|Looks like a nudibranch, Melibe leonina. Oyster Bay, near the low tide line.|
This was the largest lion's mane nudibranch (aka hooded nudibranch) I've seen so far, almost 5 inches long. Wikipedia gives their length as up to 4 inches, but my encyclopedia says they may reach 7. So this is a mature nudibranch, but not necessarily near the end of its life.
Transferred (with a stick, because of the smell* of the sand) to a nearby pool, Melibe rested a moment, then started to stretch out, expanding its cerata.
The sun was just setting. I couldn't wait to watch it turn from a blob to an active nudi; I had a long walk over the treacherous sand/mud interface before the light faded. But the tide was coming in; Melibe would survive until it did.
* Melibe leonina has a distinctive fruity smell when out of water, but here in Oyster Bay, the rotted mud stink overpowers everything. I didn't notice the fruit.