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A boat or slippers – whatever floats your boat

By Staff | Feb 6, 2009

During my childhood years, walking along the coast of New Jersey, I probably collected more of these snails than any other species and they are the only shell I can remember placing on calm water, making believe that they were boats, and seeing how long I could keep them afloat, which of course only lasted until a wave came along and tipped them over. But despite that, I remember this being a great deal of fun. When making sand castles my sister and I also used the slipper shells as mini-shovels.

Slipper shells are gastropods found on open beaches. They are very prevalent in Florida, being one of the few marine snails that are not coiled in a spiral shape. They reside in the intertidal zone in depths up to 40 feet and are about 1 inch long. Their foot can be found underneath their white shelf.

Inside this shell there is a white “deck” or seat which causes this snail to resemble a boat or a slipper, accounting for its common name. It is also known as a quarterdeck shell or slipper shell limpet.

The Credidula fornicate is a very common Atlantic Ocean species found on the eastern coast of North America. Often they are found stacked on one and other. They can also be found on crabs, rocks, dock pilings, and other shells and conform to the shape of their host shell as they grow.

The life cycle of this snail is intriguing. They are referred to as sequential hermaphrodites. Young slipper shells are mobile. Mature slipper shells stack up, one on top of each other, in stacks of about eight shells. The female snails at the bottom of the pile are the oldest and largest animals. The younger and smaller snails are the males and are found at the top of the stack. Instead of sending sperm and eggs into the water and hoping the connection takes place, they mate directly. After mating, when the female no longer produces sex hormones, the female dies and the largest of the males will become female.

The slipper shell is a filter feeder, but instead of grazing for algae and plankton, it spends it whole life in the same spot. This is referred to as sessile. Water is drawn into their mouth and the gills are used to separate the plankton from the water. Then the plankton is directed to the intestine. A suction creating foot allows the slipper shell to attach to hard surfaces. The underside of the shell is not routinely exposed to the water because it uses the surface it is attached to as its second shell and the lower surface of the shell matches the substrate to which it attaches. This tight connection prevents the slipper shell from becoming dislodged and keeps animals living in the intertidal zone from dehydrating when uncovered during low tide. This cozy arrangement protects the snail’s body from predators.

In Europe, this species is considered invasive. Slipper shells pose a threat to the oyster population. The slipper shells attach themselves to the oyster shells and reproduce. They can actually cover the entire seabed. Since both the oyster and slipper shells are filter feeders, if the oyster shells are surrounded by slippers shells, the slipper shells can actually filter out all of the food before it reaches the oyster and eventually can starve the oyster. It is also possible for the oysters to be killed directly by being crushed by the slipper shells.