In recent years, the population of crown-of-thorns starfish has grown dramatically. One reason is because there are:

Many people have seen the beautiful spiral triton trumpet shell. Unfortunately, most people can see these brown and white shells only for sale in a market. Far fewer people have seen this rare creature in its natural habitat of the coral reefs. At night, this carnivore snail can be found trudging through the reef, hunting for a meal. The triton trumpet uses its sense of smell to find prey.
In recent years, the population of crown-of-thorns starfish has grown dramatically. One reason is because there are:
more triton trumpets, the starfish's main predator
fewer triton trumpets, the starfish's main predator
more triton trumpets, the starfish's main prey
The triton trumpet is the main predator of the crown-of-thorns starfish. Starfish can be harmful to coral reefs, since they prey on the coral polyps. Triton trumpets keep populations of starfish in check.
The shells of the triton trumpet were once used for trumpets.
On the Hawaiian and other Pacific islands, the spiral shells were once used as trumpets in ceremonies.
It's okay to purchase a triton trumpet shell at a market because, by that time, it's already dead, anyway.
When you buy these shells, it simply increases the demand, and more will be taken from their habitat. Similarly, it's better not to take one you see on the beach.
triton trumpet
Scientific name: Charonia tritonis
Size: up to 20 inches long
Habitat: coral reefs along tropical coasts
Diet: sea stars (starfish)
Characteristics: brown and white spotted shell