The freshwater mussels (Unionoidea) form the largest single section of the Academy’s vast collection of roughly 10 million shells. They are found in rivers, streams, lakes and ponds all over the world, with particular richness in North America.
The Academy’s founders were all enthusiastic collectors, and several of its early scientists –– most prominently Thomas Say, Isaac Lea and Timothy Conrad –– were very interested in freshwater mussels. The collection they created has continued to grow to the present day through contributions from field collectors, environmental scientists and state biodiversity surveys, reflecting the importance of these animals to many different scientific communities.
Over their lifetime of many decades, mussels rarely move but continually add layers to their shells. The rate at which they grow can be measured in these layers down to individual days and reflects factors such as water temperature and oxygen levels. At the same time, mussels incorporate into their shells substances in the surrounding water such as metals and chemical compounds. Unlike their counterparts in the salty ocean, however, these pearly shells are quickly dissolved by the water once the animals that make them die. With their dates of collection known, therefore, the shells in our collection form a priceless and irreplaceable archive of environmental conditions stretching back into the 18th century.
The last major overhaul of the mussel collection took place in the 1950s, when the shells were moved into closed cabinets. Starting in March 2017, we implemented a large-scale upgrade program, placing the shells in archival trays within the Academy’s new cabinets and updating their systematic order to current standards. Volunteers and students in Drexel University’s Cooperative Education Program have supported this work by cleaning individual shells and labels to remove thick layers of soot.
Many of the older mussels were on display in glass cabinets or open trays for up to a century. In the days before air conditioning, windows remained open from spring to fall to counter the summer heat, and particles coated the surfaces of these significant shells. A few blocks from the Academy’s location on the Benjamin Franklin Parkway was the gigantic Baldwin Locomotive Works, which ran coal-fired foundries day and night. A short distance in the other direction lay the Reading Railroad tracks, where dozens of steam trains passed through each hour. Cheap Pennsylvania coal also powered steam tractors, ferries, ships and cranes. For more than two centuries, houses and buildings throughout the city were heated with it. The air was black with fine soot, which oozed through the tiniest gaps in cabinets and doors to settle on the exhibits.
Volunteers and students have used special pads to clean the specimen labels and display boards, and they have used paper wipes soaked in distilled water to clean the blackened mussels. The process releases distinctive fumes of anthracite coal. In most cases the thick coating has caused little damage to the shells.
Text and images by Paul Callomon, Malacology Collection Manager