Shark Eyes: The Cannibalistic Mollusk
The shark eye mollusk is an amazing marine animal that has a stunning shell and a strange, interesting life. When I first started finding these shells, I didn’t know what they were called. With a little research, I found out they are called shark eye shells, a fitting name for their appearance, with a vibrant center and circular form.
The shark eye snail is a gastropod mollusk, meaning it has one opening in a spiral shaped shell. It is also part of the moon snail family. Its scientific name is Polinices duplicatus, although it is also known as the Atlantic moon snail or Neverita duplicata. Shark eyes are commonly found on the East Coast of the United States and in the Gulf of Mexico. They live mostly under the sand and use their single large foot to move around. They are known to live anywhere from very shallow waters near the shore, to deeper waters up to 190 feet deep.
The shark eye’s shell comes in two simple base colors, orange-tan or blue-gray. Marbled into one of those colors can be orange, maroon, purple, and blue irises. When the shark eye is moving around or hunting, its body expands, covering most or all its shell by inflating with water. When threatened, it quickly expels the water so it can shrink back into its shell for safety. Then, it closes its operculum, a very thin but hard piece of shell that is connected to a part of the foot, using it like a trap door to protect it from danger. The shark eye’s shell grows with the mollusk, so if you happen upon a shark eye shell on the beach, is the same size it was when the shark eye was living in it.
The shark eye’s diet consists mainly of bivalves—mollusks that have a hinged shell with two sides, like scallop shells or clam shells. However, its most surprising dietary preference is to feast on other shark eyes. To eat other mollusks, the shark eye will latch onto its prey and produce an acid to soften the shell. Then, the hungry shark eye will drill into the shell using its sharp radula, which is similar to a tongue in humans. Once a hole has been drilled all the way through, the shark eye injects digestive enzymes and chemicals to break down and kill its dinner, slurping it up through the hole. A classic sign that a shell was drilled and eaten by a shark eye is a circular hole with rough crumbly edges. Sometimes another mollusk will eat either the shark eye drilling the shell, or the shell that was being drilled and you will find a hole that is just partially drilled through.
When a female shark eye snail lays her eggs, she deposits them in a half-moon-shaped ring called an egg collar (below). It looks like a plastic or thin rubber scrap that was left on the beach, but it actually holds unborn shark eye snails. The eggs are in a layer of slime and mucus connected to the egg collar, which protects them.
When the collar is released into the water, sand sticks to the slime and forms another barricade from the sea and other mollusks until the eggs hatch. When the egg collar is in the water with the slime cushioning and eggs still attached, it is rubbery and tough but once the eggs are hatched and the collar has dried up on shore it becomes very delicate and fragile.
This article appeared in the Beachcombing Magazine January/February 2021 issue.
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