Pliny the Elder wrote that oysters, mussels and sea urchins grow fuller as the Moon waxes and thinner as it wanes, and fishermen believed it for two thousand years.

There is something real underneath. Filter feeders eat when the tide brings food, and tides are largest at new and full, so a shellfish's condition genuinely tracks a fortnightly rhythm. Sea urchin gonads — the part that is eaten — do show a lunar cycle in several species. Pliny's mechanism was wrong and his observation was not.