Seashell Identification: One Shell or Two

Every shell on the beach answers this first: one shell or two?

The National Park Service at Point Reyes puts it plainly — univalves are animals with one shell, such as a snail, and bivalves are mollusks with two shells, such as clams and oysters. NOAA describes a bivalve as having a two-part hinged shell containing a soft-bodied invertebrate, and estimates about 9,200 bivalve species worldwide, from tropical waters to the Arctic and the deep sea.

Both build their shells the same way. An internal organ called the mantle secretes calcium carbonate, so the shell expands as the animal inside grows. That is why a shell records its own growth in concentric lines or ridges.

So start there, then follow the branch that fits: coils and an opening for a gastropod, or a hinge and a pair of matching halves for a bivalve.

A snail shell is a coiled tube, and the features worth naming are all positions along that coil: the spire at the pointed end, the whorls that wind around it, the body whorl as the largest and last turn, and the aperture, the opening the animal came out of.

The most useful thing you can check is the direction of coiling. Hold the shell with the spire up and the aperture facing you. If the opening is on the right, the shell is dextral, which covers the great majority of species. If it is on the left, the shell is sinistral, and that is unusual enough to narrow the answer immediately.

Texas Parks and Wildlife's example is the lightning whelk, usually called left-handed, with a counterclockwise spiral. TPWD records it at 2.5 to 16 inches long, off-white to tan or gray with narrow brown lightning streaks running from top to bottom and white inside, living on the bottom of shallow bays in sand or mud near shoalgrass or turtlegrass, and found along the Gulf Coast from North Carolina to Texas. It is the official Texas state shell.

With a bivalve you are usually holding half of the animal's shell, so the first move is to work out which half and what the missing one looked like.

Features to record:

• The hinge along the straight edge, and whether it carries teeth or a ligament pit
• Ribs — radiating out from the hinge, or concentric growth lines, or both
• The umbo, the raised beak near the hinge where growth started
• Interior color and scars where the muscles attached
• Outline: round, oval, fan-shaped, elongated, or wedge-shaped

NOAA notes bivalves are filter feeders that gather food through their gills, which is why so many of them are burrowers in sand and mud — and why they wash up in numbers after storms.

One caution: matching a shell to a photograph from the wrong coast is the commonest error. Note where you were. Point Reyes lists mussels, clams and oysters as the local bivalves and abalone among the gastropods, while the Gulf Coast list looks completely different.

A shell with the animal still in it is a living mollusk, and in many places taking one is regulated.

Florida's rules, from the Fish and Wildlife Conservation Commission, are a good example of how specific this gets:

• A recreational saltwater fishing license is required to harvest a sea shell containing a living organism, even from shore
• In Lee County, you may only harvest live shells if they are oysters, hard clams (quahogs), sunray venus clams or coquinas
• In Manatee County, a daily possession limit of two shells containing live organisms per species applies, apart from those same four
• All harvest of the Bahama starfish (Oreaster reticulatus) is prohibited
• Living queen conch may not be possessed; empty shells collected naturally are allowed

National parks and seashores have their own rules. The National Park Service at Point Reyes directs visitors to its fishing and Marine Life Protection Act pages for where collecting bivalves is permitted and what the harvest limits are, and notes stringent restrictions on abalone collection in northern California.

The simple habit: if anything is inside, or a hermit crab is using it, put it back. Photograph it instead — photograph it and the app identifies it.