Crab tide

Yesterday’s tide line on Penrhos beach comprised almost nothing but spiny spider crabs, hundreds, probably thousands of them. It looked like crabageddon.


Spiny spider crab shells

In fact, I am assured that these aren’t actually dead crabs, they are just the shells from the annual moult. Which explains why there were no birds taking any interest and no smells either. Somewhere out to sea there will be thousands of naked spiny spider crabs waiting for their new shells to grow.

I was puzzled how the crabs could manage to shed their shells, whilst seeming to leave the shells intact. How was that possible? Then it was explained to me that the shell can hinge open (hinging at the front) and the crab wriggles out the rear.  Once I knew that, I had to go and check it out for myself.


Here’s a freshly washed up crab shell. You can tell it’s empty by giving it a tap and it sounds hollow.


And, sure enough, if you lift the back edge of the shell, it easily opens up to show you the empty space inside:)


I also spotted this “jellied” crab…




4 thoughts on “Crab tide

  1. Sue

    This is fascinating. We visited Penrhos Beach last week and it was covered with spider crab shells. But the sea itself was (literally) alive with them – to the extent that we had to abandon our attempt to swim.

    1. Kay B Post author

      I too gave up even just walking along the water’s edge because of the number of crabs, and the jellyfish too: and I thought I definitely wouldn’t have wanted to be swimming! The water was incredibly warm and I think that might have something to do with such large numbers of crabs this year.

  2. wellhopper

    Fascinating – I noticed all the crabs too, wondered what on earth was going on, looked like a mass killing or something – thanks for the explanation

    1. Kay B Post author

      You’re welcome! I was relieved to find out it wasn’t anything untoward. It does look quite bizarre. And every day there are still more, although far fewer than the first “invasion”.


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