What is the world's largest type of seashell by length?

The world's largest seashell by length is the 'Tridacna gigas' or the Giant Clam, which can grow up to 1.2 meters (4 feet) long and weigh up to 200 kilograms (440 pounds). It is one of the heaviest mollusks in the world and can live for up to 100 years. The giant clam is native to the warm waters of the South Pacific and Indian Oceans and is an important part of marine ecosystems as it helps filter water and provides a habitat for small fish and other sea creatures.

The giant clam is a popular food item in some cultures, which has led to overfishing and endangerment of the species. Today, the giant clam is a protected species and it is illegal to harvest or trade them in many countries. Despite their protected status, giant clams face threats such as habitat destruction, ocean pollution, and climate change.

The record for the largest giant clam ever found goes to a specimen discovered off the coast of the Philippines in 1956, which weighed a massive 230 kilograms (510 pounds).