Help ID this mid-Atlantic invertebrate (no photo)

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?

can’t really visualise what you’re describing, but could it have been some kind of chiton?

Unlikely… the “shell” wasn’t really a shell of any sort. It was leathery on one side, but the leathery surface was more of a callous than a shell. The other side was covered edge-to-edge with soft, squishy white tentacles with pink tips, but they were very short compared to their diameter – maybe 2mm long and 1/2mm in diameter each. When it was not immersed in water the tentacles clumped together and deflated into a soft gelatinous mass.

I sniffed it to see if it smelled any different from any of the various things that wash up on the shores of the world. It did not.

The more research I do the more I’m convinced it’s either an echinoderm or a cnidarian, but I have no idea which direction to go.

Could it have been a Portuguese Man o’ War?


The ones I have seen have been purplish in color.

I herewith address the only portion of the OP I feel qualified to answer:

Yep.

Not likely… it didn’t have any air bladder, and I handled several of them extensively without even an itch. A Man O’ War – even a small one – should have left at least a red mark.

Also, Man O’ War jellyfish are soft all over, and somewhat balloon-shaped. The creature I handled was like a small flattened lens: tough, rubbery, and brown-gray on the barely concave side; soft and gelatinous with pink and white tentacles on the convex side.

I’m becoming more convinced that it was some kind of anemone, and the “hard” side was intended to grip rocks or the sea floor. But still, no proof!

It bore a great resemblence to Velella velella of the Porpitidae, but without the sail and much smaller. Could be that the strong northeaster pushed some of the young ones ashore out of season.

baby sand dollars? http://www.carolinanature.com/pix/sanddollar.html

Okay, now I’m kicking myself. I looked right at the Wiki graphic for the Blue Button Porpitid, and didn’t think that it could be one of those with the long frilly tendrils torn off. Of course the beach was
“experiencing a fairly serious northeast wind that was causing all sorts of stuff to wash up” but I didn’t even think that it could be part of a creature. Here is a Flickr link to some blue button jellies washed ashore, tentacled side up. They look just like the ones I found! Here is one with the leathery dark brown side showing, and tentacles intact.

Thanks for the suggestions, all.