Living Sanibel: Pen shell, Atlantic calico scallop
Pen shell
Although difficult to find alive in less than five to 10 feet of water, the two species of pen shell do become dislodged after heavy storms and can sometimes be found in tidal pools at low tide. The difference between the two species is in the ribbing along the outside of the shells. The saw-toothed pen shell has more than 30 fine ribs, whereas the rigid pen shell has 15 or fewer. The rigid pen shell tends to grow roughly an inch larger than the saw-toothed and displays a distinctive and beautiful mother of pearl iridescence at the pointed base of the shell.
The pen shell buries itself in the soft bottom in depths up to 20 feet and attaches to the limestone substrate or shell fragments though the use of byssus (pronounced “biss-us”) threads, which are commonly found on mussels and other sedentary bivalves. The pen shell has a symbiotic relationship with a tiny crab (Pinnotheres maculatus) that lives its entire life inside the filtering end of the pen shell. It is believed that these crabs help the pen shell feed, though the details of this unusual relationship are still being studied.
The pen shell is edible and is commonly made into a soup in New Zealand and Japan where it is sometimes combined with shiitake mushrooms. It is heavily preyed upon by gastropods, rays and nurse sharks, which scour the bottom looking for colonies of these large bivalves. Both species of pen shell are often found by the thousands after tropical storms or strong winter cold fronts come through. They are seldom collected.
Atlantic
Calico Scallop
One of the most commonly found shells on the beaches of Sanibel and Captiva, the calico scallop, is also one of the most beautiful. It washes ashore in a vast array of colors and is fairly resistant to damage in the rough-and-tumble world of the surf zone. The calico scallop is widely used in shell art and for decorative purposes. It has the classic shell shape made popular by the Shell Oil Company, and it is the centerpiece of one of the world’s most famous paintings, “The Birth of Venus” by Botticelli.
Although calico scallop shells are easy to find and identify on the beach, it presents a much more difficult challenge to find one alive. Because it is free swimming, it occasionally washes up alive on the beaches after a major storm. Under normal circumstances the best place to find a live calico scallop is in a tide pool, sitting on the bottom in ankle to waist deep water.
The calico is widespread and is not as commercially harvested as the bay scallop. It ranges from Delaware to Brazil. Like all scallops, the calico is a filter feeder, taking in vast amounts of seawater and removing microscopic particles of algae, diatoms, and other phytoplankton. It is eaten by rays, nurse sharks, and a number of other fish.
This is an excerpt from Living Sanibel – A Nature Guide to Sanibel & Captiva Islands by Charles Sobczak. The book is available at all the Island bookstores, Baileys, Jerry’s and your favorite online sites.