Cannibalistic mollusk: The secret life of the northern moon snail

Many beach-goers in P.E.I. have likely seen moon snails before. But these sea creatures have a range of fascinating behaviours that may not be at all apparent behind their shells.

Jeff Clements, an aquatic biologist with Fisheries and Oceans Canada, says there are multiple species of moon snails across the world, but the one found in the Maritimes is called northern moon snail.

Some say you're more likely to find moon snail shells on the beach after a full moon. That does not surprise Clements, as tides are higher during full moons and more seashells wash up on shore. 

But the moon's pull of the tides is not where the snails get their name. 

"[It] actually comes from the shape of the large opening that's on the underside of their shell, where the snail comes out," he said. The aperture looks somewhat like a half moon.

Clements said moon snails have round, spherical shells ranging from the size of a marble to that of a baseball.

Those shells can be a combination of many colours: white, grey or yellow, with dabs of purple, blue or orange.

If you see a live moon snail, it'll have a big, fleshy, purple foot that wraps around the outside of its shell when cruising on the sand.

Where to find them

You might see evidence of moon snail reproduction while walking along the beach.

When the female is ready to lay her eggs, Clements said, she covers her entire shell with her foot, burrowing just beneath the surface of the sand. 

She then coats her foot in a thick layer of sticky mucus, binding together layers of sand and eggs. The result is a hardened sand collar you can spot sometimes on the beach.

Moon snail shells can be found washed up on dry parts of the beach. But you might find live ones in submerged parts of sand flats during low tide, Clements said.

"You may actually see something almost like a bubble of sand with a little hole near it," Clements said.

"If you dig underneath that bubble of sand with the hole, you may find a moon snail. And it may actually be eating something."

Should you end up picking up a live moon snail next time you're on the beach, Clements said you should remember to put it back in the water so it doesn't dry out in the sun.

A carnivorous predator

The feeding habits of moon snails are probably the coolest thing about them, Clements said.

When they're babies, they tend to eat little bits of algae or microscopic plants called diatoms.

But as they get bigger, they turn into terrifying predators, eating other types of mollusks such as clams, mussels, periwinkles — and even fellow moon snails.

That's right: They're cannibals.

"To attack their prey, they will burrow underneath the sediment and move around below the surface," Clements said.

"And once they smell the prey, they can move toward it and ambush it."

Once the prey is caught, the moon snail will envelop and suffocate it, secreting an acidic enzyme to soften the shell of its prey. It then drills holes in the shell of the prey using a specialized organ called radula — basically a tongue lined with sharp teeth, Clements said.

Because of their strong shell moon snails have few predators, but some of them include crabs, lobsters, seagulls and other shorebirds.

Climate change and ocean acidification

Although they pose a threat to other marine mollusks, moon snails are harmless to humans.

"That rasping tongue isn't gonna penetrate through your hand if you pick these things up," Clements said.

Instead, it may be humans who are harming moon snails through the ever-growing effects of climate change.

As far as Clements knows, there's been no research on how climate change has specifically been affecting the northern moon snail. But based on studies on similar species, he said aspects of moon snail feeding may be affected by the warming oceans.

Oceans are also becoming more acidic as the water absorbs excess carbon dioxide from the atmosphere.

"This makes it harder for animals that make calcium carbonate shells, like the moon snail, to make those shells and maintain them," Clements said.

"This is a concern for more or less all shellfish that exist in the ocean."