Dive Dry with Dr. Bill

#185: The Moon Snail

This week I am returning you to the sandy bottom habitats off our island (and elsewhere in southern California, of course). Here we will take a peek at a relative of your common everyday garden snail. Unlike its strictly vegetarian terrestrial cousin, this marine species is a vicious predator. No namby pamby salad or tofu for this critter! It would much rather have a good clam chowder, Manhattan or New England style. I'm referring to the moon snail.

Although there are two species I'm aware of in our waters, the more common one is the Lewis' moon snail. My research revealed that this snail was named for Meriwether Lewis of the Lewis and Clark Expedition who found them at the mouth of the Columbia River. It is the largest of the moon snail species. This mollusc species is found on sand and mud bottoms in protected bays and to depths of 150 meters (nearly 500 feet) on soft bottoms off open coasts. Since I can't dive that deep, I located one at about 30 feet for this column. This is a widely distributed West Coast species, found from Vancouver Island in British Columbia to San Geronimo Island off Baja California.

The moon snail has a large globe-shaped shell that can be up to 5" in diameter. Their muscular foot is even larger and gives the snail the stable base necessary to remain upright on the shifting sands. The large surface area of the foot not only assists in movement, it can also fold over to grasp potential prey. Shame on the moon snail... eating with their feet! How uncivilized can you be. The foot is equipped with cilia and mucus glands. The cilia beat against the sand to propel the animal forward, and the secretion from the mucus glands creates a much slicker surface on which to glide. These snails can also move their muscular feet in waves almost like a caterpillar if faster motion is required.

The fully extended foot of the moon snail can be up to four times the volume of its shell. In fact, the "foot" may measure a foot across! Those of you who travel frequently will realize it isn't easy packing that much into a suitcase unless you jump up and down on top of it. The moon snail solves this dilemma in a different way. There is a series of spongy cavities or sinuses in the foot that allow it to take up sea water, move it from sinus to sinus and expel it. When the snail needs to draw its foot into the shell, it simply expels the excess water. If the foot won't fit entirely into the shoe... er, I mean shell... the snail may dig it into the sand or mud to protect it.

Moon snails feed primarily on clams which they are able to sense chemically at short distances. The muscular foot is used to dig into the soft substrate to reach the prey. It is then able to grasp the clam with its foot to hold it steady, and uses its radula to drill a hole into the clam's shell. However, this would not be possible without secretions from the "accessory boring organ" which apparently soften the shell so the radula can penetrate it. It is believed these secretions dissolve the glue that holds the calcium carbonate crystals of the clam's shell together. Once the hole is created, the snail can penetrate it with its feeding snout or suck the clam's tissues through it. Moon snails apparently have somewhat specific dietary preferences, choosing certain clam species over others... apparently they are very picky eaters.

The largest moon snails tend to be mostly female. Males and females mate and produce an egg mass known as a collar. The collar receives its shape because it is formed against the shell and the foot. A layer of small eggs numbering up to 100,000 is encased in two layers of sand held together by mucus. The eggs hatch into larvae that remain within the collar for several weeks until it disintegrates. One report states they settle to the bottom and another that they feed in the plankton before settling. I guess the scientific verdict is still out on that. The young snails feed primarily on diatoms and algae until they are about half a year old. At this time they switch to their carnivorous diet. Individuals may live for several years.

Moon snail shells have been found in Native American kitchen middens suggesting they were an occasional food source. In researching this column I discovered that even today they are taken for food, and that the California Department of Fish & Game had established a bag limit of five back in the late 1970's. Seagulls are another predator that feeds on them... that is if they can't snatch my pan dulce from my hand or a visitor's french fries from their plate.

© 2006 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, "or "Giant Sea Bass" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Moon snail using foot to crawl along soft sandy bottom; egg collar (lower right) on sand.

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia