This is possibly (said in a very hesitant tone of voice) a fish egg sac. It was about the size and texture of a jelly bean. I didn't try to break one open, which in hindsight might have been interesting - I'd like to have had a better look at the little confetti like shapes inside. For a couple of days the high tide mark was scattered with hundreds of these little things.
Focusing on getting a half way decent photo meant I was oblivious to other beach goers, until I became aware of someone staring at me with undisguised wariness from a few metres away. "I'm not a crazy lady taking photos of my fingers ... I promise!" My words of reassurance were followed by a horrified look and rapid scuttling off through the damp sand. It's always hard to know how others view you, and in this instance my words were clearly not reassuring!
How many people had strolled past, lost in their own thoughts or worries, with no idea that the battle to live had been lost by so many creatures under their feet, yet leaving a trail of delicate beauty for us to wonder at?