Sanibel Sea School Blog

The Beachcomber’s Guide: It Looks Like Snake Skin!

October 18, 2019
Egg Cases On Sand

By Sam Nowinski

Is it snake skin? Is it a washed-up lei? No, it’s an egg casing!

Segments of a lightning whelk egg casing.

The snake skin-like appearance of this common beach find can be very misleading. These long, curly clusters are actually egg casings from a gastropod called the lightning whelk. Many people are familiar with lightning whelks, a marine snail with a distinct lightning bolt pattern. Female individuals can reach almost 16 inches in length. Lightning whelks are also one of the rare “left-handed” snails, meaning they spiral in a counterclockwise direction with their aperture, or opening, to the left. But what may come as a surprise is how the female whelk lays its string of eggs in such an intricate casing!

Each pouch along the egg casing contains hundreds of tiny shells.

When the female lightning whelk reaches sexual maturity at around 9 years old, she deposits her eggs in these casings, which can be up to 3 feet long. The ideal place for the female to lay her eggs is in shallow water, often in or near seagrass beds. Each casing is one long strand with up to 200 individual pouches attached. Each one of the pouches on the strand contains up to 100 eggs, all surrounded by an embryonic fluid that gives them nourishment as they develop. The female anchors one end of this strand to the ocean floor, in an attempt to prevent the casings from washing ashore and drying out.

An empty adult lightning whelk.

When the juvenile lightning whelks are ready to brave the ocean on their own, they poke through the pouch and crawl through to begin their journey. These whelks will grow and repeat the life cycle – some of them will lay egg casings of their own! The remaining egg casing will eventually detach from the sand and drift ashore. So next time you are on the beach and find a lightning whelk egg casing, pick it up and examine it up close. Are there any tiny shells remaining inside?

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