Stony Brook Harbor, NY, c. Patricia Paladines

Discovering the Common Slipper Shells on a Winter Beach Walk

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View over Stony Brook Harbor

The local beaches in December can be just as magical as they are in the middle of July, and you don’t have to battle the crowds for a spot in the parking lot. Settling down on your beach chair in the sand may not be the most comfortable thing to do but a nice old-fashioned beach comb can reveal some of the most resilient life found along the intertidal shores.

By the time the fall months arrive one of Long Island most fascinating and ancient animals, the American horseshoe crabs (Limulus Polyphemus), have moved to deeper waters. Live ones will not appear near shores again until around the full moons of May and June. But that doesn’t mean you will not find any signs of horseshoe crabs on a winter beach walk. The fall and winter high tides often leave behind the shells of horseshoe crabs, either molts, or shells of crabs that have died. Molts are easy to identify by their lightness and are usually found as complete shells. Looking for a clean split along the front of the carapace can also help identify a molt, that is where the animal slipped out of the old shell when it was ready to grow. Horseshoe crabs will produce molts between 12 to 15 times during the first 10 years of their lives so one can find varying sizes of molted shells on the beach.

The darker, heavier and usually much larger horseshoe crab shells are from “crabs” that have died. These shells often have Common Slipper Shells (Crepidula fornicata) attached to them. The Slipper Shells are marine snails categorized under the class Gastropoda, meaning their body is mostly a stomach and a foot. Their relatively large foot has evolved to behave as a suction cup for attaching themselves to other Slipper Shells, rocks, and sometimes the shells of horseshoe crabs.

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The carapace of a dead Horseshoe crab with live Slipper Shells

Slipper Shells holding fast to something that has become stranded high up on the beach are still alive. If they are out of the water for long periods of time they will become weak and fall off. Their bad luck provides opportunities for gulls and other beach scavengers to feast on them. During the cold months of winter many animals rely on these accidental strandings as supplements to their diets because regular food sources may be scarce. Luckily the population of Slipper Shells around Long Island seem to be doing well.

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The wrack line at West Meadow Beach

Slipper Shells have an interesting life history, females release thousands of microscopic swimming larvae (veligers) into the water. The larvae are all males. Many of these microscopic swimmers will become food for other marine animals before they land on a surface where they will remain attached for the rest of their sedentary lives. Through chemical cues Slipper Shell larvae find each other and begin to form stacks. The lower ones on the stack transform into females, a process called sequential hermaphroditism, and become larger. The males at the top of the stack can fertilize the eggs of the females below no matter how far apart on the stack they are. Their scientific name, Crepidula fornicata supposedly arose because Linnaeus (1758), the taxonomist, thought the shells looked like the arches (fornix) of the Roman aqueducts. Nonetheless many wonder whether Linnaeus was referring to the arches or the sexual proclivity of the snails when bestowing this memorable, though eyebrow raising, species name. Many marine biology students have no difficulty remembering the scientific name of the Common Slipper Shell.

The sedentary Sipper Shells are filter feeders, that is while submerged, they lift slightly off from their support, whether rock or other shell and feed on microscopic organism in the water that flows through the gap.

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A clump of Slipper Shells at the inter-tidal

If you come across a stack of Slipper Shells at the wrack line during a winter beach walk help them live a little longer by moving the stack back into the water then enjoy the colorful diversity of life found at this time of the year.

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Red, green, and brown algae among living and dead Slipper Shells

This piece appears in Winter 2018 – 2019 edition of the Four Harbors Audubon Society Chapter newsletter. Visit the Chapter’s website fourharborsaudubon.org to learn more about their activities. I will lead a Family Beach Walk at West Meadow Beach on Saturday, December 29th, 2018, at 11am. 

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