But on closer inspection, it belongs where it lies; it's a part of the departed crab, left behind for the gulls and the water-line isopods.
When the crab molted, he took away his soft parts: legs and eyes, his brain, memories and all, his antennae, all his mouthparts, and his gills. Besides the hard "shell", he abandoned his stomach lining, his esophagus and shreds of his intestine.
The gills are different; he takes them, but leaves the outer casing, as apparently intact as the calcified parts. Older remains left on the shore are whitish and translucent, but fresh, they're - well, fruit-coloured.
|"Ghost gills" on the half-shell.|
|A second crab molt; looks like the gulls already took a nibble.|
|A third molted crab on the same stretch of shore line.|
A molted crab, from above, looks intact. But check out the eyes!
|Eyestalks, lens, black pupil spot. But they're empty windows: the watcher inside is long gone.|