Snails of the tide line

The shore has snails that never come inland, and the intertidal zone shows, very plainly, what a shell is for. Periwinkles sit on rock through the ebb. A thick shell resists a crab or a gull, and a tight grip, or an operculum pulled shut, resists drying for the hours the tide is away. Dog whelks share that zone as predators, working through barnacles and mussels by drilling or prying. They are snails too. The hunting is slow, which suits a prey that is not going anywhere.

Limpets are the conical relatives people forget to count as snails. A limpet’s low shell and broad foot grind a home scar into the rock. It leaves the scar to graze a film of algae and returns to it. The fit between shell and stone is the seal that holds water through the ebb. Pry one off to look at the underside and you have broken that fit. A limpet dumped back onto a flat piece of rock, slightly aside from its scar, is easier for a gull to take than the animal you found. Looking does not require the pry bar of a thumbnail.

Shells in the strandline are a catalogue of what has died, been eaten, or been tumbled out of a population. They are not the population. Live snails are the ones still clamped, still moving in a pool, or still sealed with the operculum drawn in. If you turn a stone to see them, turn it back the same way up. The community underneath is arranged around shade and moisture. An overturned stone is a worse habitat until the sea rearranges it, and the sea may not come as high tomorrow.

Salt water is the medium these animals are built for, and the same salt kills a garden snail. One common name covers both. The name is older than any careful classification, which is a useful thing to remember before you treat all snails as one kind of problem.