I recall a short story in a forgotten anthology,
"The Margenes" by Miriam Allen deFord, first published in the February 1956 If (available at the Internet Archive), reprinted in The First World of If, the deFord collection Elsewhere, Elsewhen, Elsehow, and most recently in the 1978 anthology Demons Within & Other Disturbing Tales edited by Helen Hoke.
in which two guys discover an odd type of seafood on the beach.
One of the two guys was a gal. The grunion were running:
They were doing that on an April night in 1980. In the midst of the excitement of the chase, only a few of them noticed that something else was riding the waves in with the grunions.
Among the few who stopped grunion-catching long enough to investigate were a girl named Marge Hickin and a boy named Gene Towanda. They were UCLA students, "going together," who had come down on Saturday from Los Angeles for the fun.
"What on earth do you think these can be, Gene?" Marge asked, holding out on her palms three or four of the little circular, wriggling objects, looking like small-size doughnuts, pale straw in color.
"Never saw anything like them," Gene admitted. "But then my major's psychology, not zoology. They don't seem to bite, anyway. Here let's collect some of them instead of the fish. That dingus of yours will hold water. We can take them to the marine biology lab Monday and find out what they are."
Marge Hickin and Gene Towanda had started a world-wide economic revolution.
One guy dares the other to taste an odd round thing in the water, and the taster declares it, “Delicious!”.
They are not quite reckless about it, they wait for the lab analysis:
Marge and Gene were interested enough to come down again the next weekend to find out what, if anything, had been discovered. Not much had: but one of the biochemists at the laboratory casually mentioned that chemically the straw-colored circles seemed to be almost pure protein, with some carbohydrates and fats, and that apparently they contained all the essential vitamins.
College student that he was, Gene Towanda immediately swallowed one of the wriggling things down whole, as a joke.
It tickled a little, but that wasn't what caused the delighted amazement on his face.
"Gosh!" he exclaimed. "It's delicious!"
He swallowed another handful.
That was the beginning of the great margene industry.
The punchline of the story:
As for the margenes themselves, out of the untold millions that had come, only a few thousand were lucky enough to survive and find their way back to their overcrowded starting point. In their strange way of communication—as incomprehensible to us as would be their means of nourishment and reproduction, or their constitution itself—they made known to their kin what had happened to them. There is no possibility, in spite of the terrific over-population of their original home and of the others to which they are constantly migrating, that they will ever come here again.
There has been much speculation, particularly among writers of science fiction, on what would happen if aliens from other planets should invade Earth. Would they arrive as benefactors or as conquerors? Would we welcome them or would we overcome and capture them and put them in zoos and museums? Would we meet them in friendship or with hostility?
The margenes gave us the answer.
Beings from outer space came to Earth in 1980.
And we ate them.
