Archive for the ‘Crustacean’ Category

The Coconut Crab – A Giant Land Crustacean

December 21, 2008
Two coconut crabs on some palm trunks

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 didnt believe me!

    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.

    rtsr

    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

    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

Coconut crabs moving through a forest

A captured coconut crab probably being taken to the market

A captured coconut crab probably being taken to the market

A female coconut crab carrying her eggs

A female coconut crab carrying her eggs

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

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)

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The Mantis Shrimp – Amazing Eyes, Violent Claws

December 8, 2008

A brilliantly colored tropical mantis shrimp

Mantis shrimp are among my favorite animals, and for good reason. Otherwise known as stomatopods, they diverged from other crustaceans over 400 million years ago during the Cambrian period, so they are only distantly related to the likes of shrimp and lobsters. Many are very brightly colored, like the one above. Some are also fluorescent.

Special attributes:

  • Eyesight – Mantis shrimp have the most complex eyes of all animals. Each eye is on a stalk and can move independently of the other. They are each separated into three bands (trinocular vision), allowing each eye to see objects from three different perspectives, giving both of them highly-developed depth perception. They can detect wavelengths from the infra-red to the ultraviolet; in all they can see ten times more colors than humans. Moreover, they are the only animals that can detect circular polarized light (CPL). Before it was discovered that mantis shrimp could see CPL, there were only three known modes of sight – black and white, color, and linearly polarized vision. So mantis shrimp occupy a quarter of the known ways of perceiving light alone.
    Mantis shrimp eyes have special filters that convert CPL into linear polarized light. To humans, linear polarized light appears as glare; but for mantis shrimp, the polarized light is used in mating displays. So the ability to detect CPL probably arose from sexual selection. One theory speculates that the CPL displays allow mantis shrimp to communicate and perform mating rituals without attracting the attention of predators, which can’t see the CPL they reflect.
    For more information on circular polarized light, see this Wired article or this Science Daily article.

    This picture vividly displays the three bands of vision in the mantis shrimp eye

  • Claws – Mantis shrimp are renowned for being amazingly violent (they are sometimes called “thumb splitters”). They have been known to break glass aquarium walls with their strikes, which are among the fastest in the animal kingdom and have the power of a .22 caliber bullet. Their claws look very much like preying mantis claws (hence the name “mantis” shrimp). According to Wikipedia, mantis shrimp can be categorized by the type of claw they have – spearers and smashers. Spearers, like the one below, have toothed claws that they use to impale prey. Clubbers have knobby claws that they use to bash prey.
    The fearsome claws of a large mantis shrimp

    The fearsome claws of a large mantis shrimp

    This mantis shrimp has clubs

    This mantis shrimp, Odontodactylus scyllarus, has clubs.

Stomatopod anatomy

Stomatopod anatomy - mantis shrimp have a typical decapod body plan, like that of crabs, lobsters, and shrimp.

A female mantis shrimp guarding her eggs

A female mantis shrimp guarding her eggs

This mantis shrimp, Lysiosquillina glabriuscula, was photographed in blue light with a yellow filter to show only the fluorescence. Scientists believe that mantis shrimp

This mantis shrimp, Lysiosquillina glabriuscula, was photographed in blue light with a yellow filter to show only the fluorescence. Scientists believe that mantis shrimp use the fluorescence to communicate with each other.

More Information:

External Anatomy and Explanatory Notes [Roy’s List of Stomatopods for the Aquarium, UC Berkeley]

The Magnificent, Ultra-Violent, Far-Seeing Shrimp from Mars [Wired article]

Shrimp Eyes May Hold Key to Better Communications [Wired article]

Fluorescing Mantis Shrimp Catches Fish [from Night Sea article]

Image Sources:

Wired Blog – The Magnificent, Ultra-Violent, Far-Seeing Shrimp from Mars (by Brandon Keim, Image: Justin Marshall)

Wired Blog – Shrimp Eyes May Hold Key to Better Communications (by Brandon Keim, Image: Roy Campbell)

Trek Earth – Mantis Shrimp Photo (Image: Rabani HMA, 2006)

Museum Victoria Australia – Mantis Shrimps (Image: Museum Victoria Australia)

FishForums – Stomatopods (Image: Roy Caldwell)

Flickr – Peacock Mantis Shrimp (Image: NG Richard, 2006)

UC Berkeley – Mantis Shrimp Fluoresce to Enhance Signaling in the Dim Ocean Depths (by Robert Sanders, Image: Roy Caldwell)