When I was in Cape Cod last week, I kept seeing these little Styrofoam-like "eggs" on the beach. They are about 4 inches long and fairly hard. I have been going to the Cape since I was a child, and I have never seen/noticed these things before. I asked the ranger at the Visitor Center, and she said they are brown algae; I broke one apart to see the insides and to take a whiff. They are definitely plant-like, that is true. They are like hardened foam on the outside, spongy green on the inside. The smell is a bit like low tide; not horribly unpleasant, but very biological.
I did a hurried web search to read up on them; I discovered their Latin name and also that there is a second "sea potato" that is actually a species of urchin. So much for common names (thanks again for the binomial nomenclature, Linnaeus!).
But I am left deeply unsatisfied with the information out there on the web about these objects. I want to know why the brown alga creates this structure and why there suddenly seem to be so many more of them. Do they bunch together like this in response to some sort of environmental stresses, or do they naturally grow like this? I must research further.