With a hat tip to M.F.K. Fisher, author of “Consider the Oyster,” why do California mussel shells change color over their lifespans?
It’s not unusual to see dozens of half-inch long, pale gray striped shells on the littoral beach, along with a few adult shells. Typically, the shells have parted with their other halves.
Mature mussel shells are hard to miss. They’ve morphed into an appealing striation of marine and navy blue, gray and black, iridescent even when dry. Some are tinged brown with the addition of a sea algae. The color change over the mussel’s life cycle is due partly to attacks by endoliths, which dissolve the black shell surface and leave the blue and gray layered surface. The lighter colors help mussels cope with warming water temperatures.
Eating is a complicated exercise, particularly for those mussels on rocks exposed at low tide. When back under water, these filter feeders open their shells just enough to admit water carrying tiny food scraps which little hairs capture and filter. Their diet consists of detritus and phytoplankton.
To make it to adulthood, mussels filter two to three quarts of water per hour. They take about three years to mature.
In addition to being enjoyed at the table by humans, sea stars also find mussels irresistible. Here’s a pithy description of the process: “A sea star envelops a mussel with its arms and, by applying many tube feet to each valve, is able to pull the shells slightly apart, no easy feat. It then everts its stomach out through its mouth and inserts it into the shell, eventually digesting the mussel and absorbing its nutrients.”
At low tide, shore birds, crows and gulls also enjoy munching on the bivalves.