Ocean Watch

By Susan Scott

These snail biters
are both boring
and interesting

Last week, on the beach I found two small seashells, both missing their inhabitants. One was a nerite, a black snail about a half-inch long with fine spiral lines on its shell. These common algae grazers are easy to find clustered together in cracks and under ledges in the ocean's splash zone.

The other shell I found formerly belonged to a worm snail. These creatures live in tubular shells, often spiral-shape, and spend their entire adult lives cemented to rocks. To feed, they cast out stringy, mucus nets that snare passing food particles.

Like nerites, worm snails are also common here, ranging from tide pools to about 40 feet down. If you go to the beach, you've seen both these snails.

I picked up the still-perfect shells and discovered that each had a neat hole drilled in it. "I wonder who made these holes?" I said when I got home.

"Moon snails," my husband said, examining the cleaned-out shells.

"What's a moon snail?" I asked.

"Snails that drill holes."

I drew a blank. "Do I know about moon snails?"

He shrugged. "I don't know. But I do."

I hate it when he does that.

Moon snails do indeed drill holes in, and eat, other snails. These carnivores live beneath the sand, plowing along in search of other shelled animals to eat. When the moon snail finds a meal, it secretes a chemical substance onto the prey's shell to weaken it. The predator drills a hole into the thinned shell with its long, tooth-bearing tongue and devours everything inside.

But moon snails didn't quite fit the picture of my dead nerite and worm snail. Most of Hawaii's moon snails are either rare or found in deep water. And the most common one here specializes in eating clams.

I called the Waikiki Aquarium. "Drupes," said education specialist Carol Hopper.

"What's a drupe?" I asked. (OK, so I don't know my snails.)

"A family of boring snails," she said. Carol, having once studied these creatures, filled me in.

Drupes, sometimes called rock shells, are common in shallow water and on beaches. Again, if you go to the beach, you've seen drupes. Some of these inch-or-so-long snails have beautiful purple splotches around their openings. Hawaii hosts at least 27 species of these little snails.

Drupes eat, among other things, nerites and worm snails by drilling neat holes in the prey's shells. So, hah, the attackers of my beach shells were not moon snails. They were drupes.

He hates it when I do that.

Researchers have extensively studied another family of boring snails called murexes, because they wreak havoc in commercial oyster beds.

Both American and Japanese murexes have accidentally gotten around the world in shipments of oysters. These drills, as they are sometimes called, turn the front of their foot inside out and apply an acid substance that dissolves the oyster shell's hard minerals. The snail drills for about one minute and then applies acid for about 30 minutes, repeating the cycle until the shell is pierced. This can take up to eight hours.

Once the murex penetrates the shell, the oyster is toast.

Hawaii hosts only eight murex species, and all usually live in deep water. Still, these snails are familiar, even to those of us who don't know their snails. Murexes are those fancy shells covered with long, spiny projections.

I found boring snails quite interesting. Now I'm off to the beach to look for something my husband never heard of.

Marine science writer Susan Scott's Ocean Watch column
appears weekly in the Star-Bulletin. Contact her at

E-mail to City Desk

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