I had a computer glitch recently that forced me to reorganize my photos. But with that time-consuming work came a bonus: I found an astonishing picture I forgot I had. During our last snorkeling trip on the Great Barrier Reef, Craig found a snail eating a sea cucumber.

It was an extraordinary scene. Finding a full-grown Triton’s trumpet is rare, and the poor sea cucumber the snail had in its grip was white rather than the more common brown or black.

Triton’s trumpet is the second-largest snail in the world, growing 20 inches long (the largest is Australia’s 28-inch-long giant welk), but the largest in Hawaii. The species is native to the Indian and Pacific oceans. A close relative is found in the Atlantic.

In Greek mythology Triton is the messenger of the sea, a merman with the upper body of a man and the lower body of a fish. To make the waves either rise up or lie down, Triton blows a trumpetlike shell.

Ancient Hawaiians used shells as ceremonial horns, as did other islanders throughout the species’ range. The well-named trumpet shell suited the purpose, either with the pointed end filed off or a hole drilled in the shell’s side.

Although often covered with encrusting algae, the trumpet shell’s exquisite colors and patterns remind some people of pheasant feathers. The inside of the wide opening is reddish-orange with brown stripes similar to tiger fur. A trap door called an operculum slams shut when the animal is startled.

The trumpet snail’s body is as remarkable as its house, having stalked eyes and a tube mouth. At night the snail prowls around on a foot that looks like a leopard skin rug hunting for sea urchins, sea cucumbers and starfish.

Trumpet snails are one of the crown-of-thorns starfish’s few predators. The snail grabs its prey with the muscular foot, injects toxin with the mouth and then bores a hole in the tough skin to suck out the soft insides.

As you might expect, people have killed trumpet snails for ages for their large, lovely shells. The animals are now rare in Hawaii and other parts of the snails’ range.

Even though trumpet snails are legally protected in Indonesia, Australia, Fiji, India and other places (but not the U.S.), enforcement is nonexistent, and trade in trumpet shells thrives. The internet is a pretty snail’s worst nightmare. You can help Triton’s trumpet snails by not buying their shells.

Because our Australian trumpet was 25 feet deep, I might have missed it. My free-diving husband, however, found it, surfaced to fetch my camera and headed back down to take some amazing pictures. When we got home, I bought Craig an underwater camera of his own.

Reach Susan Scott at susanscott.net.