutionary history. As anomurans with a relatively recent transition to a free-living, fully calcified body plan, king crabs may not have the same uniformity across populations that longer-established true crab lineages exhibit. Their biology still carries the stamp of a lineage in evolutionary transition.
True Crabs Have Their Own Convergence Story
While king crabs are famous for converging on the crab body plan from the outside, true crabs have their own parallel story of repeated convergence, particularly in their relationship with land. Research estimating how many times true crabs independently left marine environments has found that the transition happened at least seven and possibly up to seventeen times, with at least two instances of crabs returning to the sea from non-marine habitats.12PubMed Central. Convergent Adaptation of True Crabs (Decapoda: Brachyura) to a Gradient of Terrestrial Environments Moving from intertidal zones onto fully terrestrial ground is much harder than becoming semi-terrestrial: it takes roughly eight to thirty-six times more evolutionary “effort” to reach true land-dwelling grades compared to simply adapting to intertidal life.
This pattern echoes the carcinization story in an interesting way. Just as the crab body shape keeps evolving independently in different lineages, the transition from sea to land keeps happening independently in different true crab families. Evolutionary biologists see both patterns as evidence that certain ecological niches exert powerful selective pressures that pull diverse lineages toward similar solutions. The crab shape is an answer to a common set of marine problems; terrestriality is an answer to an entirely different set of opportunities on land. In both cases, the “answer” is arrived at again and again by separate groups working from different starting points.
Other Animals That Are Not What They Seem
King crabs are far from the only case of an animal named for a group it does not belong to. Horseshoe crabs are not crabs at all; they are more closely related to spiders and scorpions. Coconut crabs, despite their name, are actually the world’s largest terrestrial arthropod and are anomurans like king crabs, specifically a type of highly modified hermit crab. Electric eels are not eels but knifefish. Koala bears are marsupials, not bears. Common names in biology are notoriously misleading because they are based on appearance and folklore rather than evolutionary relationships.
What makes king crabs particularly interesting in this context is that their resemblance to true crabs is not superficial or accidental. It is a deep, whole-body convergence driven by natural selection over millions of years. A horseshoe crab does not really look much like a crab once you examine it closely. A king crab, on the other hand, looks extraordinarily crab-like because evolution has sculpted it into almost the same shape through the same functional pressures. The mimicry is so thorough that it fooled taxonomists for centuries and still fools anyone who has not been told to count the legs. That makes king crabs not just a taxonomic curiosity but a vivid case study in how evolution can produce near-identical outcomes from fundamentally different starting materials.