Oyster
Oysters are usually harvested before they are fully grown. Left to themselves, they do grow quite large. A recent example was an oyster which was 11 inches long (29cm) and 3.7 lbs in weight (1.7kg). The BBC report says "The family (firm) estimated it could be between '15 and 20 years old' due to its size. The world's largest oyster was 13.97 inches long (35.5cm) and  4.21 inches (10.7cm) in width. It was found in Denmark in December 2013, Guinness World Records said".