How on earth did those stones end up there

This picture was taken at a strait/channel in Norway with VERY strong current. There was only about 10min around the turn of the tide where it was at all possible to not hold on to something. I saw several of these. The stones are stuck. Is this deliberate - trying to be heavier?

this is a very common urchin behavior. They collect debris from the local environment and stick it to themselves as an attempt at camouflage.

Urchins have tentacles with suction cups on them (the tentalces are calld tube feet) all over their body that they use to hold on the their treasures.

Interesting, thanks. Haven't noticed that behaviour before. I suppose stones that size was the only thing they could find (given the current) and they just had to do it.