Creature Feature: The Moon Snail, Conqueror of Clams

by Casey Cruikshank

The Moon Snail (Euspira lewisii) is one of the largest intertidal snails that can be found in the Northwest. Found in mud flats and sandy intertidal areas from Ketchikan, Alaska to Baja California, Mexico, their shell can get up to 5.5 inches and the extended body can be up to 12 inches long. Males are smaller than females and they can live up to an impressive 14 years, which is often attributed to their lifestyles as voracious predators and their ability to quickly protect themselves against predation. 

The charismatic Moon Snail on the move. Photo by Casey Cruikshank.

Similar to most other marine snails, the Moon Snail has a muscular foot that is used not only to glide on top of the sediment, but also plow below the surface. Movement is accomplished through waves of muscular contractions passing down the foot. They move with their foot partially extended in front of the shell, acting like a snow plow to push through the sediment and search for prey. This fleshy foot can also do something that most other snails can’t; it fills with water to expand, practically covering it’s large bulbous shell. When they sense danger or are disturbed, they release water from sinuses in the mantle and withdraw the foot into their shell. They seal the opening with something called an operculum (basically a hardened door) so that their vulnerable fleshy foot is fully protected. The moon snail can’t stay fully wrapped up inside of its shell for long because it cannot breath while fully enclosed. 

Moon Snail front. Photo by Casey Cruikshank.

Moon Snails feed on clams but do not seem to search for cockles, which is thought to be related to their thicker shells. When they find a clam they often drag it farther into the sand and envelop it in their inflated foot. A gland on their proboscis secretes enzymes that helps the radula with seven rows of teeth to burrow a hole into the clam shell. They can bore about 1/2mm per day and suck the clam tissues out of the shell over a period of days. Clam shells are often found with a tell-tale “countersunk” drill hole, however it is also thought that the snails sometimes feed on clams by wrapping their foot around the entire shell to suffocate them enough to come out. Their feeding on clams during a red tide can result in an accumulation of poisons in the Moon Snail that can cause paralytic shellfish poisoning to humans if eaten. Hunting of Moon Snails by humans in the past has been prolific, so in 1984 they were protected by California Law: the daily limit is 5 and none are to be taken north of the Golden Gate Bridge. 

A ring of Moon Snail eggs. Photo by Casey Cruikshank

They are large enough that they do not have many predators except for the occasional Sunflower Star attack, though the Moon Snails themselves will occasionally turn cannibalistic. Moon snails emerge from deeper waters to the shallow intertidal habitats in the summer to breed on sandy shorelines. Their eggs are laid in sandy, firm gelatinous collar-shaped masses that contain thousands of embedded eggs. In midsummer the eggs hatch into free-swimming larvae which feed on plankton and ironically often become food for bivalves in the area. These goopy, slow moving friends are quite the site to behold in the intertidal habitat.