The Coconut Crab – A Giant Land Crustacean

Two coconut crabs on some palm trunks

Coconut crabs, Birgus latro, are among the most amazing crustaceans in the world. They are not actually true crabs but terrestrial hermit crabs. When they are young and still the size of your thumbnail they live in snail shells (and occasionally broken coconut shells); however, as they age their carapace hardens with chitin and chalk and they begin to not really need protective covering. They go through periodic molts that leave them vulnerable for up to a month, and they curl their tails under their bodies as true crabs do.

Special attributes:

• Giant size – They are the largest of land arthropods. They can grow up to 16 in. in length,weigh up to 9 lbs., and have a leg span of up to 3 ft. Their size probably contributes to their slow aging process. Coconut crabs take 4-8 years to become sexually mature, which is very long for a crustacean. They can also live to be over 30 years old and never stop growing through their life. Their claws can lift objects weighing up to 64 lbs., so it’s no wonder they use them to crack open and eat coconuts!
  

  A picture of two coconut crabs opening a coconut, in case you didn't believe me!

• Branchiostegal lungs – Coconut crabs have branchiostegal lungs, which are a unique adaptation to land and are considered to be a developmental stage between gills and lungs. The crabs use their tiny fifth pair of legs to keep the lungs, which still require water to function, clean and moist. Though coconut crabs have some small gills, they will drown if submerged in water for a few minutes.
  

  The tiny, slender fifth pair of legs, which clean the crab's lungs, are visible in this diagram

• Sense of Smell – in another unique land adaptation, coconut crabs have evolved very different sensory organs than their aquatic ancestors. Normal crabs have special organs on their antennae that pick up both the concentration and the direction of chemicals in the water. Coconut crabs, however, have evolved very sensitive sensory organs that can detect smells in the air, and they flick their antennae to pick up scents much as insects do.
• Other Stuff – just a few miscellaneous facts about coconut crabs
  – They are also known as “robber crabs” and “palm thieves” because of their tendency to steal food from each other as well as because they are known to steal shiny objects from people.
  – Coconut crabs are often considered to be delicacies in the places where they are found, and their flesh is thought to be an aphrodisiac.
  – Coconut crabs are widely distributed throughout the islands of the Pacific and Indian oceans. Though adult crabs suffocate in water, they lay their eggs in the ocean and their larvae float on the water for up to month, allowing them to spread to far away islands.
  

  The range of the coconut crab

  – Probably due to their wide distribution coconut crabs range in color from red to blue to brown.

Here’s a short clip of a coconut crab walking around:

Coconut crabs moving through a forest

A captured coconut crab probably being taken to the market

A female coconut crab carrying her eggs

A coconut crab apparently found on a garbage can on Christmas Island

More information: Wikipedia actually has a very decent page on coconut crabs that satisfied most of my curiosity.

Coconut Crab [Wikipedia]

Coconut Crab (Birgus latro) [ARKive]

Image Sources:

Wikimedia – Coconut Crab (Image: Mila Zinkova)

ARKive – Coconut Crab (Birgus latro) (Image: Green Cape Pty Ltd.)

Wikimedia – Coconut Crab (Image: Charles Orbigny, 1849)

Wikimedia – Coconut Crab (Image: Chris 73)

ARKive – Coconut Crab (Birgus latro) (Image: Jean-Paul Ferrero)

WorldTeach Marshall Islands – Photos (Image: source unknown)

ARKive – Coconut Crab (Birgus latro) (Image: Jean-Paul Ferrero)

Rifftrax – It’s time to admit (Image: source unknown)

Tags: branchiostegal lungs, coconut crab, giant crab, huge crustacean