Biology/Natural History: This snail, which is one of the largest to be found intertidally in the Northwest, is an active foraging predator on mud flats and sandy bottoms. When the animal is fully expanded out of its shell it is so large that it covers almost the entire shell and looks as if it couldn't possibly fit into the shell. Much of the expansion is due to the uptake of water, however, and if the animal is gently disturbed it will slowly release water from sinuses in the mantle and foot and contract into its shell, finally closing a horny operculum. It does not usually stay inside the shell long because it cannot breathe. It crawls across sandflats and mudflats with its huge foot partly extended in front of the shell like a snowplow, pushing through the sediments in search of clams. It may burrow at least 10 cm into the sand. Bivalve prey include Mya truncata,Tresus capax, Saxidomus gigantea, and Protothaca staminea clams but don't seem to seek out cockles Clinocardium nuttallii which are found in the same areas, probably because their shells are thicker. The moon snail can bore about 1/2 mm per day. Some clams are found with characteristic, "countersunk" drill holes through the shell made by the radula; while with others the snail seems to simply surround the clam with its enormous foot and wait until the clam suffocates enough to open up. Predators on Euspira lewisii include the sunflower star Pycnopodia helianthoides. It quickly withdraws its foot when it contacts Pycnopodia. Enteroctopus dofleini may be another predator, and the moon snails themselves may sometimes be cannibalistic. Eggs are laid in a distinctive gelatinous coil or collar shaped as if it were formed around the top of a canning jar. The egg mass is a firm gelatin with thousands of embedded eggs. Sand is also embedded in the mass, giving it a sandy texture. The coil is left on the sand. Eggs hatch into free-swimming, nonfeeding veliger larvae in midsummer. Males are smaller than females, and females can live up to 14 years.

Shells from these snails are abundant in shell middens left by native Americans. The species can become toxic if they consume toxic clams during a red tide. They are also a favorite shell for large hermit crabs.