The Daily Blue Soldier Crab Migration

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As the tide comes in along the quiet bay sand shores of Bribie Island a twice daily miniature migration occurs that is interesting to observe. Blue Soldier Crabs, in their thousands moving en-mass up the sand to the safety of their burrows.

The crabs are skittish, as you approach the swarm clearly alerted by the vibrations of footprints in the sand moves away. From the back it’s a mass of tiny round blue blobs. From the front, its more a scene from a sci-fi movie. Led by bigger (presumably male) crabs, the mass dutifully follow. The pressure of the incoming tide eventually overcomes fear of the guy with the camera and they advance until focusing becomes impossible.

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Endemic from Australia through to India Soldier Crabs spend much of their time buried in the sand. They usually emerge to the surface a few hours before low tide, with the process of emergence taking anywhere from five minutes to an hour. After emerging the crabs perform an aerobatic grooming process designed to rid themselves of any sand entrapped in their shell. Then its down to the water to feed on micro organisms in the sand.

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Watching these little guys swarm up the sand in precision reminded me of those science fiction movies where big aliens take over the earth. The image is the same, just the scale is different and in this case it’s us humans that are the monsters. For soldier crabs it’s all about the tides and getting back in their burrows alive.

 

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