Blood Cells Build Seashells

For more than 30 years, scientists thought they knew how oysters and other soft-bodied mollusks make their calcified armor. But new research reveals that as oysters build their shells they get help from a surprising source: their blood cells.

Construction site. Oysters' blood cells (green) ferry raw materials to the site of shell formation. CREDIT: SCIENCE

In the conventional view, oysters build their houses by secreting a gelatinous mixture of organic molecules from their mantle, the layer of skin that coats the inside of their shells. That gel contains tiny crystals of calcium carbonate that grow to form new shell. Looking at oyster tissue under a microscope, a team led by zoologist Andrew Mount of Clemson University in South Carolina recently came across another source. They found calcium carbonate crystals inside the mobile, amorphous blood cells of the Eastern oyster, Crassostrea virginica. The embedded crystals refracted light in a characteristic way, allowing the researchers to distinguish blood cells with cargo from ones without.

To test if the blood cells actually bring the crystals to the site of shell formation, called the mineralization front, the team carved a notch in the shell to induce repair. Forty-eight hours later, they found a 10% increase in refractive blood cells at the mineralization front. A closer look with a scanning electron microscope revealed that crystal-bearing blood cells were clustered around crystals secreted onto the new shell bits, as they report in the 9 April issue of Science. "It looks like the cells are contributing the crystals," says Mount, who figures the same process is at work in all shell-making mollusks, the phylum that includes oysters, snails, and abalone.

Finding a role for blood cells in shell production is an important discovery, says biologist Dave Brushek of Rutgers University in New Jersey. Because blood cells are also involved in immunity, finding them at work in this process "may change the way we're approaching studying resistance to disease in oysters." He adds that the pearl industry will be keen to hear this news, as they'd like to manipulate mineralization to make bigger and better gems.

--MEGAN MANSELL WILLIAMS

Related sites
More information about pearls from the Chicago Field Museum
The Haskin Shellfish Research Lab at Rutgers