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Limpets

For many years, I assumed nearshore rocks and boulders became pockmarked from millennia of steadfast hammering by the surf. I never imagined that very small, seemingly inconsequential mollusks called limpets are responsible.

La Jolla Cove hosts a variety of life that clings to the rocks, and these include 2-inch-long owl limpets (Lottia giagantea) that occupy home scars (depressions) on rocks. The depressions may be a good fit or exceed the circumference of their shells.

How can soft-bodied animals make such an impression on a rock's surface, and why are the grooves different sizes and depths? The answer to the first question is succinctly explained by their name: "Limpet" means lick-rock, and that's exactly what they do. Sounds impossible, and it would be with a human tongue, but owl limpets are endowed with 150 rows of teeth on their oral appendage. Called a radula (sandpaperlike tongue), it has the necessary tools to rasp away at surfaces.

The limpets' recessed abodes vary in depth and diameter compared to the animals' sizes because limpets don't necessarily get their groove on from scratch. Hailing from the Cretaceous period (around 100 million years ago), limpets take advantage of their long history by leaving a legacy to the next generation once their life reaches its inevitable conclusion (10 to 15 years). Over the years, successive limpets add to the depressions with their own signature rasping. This changing-of-the-guard strategy boosts survival of future limpets because younger, smaller limpets clamp down on a recessed scar instead of facing the wrath of a fully exposed rock face. Consequently, they are less likely to be yanked off the rock by relentless pummeling from breaking waves. It also means that larger, deeper concavities are likely thousands of years old.

Owl limpets also use their radula as a gardening tool, feeding utensil and property defender. They farm their own food, an algal species they staunchly protect against other limpets. Clearing their territory of anything besides their food source encourages the algae to develop into a thick film just right for grazing. The farmers run a fine line between filling their bellies and overharvesting the crop, and this must take into account loss from stealing. Smaller limpets lacking their own territory raid the plots of those who do until the proprietor sidles over and pushes out the freeloader "” using its radula, of course.

Not all limpets face the same daily struggle integral to surf zone existence. Kelp limpets (Acmaea insessa) enjoy resort living compared with their rock-rasping brethren, but they still require a radula for survival. Only a half-inch long, kelp limpets settle onto feather boa kelp (Egregia menziesiis), a large, brown seaweed found a bit further off shore, and rasp away on the central stemlike part called a stipe (compared with rasping rock, this must be like licking an ice cream cone). When I come across a feather boa, invariably I find kelp limpets in depressions all along the stipe interspersed with holes eaten through by limpets. Kelp limpets get a two-for-one with this approach: The feather boa acts as both a home and food supply. No territorial farmer here, once a limpet eats a hole through the stipe, it creeps off to fresh terrain. 

As with owl limpets, young kelp limpets grow faster and survive longer if they settle on scars made by past limpets. This limpet species then benefits from enhanced security against surge and current.

On the surface, limpets are mundane-looking creatures, so they don't get much attention from most tidepoolers. Personally, I'm a huge fan because I appreciate their physical capabilities and resulting handiwork. In fact, every time I notice rocks and boulders with telltale scars, I am compelled to trace my fingers along an empty depression or two, knowing I have encountered a true archeological find.

— 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 jgarfield@ucsd.edu.