ProsobranchiaProsobranchia

Every shell you can name, the conch, the cone, the helmet, the murex spined like a crown, was built by an animal in this group. Prosobranchia is an old label, coined for gastropods whose gills sit in front of the heart, and modern work has taken it apart: it is not one branch of the family tree and no working biologist uses it as one. What it still describes is real enough, the sea snails that kept the coiled shell and, in most cases, the trapdoor that seals them into it.

Almost none of them are grazers. The shelled snails of a coral reef are hunters, and their weapon is the same toothed ribbon other molluscs use to scrape algae. A murex drills a neat round hole through the shell of another mollusc, softening the carbonate chemically and cutting with the radula until it can reach the flesh inside. A helmet shell hunts sea urchins. A triton's trumpet, which grows longer than your forearm, is one of the very few animals that will eat a crown of thorns starfish, spines and all.

The cone snails took it furthest. In a cone, one radular tooth has become a hollow barbed harpoon. The animal loads a single tooth into the tip of a stretchable proboscis and fires it into passing prey with venom running down the shaft, a mixture of dozens of small proteins tuned to shut a nervous system down in seconds. Some fish hunting species also release insulin into the water first, crashing the blood sugar of the fish before they strike. That venom is a pharmacy: one painkiller in hospital use today is a synthetic copy of a cone snail peptide. The larger fish hunters, the geography cone among them, have killed people, and every cone in this sea is a photograph rather than a souvenir.

Thirteen species are listed here, most of them nocturnal, most of them under the sand by day. What you usually meet is the evidence: a plowed track across a sand flat at first light, a drilled hole in an empty valve, or a big shell moving briskly in the wrong direction, which means a hermit crab owns it now.