Habits: Although the robber crab hatches in the water, it spends most of its life on land, where it lives in rocky crevices or shallow burrows. The robber crab climbs trees to escape predators or to find shade when it is very hot. The crab has no special adaptations that enable it to climb. It simply grasps the trunk with the sharp, pointed claws located on each leg and hauls itself up. The crab climbs backward down trees, and it can walk backward on land as well. It was once believed that the robber crab fed on coconuts by cutting them down with its pincers and then breaking the shell. But it is now known that the crab cannot break open coconuts. Instead, it feeds on those that are on the ground and already split apart.

Breeding: Robber crabs mate on land. The female carries the eggs under her abdomen, waiting until low tide to release them in shallow water so she won’t be swept away by the tide. When the eggs hatch, the larvae barely resemble crabs. They spend much of the time floating among plankton. Most of the larvae are eaten by aquatic animals that feed on plankton, so only a tiny proportion survives. 

The larvae eventually shed their skin, or molt, and begin to resemble crabs. They crawl out of the surf and onto the beach, where they remain for the rest of their lives. At this stage their bodies are still soft, so they crawl into tiny, empty mollusk shells for protection. After molting several times, they become fully mature and abandon their shells.

Food and Feeding: Robber crabs feed on carrion (dead animal mater), as well as various types of fruit. They locate food primarily by smell, sometimes from several yards away. They even consume the remains of other robber crabs. Sometimes they chase other crabs to their burrows, where they pull off and eat the claws that the crabs use to defend themselves. The meat of the coconut fruit is one of the robber crab’s favorite foods. When the fruit is plentiful, the crab eats enough to grow to its maximum size. The robber crab often carries its food great distances to a hiding place before eating it.

Special Adaptations: The robber crab is uniquely adapted to life on land. It withstands extreme drought by absorbing water from the moist earth and by sealing its burrow to keep it damp. The robber crab breathes through gills, like other crabs. But its gills are modified for use on land. They are surrounded by spongy tissue that contains many blood vessels, similar to lungs. The crab keeps its gills moist by dipping its brushlike hind legs in water and stroking them over the bronchial tufts within the gill cavity. The robber crab drinks by picking up water droplets with its claws and transferring them to its mouth.

Key Facts:
Sizes:
Width of body: 18 in. Including tail, 30 in.
Weight: Up to 6 lb.

Breeding:
Mating: On land, year round
Eggs: Several hundred laid at a time
Hatching time: Several weeks
From hatching to leaving the sea: Several months

Lifestyle:
Habit: Terrestrial. Mainly nocturnal on inhabited islands
Diet: Fruit and carrion (dead animals)
Lifespan: Unknown, probably long lived

Related Species: The robber crab’s closest relative is the hermit crab, which it resembles when young because it lives in other animal’s shells.

Distribution: Found mainly on islands in the Indian and Pacific oceans. Range extends east from Zanzibar to the Gambier Islands.

Conservation: Numbers are constantly decreasing on heavily inhabited islands because of loss of habitat. On more remote islands, pollution is the greatest threat to the crab’s survival.

Features of the Robber Crab: As it matures, the robber crab’s abdomen becomes hard and well protected. The crab has a thick bony covering, called an exoskeleton, which helps conserve precious water.

Did You Know: 
The robber crab has become so adapted to living on land that it will drown if it is submerged in water for more than a few minutes.
The robber crab grows much larger than the hermit crab, possibly because it does not need to find a shell to accommodate its growing body as it matures.
The world’s largest crab has a leg span of over six feet. It lives in the deep ocean and never comes to the surface.
Robber crabs steal cooking utensils and cutlery from campsites. One was even seen dragging a camp stove into the bush, while another was found pulling a whiskey bottle behind it.

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