Huge hermit crab migration

This video by Steve Simonsen shows an amazing migration of hermit crabs on St. John in the Virgin Islands:

This morning I received an urgent telephone call from my good friend Pam Gaffin. She was terribly excited about an event that was happening before her eyes. Pam told me it was a migration of soldier crabs also called hermit crabs and there were millions and millions of them she likened it to the migrations of Serengeti. I didn’t need to hear anymore, I loaded my car with cameras and was out the door. Pam told me that this began this morning at sunrise at Nanny point near Concordia. I have heard about this migration for years and knew that it occurred in August, I can’t tell you how happy I am that Pam called. Pam you’re my hero.

Apparently this migration is for reproductive purposes. On St. John it’s an annual event:

The Hermit crab is predominantly a land creature, but it still has close ties to water and the sea. In fact it can never be entirely away from water, and solves this problem by always carrying a supply of water in its shell. Moreover, the hermit crab returns to the sea in order to reproduce. At certain times of the year hundreds of these creatures scramble and tumble down the mountainsides and make their way to the seashore where the females crawl to the water’s edge. They then cast their fertilized eggs in the sea where the newly born hermit crabs spend their next few months before returning to land.


  1. Posted September 5, 2012 at 1:12 pm | Permalink

    It’s not for reproductive purposes! I hear there’s a crab that’s calling himself the son of god. Apparently he does some great magic tricks and has announced the end is nigh.
    He’s threatening the other crabs. He’s told them that if they don’t follow him, they’re going to end up on a fiery grill- hence they massive migration 😀

  2. suwise3
    Posted September 5, 2012 at 1:36 pm | Permalink

    Shouldn’t there be lots of predators taking advantage of this?

    • Posted September 5, 2012 at 1:41 pm | Permalink

      My thoughts exactly….


      • SnowyOwl
        Posted September 5, 2012 at 2:15 pm | Permalink

        Ah, yes indeedy.
        In eastern North America — in the less exotic Delaware Bay, and in the very less exotic state of New Jersey — an exactly similar event happens in Spring.
        The very ancient Horseshoe Crab ( comes ashore ‘in hooves’ to lay eggs.
        The Red Knot (Calidris canutus) times its very long journey to this egg feast. As do other shorebirds arcing from (nearly) the Antarctic to the Arctic:

        As this is Delaware Bay though, this natural happening is happening less and less due to environmental degradation… from human activity (que surprise).
        On the other hand, New Jersey Audubon Society has been working tirelessly for a couple of decades to turn this around… in favor of the crabs/knots. And of course, this favors us (beyond the next quarterly stock report).
        Tom Carrolan

        • Ken Pidcock
          Posted September 5, 2012 at 6:08 pm | Permalink

          Well, they are useful, aren’t they? My grandmother retired, from Kensington Philly, to Villas, on the New Jersey side of the bay. As a child, I dedicated many hours on the shoreline tending to horseshoe crabs who had turned turtle.

    • infiniteimprobabilit
      Posted September 5, 2012 at 10:52 pm | Permalink

      Well, they’re probably quite hard to extricate from their shell since they retreat and jam the door shut with nothing visible but big bony claws, and there’s probably not much good eating on them anyway. So I’d guess any predator that can’t just crunch them in his jaws then spit the bony bits out, wouldn’t be very interested. And for predators big enough to do that, I suspect the tiny edible part is probably not worth the effort. Just my guess.

  3. abrotherhoodofman
    Posted September 5, 2012 at 1:42 pm | Permalink

    It’s like they don’t have homes to live in.

  4. JDStackpole
    Posted September 5, 2012 at 1:55 pm | Permalink

    Since (I understand) those guys don’t grow their own shells, there sure must be a lot of displaced “snail” critters around.

    Is there a central supply dump where the original owners of the shells leave them, when outgrown, to be recycled? Or perhaps the parents will their borrowed shells to the kids, anybody’s kids.

    • mday
      Posted September 5, 2012 at 2:18 pm | Permalink

      I was thinking the same thing. And, they each use multiple shells as they grow.

    • Posted September 5, 2012 at 5:27 pm | Permalink

      Soldier crabs must trade up to bigger shells as they grow, and each shell is probably used by multiple crabs. The most commonly used shell is Cittarium pica, known in the islands as a whelk. It’s the most common shell in the video. In Bermuda, the snail has gone extinct, but it is still the most popular shell among the soldier crabs, who use the fossil shells.

  5. Marella
    Posted September 5, 2012 at 1:56 pm | Permalink

    Amazing. Didn’t need the music though.

  6. MadScientist
    Posted September 5, 2012 at 1:59 pm | Permalink

    What a pity, I can’t see the huge hermit crabs. 😦

  7. Posted September 5, 2012 at 3:14 pm | Permalink

    Excellent. I agree with Marella – the music doesn’t add anything. The video is beautiful without it. The clicking claws and sliding shells are amazing, and the gentle surf is music enough.

    • Nick
      Posted September 6, 2012 at 1:32 pm | Permalink

      Ditto – the music actually detracted from the video for me and I turned it off after less than a minute. I actually watched it with the intent of hearing the clack of claws and shells across rocks.

      Perhaps someone who use Facebook can contact Simonsen and ask him to release his video without the music.

  8. Filipe
    Posted September 5, 2012 at 9:54 pm | Permalink

    _Coenobita clypeatus_, interesting animals but a bit creepy. Very cool video, and I don’t see anything wrong with adding the music.

  9. Dominic
    Posted September 6, 2012 at 2:03 am | Permalink

    This is an interesting example of a form of selection that does not involve mate choice, is it not? The females presumably just spawn & random males – who survive the journey – fertilize the eggs. What are the implications for genetic change? Thinking about when mate selection did or did not happen in ‘lower’ forms of life & the changes that this caused in how those creatures did or did not evolve, i.e. reach a form of stasis. Do you need sexual selection for ‘progression’? Anyone?

    • Dominic
      Posted September 6, 2012 at 2:27 am | Permalink

      Thanks to Twitter, Darren Naish points to this paper by Levitan saying there is sexual selection in mass spawners –
      The Distribution of Male and Female Reproductive Success in a Broadcast Spawning Marine Invertebrate

    • MAUCH
      Posted September 6, 2012 at 7:55 am | Permalink

      The eggs are fertilized by males that are tough enough to get to shoreline to do their appointed task. Think of it as a giant singles bar.

      • Dominic
        Posted September 7, 2012 at 1:53 am | Permalink

        Yes – but my point was that the male does not choose a particular female & the female does not choose a particular male. So your singles bar is one where everyone is blindfolded!

  10. Posted September 6, 2012 at 11:58 pm | Permalink

    Hey I din knew crabs looks like this also, there shells are like soldiers uniform only.
    And the video my god I havent seen so may creabs at a time..

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