17 April 2010

Box of Surprise

What is this odd little lump on the sand?
It's a crab! One I've not seen before!

Ivan found this cute little fellow (about 4cm across) on the sandy shore of the lagoon. At first he thought it was a snail as it certainly doesn't look like a typical crab. The photo above is what it looks like from on top. The photo below is what it looks like from the side.
This is what a box crab (Family Calappidae) looks like. Box crabs hold their large flat pincers tightly in front of the 'face' so the entire crab looks squarish. They are also sometimes called the 'shame face' crabs because they appear to cover their faces in shame.

It is only when you look at the underside that its crabbiness is revealed. Like other crabs, it has 4 pairs of walking legs. In the box crab, these are tucked up under its large shell.
When I put the crab on soft sand in a pool of water, it proceeds to rapidly bury itself by digging in backwards.
Here's a closer look at the adorable little eyes on very short stalks. The eyes have a star-shaped pattern. Its back has lots of bumps, and it blends perfectly with the surrounding sand.
The pincers of box crabs are specialised for cracking open snail shells. The left and right pincers look different and have different functions.
The snail is gripped in the left pincer which has pointed claws. With the right pincer, which is stronger, the crab cuts pieces of the shell from the shell opening. Once the gap is big enough, the crab can enjoy its snail meal. There seems to be little hairs on the left pincer of this crab.
Box crabs are awesome!

I've seen the Spotted box crab (Calappa philgarius) at Changi before (though, sadly, not recently). This doesn't look like the Spotted box crab.

Could it be Calappa hepatica? Which is recorded for Singapore and listed as Vulnerable.

Thanks to Ivan for finding the crab!

2 comments:

  1. Yes, C. hepatica (from the outline of the carapace). Very nice!

    ReplyDelete
  2. Thanks Joelle for the ID confirmation! Yes, I think it's a very nice crab too!

    ReplyDelete

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