What food Alain eats on the island in "The French Connection"?

Early in the 1971 film "The French Connection", before departing to America, Alain Charnier arrives by boat to Château d'If, offshore in the Bay of Marseille, to meet with hitman Pierre Nicoli. Upon his arrival to the island, he picks some food from a small puddle of water on the island. He opens it with a knife and eat its contents. I'm attaching a screenshot of that food. What is this food?

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Some kind of shellfish, obviously, but I don't know which kind, exactly? It's an oyster or clam or perhaps something more exotic like tartufi?

Dunno? I don't eat much seafood.

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Not sure, but I am sure Andrew Zimmern would describe it has having an earthy, musky, gamey flavor

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I thought they might Be some kind of sea urchin or squid thing but googling around someone suggested they were 'taratuffi' a Mediterranean speciality prized in Italy. Google some pics , they look like it.