Hanging in the water, the shell starts to form. The mantle inside is absorbing the calcium ion from the seawater. It secretes the conchiolin, which in time becomes calcified and forms the shell. Since much of the conchiolin secretion occurs around the mantle’s edges, the shape and position of the mantle determines the shape of the shell.
The shell is important for many reasons, not least a defence against predators. Crassostrea is better adapted to handling muddy waters and even pollution than its cousin ostrea, which may be one reason for its success. Ostrea’s flatness leaves it exposed to tiny fluctuations in tidal debris, where crassostrea will typically be a few centimeters off the ground. Also being larger there is more mantle to filter out impurities. And crassotsrea can close valves in the face of pollution.
But the shell is only part defense. A star fish can wrap itself around the oyster and rip it apart. In the Thames, there used to be a side industry collecting starfish off oyster beds and selling them on as manure for gardens. An American sea snail (Odostomia Impressa) is more wily attaching itself along the outside margin of an oyster shell. When the oyster opens its shell to feed, which it will be doing for nine tenths of its life, the snail inserts its snout and pierces the oyster’s mantle to suck the flesh dry. Another snail (urosalpinx), which wiped out young oysters in the Blackwater estuary in Essex, has been seen to be so determined that even if it is moved a few feet away from its prey and tipped on its back, it still returns like a dog for a bone many times over.
Similarly, but less instantly fatal, is the tiny pea crab which climbs inside the shell and sets up home along the mantle grabbing the plankton as the oyster is processing it for itself. The plover-like oystercatcher forages along the low tide mark slipping its bill inside the shell and snipping the adductor muscle, although it seems as happy to eat mussels and limpets too and any other shellfish despite its name. Crabs can crack open an oyster shell with their claws. Stingrays too are partial to oysters.
We are not the only species to covet the flesh of the oyster.