What causes.....

This

Is it a boat dump or what?

I have seen it before and presumed it was a by catch dump or something, but this time, the remains are mixed heavily with mussels (mussel bed on mark 60 yrds out) so the bottom was heavily churned

Anyone the definitive answer?

Literally thousands of crab - maybe hundreds of thousands, spanning miles.

Get up early enough and you 'll see a load of boats within spiting distance working up and down that beach.

I seen this back in the 80's and it was proved to be the Mussel dredgers from Wexford dredging the coast up as far as Dublin even the back of Dollymount strand along red rock caused a lot of damage to the mussel beds all along the east coast and wiped out the cod fishing along the Wicklow beaches from Wicklow to Greystones It has never returned /A lot of complaints at the time but never proved it was them so nothing was done.
mickser

I have this on facebook, and they are all convinced its a mass spider crab molt, so im presuming, the crabs molted in the mussel bed, the the beds were dredged and everything churned up to the surface?

Yes I have seen this but I am still keeping a watch for those dredgers if I remember right they dredged up an old underwater telephone cable and caused some damage to it bad enough taking our Mussels but leaving us with no communications with our neighbors was uncalled for
mickser

I've seen this around cahore beach over the years, shocking amount of crabs hundreds of thousands all washed up.

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So a few people have mentioned spider molts, its a safety in numbers thing apparently, they all huddle together and shake of the shells. Im going with that explanation. Makes sense!