On most of the beaches I frequent, Common Slipper Shells (Crepidula fornicata) are by far and away the most common seashell I encounter. Each shell is an oval, approximately an inch to an inch and a half long. But on some beaches, particular those along Buzzards Bay, these relatively small shells can be so numerous as to form significant mounds covering entire areas of the beach.

Although usually referred to as slipper “shells”, while the animal is still alive I think it’s more appropriate to refer to them as slipper “snails”. Slipper snails are, in fact, a type of gastropod, or snail. However, instead of the typical spiral-shaped snail shell, their half-egg-shaped shells have an inside platform extending about half way across the shell’s opening that helps to support the animal’s body. The shells thus resemble a small slipper, from which their name is derived. 

Slipper snails are sequential hermaphrodites (each individual contains both male and female reproductive organs, but they operate at different times of the animals’ lives). The younger, active snails are males, which will become females and more sedentary as they mature. Many times, a large pile of slipper snails can be found living on top of each other. The larger shells on the bottom of the pile are the females. The smaller individuals on top are males. In addition to reproduction, a lack of hard substrate will also instigate these stacked aggregations as the snails cling onto the only hard surfaces they can find: each other.