spotted this during my phuket trip…….. its amazing when you can witness what you normally see in TV!
i believe its called sand bubbler crab right? there’s the crab, in the centre of the spiral structures.
when its low tide, these crabs come to the surface and use their feelers to collect sand, and then sift and sieve through each and every sand grain for microscopic food – called detritus. whatever they dont need, which is most part of the collected sand, they mould into a ball and dump it behind them. so you can safetly assume that balls of sand are clean as hell! check out the video here.
seems that the more intricate the patterns, the longer the tide has been receding. i guess thats when they have a scumptuous meal. when tide is up, the waves bring along a whole cuisine of microorganisms that are deposited on the sandy shore. so when the tide goes down, these crabs have a time of their life eating all they can get, for at least a few hrs, before the tide comes back up.
sand bubbler crabs spend their time cleaning their burrows, but i dont know why they make such nice patterns.
from what i found out, the balls show the circular outline at which the crabs feasted upon. and then they venture outwards, one outline at a time. usually you can find a straight line leading back to the burrow. thats for when the crab senses danger, so that home is just a straight road away.
unless of course the shore birds are too fast for them. by then, they’d have become the delicacies of other predators. :)