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Moon snail 'unphased' by own fancy footwork

Did someone dump a boatload of broken gaskets off La Jolla Shores? The rubbery-looking things do resemble refuse from human activity, but actually they are the result of many moon snails' (Euspira lewisii) reproductive efforts. A common resident of the local sand community off La Jolla Shores, the moon snail is the largest of its species, reaching about 6 inches long in these parts. The snail's thick, globular, shell (to about 2 inches across) varies in color from cream, to tan, to yellowish, to orange-yellow. Thin brownish striations overlap the base color. Other pastel hues often sneak into the inner spiral regions.

I usually see the snail plowing half-buried through the sand using its impressively large pale-pink foot, a blob of soft tissue about four times the volume of its shell. Because of this size discrepancy, the shell is barely discernible when the foot is fully extended. Unexpectedly then, if danger is afoot, the snail can fully retreat into its shell. I kid you not. Cinderella's stepsisters would envy the moon snail's successful fit into its petite slipper. The feat of the foot is due to the snail's ingenious internal sprinkler system. When the snail contracts, water is squeezed out pores located on the foot that open to the outside. The only flaw in this arrangement is the limited time a snail can be confined because the tight quarters don’t include breathing room.

The foot is also responsible for propelling the animal forward over shifting and ripply sand bottom, a task made possible by tiny hairs (cilia) covering the bottom of the foot. As the cilia beat forward to backward, the animal forages ahead at about the same rate sand particles are swept back over the foot. Muscular waves, contracting muscles on the sides of the foot to push the animal forward, are another form of snail transport. A snail may use the former, latter, or both tactics simultaneously but whatever moves it, mucus glands (also covering the foot's underside) are there to pave the way. Secreting mucus eases a snail's trail so the mollusk glides along with minimum effort over what would otherwise be bumpy, scratchy sand grains.

When prey is nigh, a moon snail's versatile foot transforms into a grasping appendage. Clamping over the victim's shell (adult moon snails have a special fondness for clams), a moon snail suffocates its prey, after which it drills a hole through the shell using a specialized accessory organ. An enzyme released to soften the prey’s shell aids the process. With all this demolition underway, it’s only a matter of time (hours to a few days) before the moon snail gets to the goodies. Telltale evidence of a moon snail meal is an empty shell punctured by a countersunk hole with clean, beveled edges.

Now is the season for moon snail reproduction, so they and their collars are plentiful. Beach walkers are likely to come upon a collar due to the recent uptick in surf and surge. While sand collars look architecturally exotic, they are fundamentally as sophisticated as an egg sandwich, though not the kind assembled at your local deli. This recipe calls for thousands of microscopic-sized eggs pressed into a single layer between two layers of sand grains and cemented together with mucus. Hold the tomato and mayo.

Now about the reason for the collar's “broken” appearance. The sand-mucus-egg concoction is extruded as a wide strip that follows the circular perimeter of the snail's body. Because the excreted mucus hardens quickly, the end product — the collar — holds its curl. In other words, a sand collar is never a closed circle and so is not broken. Externally, the collar looks inert while the inside environment is a hotbed of activity because moon snails don't leave the collar immediately after they hatch. They spend a few weeks swimming between the sand grain layers and continuing to develop. Once the collar disintegrates, tiny moon snails with transparent shells emerge. Survivors seek out green algae to settle where they graze on diatoms coating the algal surface. Later on, they eat the algae itself. When moon snails reach the half-year mark, they shift from being vegetarians to carnivores, taking up sand travel to satisfy a newly developing yen for clams.

— Judith Lea Garfield, naturalist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. Send comments to