I'd love to know what these are: we saw thousands of "sacks," in shades of blue and purple, at the high water line on a beach in Tenerife. About 3 to 12 cm in length. Some seemed to have leaked a kind of jelly. They popped like bubblewrap if you trod on them. At first, I didn't think they were natural things, but had to accept they couldn't be manmade. Though the colour loooked unnatural. Photo attached. Can yoyu tell me what they are?