Sea urchins. The modern Greek word comes the ancient word echinos, for hedgehog.
They thrive in pristine waters, and especially by rocky beaches, and stepping on one and having to painfully remove a spine or two from a foot was long a part of a Greek holiday. The Greeks get their revenge by making their red roe into a dip called αχινοσαλάτα (achinosalata), although these days so many have been taken from the wild that they are endangered, and it's illegal to take them, so source your sea urchin roe from a farm.
Image by Filip Maljković