Come to West Meadow spit at a good low tide and walk along the beach, especially out on the broad sand flats that are exposed in front of West Meadow Beach and in front of the beach to the south. If you look down you will see millions of shells on the shoreline. The shells are oval in shape, usually about 4 cm (about 1 1/2 inches) long and have an obvious shelf inside. These are slipper shells of the common slipper shell snail Crepidula fornicata. Another, the eastern white slipper shell, Crepidula plana, has a white and very flattened shell and can be found attached to the insides of large whelk shells and occasionally horseshoe crabs.

Crepidula fornicata has become very abundant in the past few decades, and one can’t help but wonder if they are taking the place of surf clams, hard clams and soft-shell clams, which have been badly overexploited by clammers on Long Island. They also have become extremely abundant in the eastern end of Peconic Bay, where scallops disappeared because of brown tide and increasing temperatures owing to climate change.

If you look out on the sand flats you will find thousands of live slipper shells. If you look closely you will notice that they occur in a stack of 2-5 individuals, usually, with the bottom one attached to a small rock. The stacks tell you much about their life history. The bottom individual is larger than the ones at the top of the stack and is inevitably a female, but the top individuals are smaller and males. What is not obvious, is that every individual common slipper shell starts life out as an immature snail, then matures into a male, then loses the male function and matures into a female! If a new slipper shell comes on top of the first and lowest animal in the stack, it will have male function (copulation occurs by means of a penis), until another individual comes on top of him. He will then change sex to female function. Sex change, therefore, is not precisely timed but depends on the presence of other individuals in the stack.

Slipper shells are snails, but they are unusual. Their very large gills are covered with a layer of mucus. This layer traps phytoplankton particles in the water and the material is transferred to the edge of the gill and gathered together eventually to be eaten. See this video for an explanation. They therefore feed in more or less the same way as mussels and clams.