Keeping both your valves, or shells, keeps you safe. You’re quite hard to eat, and that gives you enough time to grow and reproduce.

As a strategy, this is a success. as long as the mineral content of the sea remains constant. You spread throughout the oceans, from the top to the bottom, in a huge variety of shapes and sizes comprising mussels, clams, oysters and many more.

You survive to the present day, happily filter feeding from within your open shell, then closing up when threatened.

The only threat to your existence has come from a rather unexpected quarter: humans. As the humans colonised the planet, and increased the carbon in the air by curning coal and oil, some of that burnt carbon - carbon dioxide - dissolved into the sea. That makes carbonic acid, which makes the whole sea slightly acidic, which erodes your valves, making them weaker.

Who knows, you may evolve some more, to make a thicker shell. Or the rising acidity of the sea may kill you all.