Crab

Crabs are nature's experiments in housing economics—organisms that evolved the same body plan at least five separate times through convergent evolution, a phenomenon biologists call 'carcinisation.' Something about the crab form is so advantageous that evolution keeps rediscovering it: a wide, flattened body, eyes on stalks, sideways walking, and claws for manipulation. The crab shape represents a local optimum that multiple lineages independently climb toward.

The Molting Economy

True crabs face a fundamental growth constraint: their exoskeletons cannot expand. Growth requires molting—shedding the entire outer shell to expand before the new shell hardens. This creates predictable vulnerability windows. A soft-shell crab is protein-rich and defenseless, commanding premium prices in human markets (up to ten times hard-shell crab prices) precisely because the molting phase is brief and dangerous.

The molting cycle imposes strategic trade-offs. Energy invested in rapid shell hardening comes at the cost of growth. Energy invested in growth extends the vulnerable soft-shell period. Crabs that lose limbs face an additional trade-off: autotomy (self-amputation) saves the individual from predators but regenerating the limb requires resources that could otherwise fuel growth or reproduction. Research shows crabs preferentially allocate energy to limb regeneration when the lost appendage affects feeding, but delay regeneration when it's a rear walking leg.

The decision to regenerate now versus grow now mirrors capital allocation choices: repair degraded assets versus invest in expansion.

The Vacancy Chain: Hermit Crab Housing Markets

Hermit crabs (not true crabs but close relatives) cannot produce their own shells and must scavenge gastropod shells. This creates genuine housing markets with vacancy chain dynamics. When a large shell becomes available, hermit crabs gather and queue by size. The largest crab takes the new shell, vacating its old shell for the next-largest crab, cascading down until multiple crabs upgrade housing from a single vacancy.

The vacancy chain produces something unusual in animal competition: positive-sum outcomes. A single new resource enables 2-3 individuals to improve their situations on average. The late Frank Kristof, studying New York City housing, discovered humans exhibit identical dynamics—each new luxury apartment triggers approximately 2.4 families to upgrade housing as vacancies cascade through the market.

Specialized Strategies Across the Crab Radiation

The crab body plan enables diverse strategic specializations:

• Decorator crabs attach algae, sponges, and hydroids to their shells for camouflage—wearable disguise that changes with each molt
• Boxer crabs carry stinging anemones in their claws as weapons, a symbiosis so critical that crabs will fight to steal anemones from each other
• Fiddler crabs use massively asymmetric claws for signaling—the enlarged claw serves display functions but cannot feed, forcing the small claw to do all foraging work

Each specialization represents a different resolution of the growth-protection-reproduction trade-off that molting imposes. The crab radiation demonstrates that a single body plan can support radically different life strategies through differential investment.