I found about six of these things in a half-hour on a Delaware beach. The beach is an Atlantic Ocean beach (not the DE Bay) and was experiencing a fairly serious northeast wind that was causing all sorts of stuff to wash up – including the black plastic bottom from an old two-liter bottle (remember those?). In addition to antique garbage, I found some invertebrates, and despite living at the beach for almost 20 years, I had never seen anything like them.
They were small and round, about the size of a stack of two or three U.S. quarters, or an English one-pound coin, with dozens of short 2mm pink-and-white tentacles covering one side, a bright blue rim, and a semi-hard back side with brown and grey radial ridges, not much deeper than the designs engraved on a coin. One of the creatures I found had a notch torn radially into its body, and I got a good look at the cross-section of the hard back. It was not a shell – it was just the leathery exterior surface of a complex structure like the inside of a bird’s bones. The leathery texture of the hard plate reminded me of the “foot” plate on a univalve like a conch, but it was not actually a shell, or else it would not have torn. Less than a millimeter in from the hard/leathery side, the structure was clear stiff gel in an open-cell foam. There was no visible mouth, no visible sensory organs of any kind. It was as if a jellyfish, an anemone, and a sand dollar got together and made sweet sweet interspecies love.
It was like no jellyfish, univalve, or bivalve I had ever seen. But maybe one of you Dopers knows what I’m talking about?