Gaskets. At least that’s what they look like. Is this yet another example of litter dumped on the ocean floor? Happily, in this instance, the rubbery-looking things aren’t refuse from human activity but instead house the next generation of a distinctive marine snail found along the sand flats off La Jolla Shores. While not really rubber, gasketlike “sand collars” are made up of an egg sandwich creation not assembled at any deli counter: Press a layer of eggs between two layers of sand cemented together by mucus. Hold the tomatoes and mayo.
Lewis’s moon snail (Euspira lewisii), a common resident of the La Jolla sand community, has a thick, globular, yellowish-tan shell generally no more than 2 inches in diameter. I find it plowing half-buried through the sand, which it does with an impressively large foot that nearly envelopes the shell. When danger is at hand, it doesn’t seem possible for the moon snail to tuck into its shell, but it does. Considering that it has up to four times as much soft tissue volume as the shell, it seems a trick only solved by Houdini were it not for the snail’s ingenious sprinkler system. When disturbed, the snail contracts, squeezing water out of pores found at the back of the foot that open to the outside. The system is self-regulating, with the snail deciding whether seawater will be taken in, squirted out or moved around the tissues. The only flaw in this arrangement is the limited time a snail can be cooped up in the shell, as tight quarters don’t permit breathing room.
The multitasking foot is also covered with tiny hairs (cilia) and mucus glands, which propel the animal forward over the shifting bottom. The cilia beat from forward to backward so the animal forges ahead at about the same time sand particles are swept back over the foot. In another mode of travel, a moon snail uses muscular waves by contracting muscles on the sides of the foot to push forward. Both forms of locomotion may be employed simultaneously.
The versatile foot can also transform into a grasping appendage to hold prey. Adult moon snails are strict carnivores of various bivalve mollusks but have a special fondness for clams. The telltale sign of a moon snail victim is a clam shell displaying a relatively large, countersunk-looking hole with clean, beveled edges.
To find a moon snail in the act, I follow its zigzagging tracks. Why the snail’s sand tracks do not follow a straight line I do not know, but once prey is found, a moon snail doesn’t vacillate. It clamps its foot around the shell and drills a hole using a special accessory organ. Alternately, it rasps away at the shell using a long, ribbonlike tongue layered with teeth (a radula) that projects from its mouth opening. To aid the process, a moon snail’s drilling device also releases an enzyme to soften the prey’s shell. With all this demolition under way, it’s only a matter of time before the bivalve flesh is available for consumption.
A moon snail is particularly ubiquitous during spring and summer when it reproduces. On my last dive, I saw no less than eight moon collars scattered on the sand bottom and at least as many snails. Because the sand collars are extruded as a wide band that curves around the snail shell and foot, the collar is twice the diameter as the animal that laid it and is an open collar, giving the appearance of being somehow broken. The collar holds a layer of a half-million or so microscopic eggs that hatch to larvae while still in their sand””mucus sandwich. Here they swim around while continuing to develop until the collar disintegrates. The newbie snails then settle out on green algae to first graze on diatoms that coat the algae’s surface and later to eat the algae itself. After about a half-year, young moon snails shift from being vegetarians to carnivores, whereby they take up sand travel to satisfy a newly developing yen for clams.
” Judith Lea Garfield, biologist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. www.judith.garfield.org. Questions, comments or suggestions? E-mail [email protected].