Sure, I’ve found money underwater. For instance, I nabbed a dollar bill off the Cove and pocketed another greenback off the Marine Room, but that’s chump change next to the thousands of dollars strewn across the sand off La Jolla Shores. I refer to the local sand dollar, Dentraster excentricus, a disk typically a couple of inches across. Likened to a silver dollar in shape and size, a living dollar is thickly covered by short, fine, mauve-colored spines that look velvety. By moving the spines together in waves (picture a field of grain slowly blown by wind), a sand dollar can, bit by bit, travel across the sand. As the dollar’s entire side plows through the sand like a shovel, it leaves a distinctive wide, shallow track.
Because no official name exists to describe a group of sand dollars (such as herd, flock, et cetera), I choose “mint.” From a distance, mints of sand dollars appear as long, dark swaths against the pale sand. Find them about 150 yards offshore at depths of about 30 to 55 feet, where, close up, individual dollars are aligned parallel to the shoreline. Their proximity to each other is critical for successful reproduction, because when eggs of some and sperm of others are released directly into the water, they need to find each other for fertilization to take place.
Currents mostly determine whether sand dollars stand up or lie down. In light current, they bury the bottom third of their disk into the sand in an oblique position and slant their exposed upper disk toward the current. When I first noticed the mint members all leaning identically, I thought some divers must have had too much time on their hands. Not true. The dollars’ military formation is a good strategy, because a higher profile permits them to better capture food as it whisks by. However, it’s as fruitless for sand dollars to try standing in a strong current as it would be for us to comfortably stand in a 50-mile-an-hour wind. To resist being blown away, the dollars temporarily abandon their soldier stance and lie down, which morphs their orderly appearance into one of poker chips strewn haphazardly on a table.
Dealing with current isn’t the only challenge for sand dollars. They are easy marks for predators such as fish and sea stars. Even snails a fraction of a dollar’s size take advantage of the nearly immovable feast. One parasite, a tiny, white eulimid snail, lives exclusively on the sand dollar. Using a noselike organ, it drills through a dollar’s skeleton to reach the dollar’s soft body, and then enjoys a lifetime of nutrition. Human predators hunt sand dollars not for consumption but for curios. When dead, their patterned, white skeletons are in demand. Consequently, a sand dollar’s worth has risen over the years, making it the only local dollar presently increasing in value.
If a sand dollar can navigate around life’s pitfalls, it may live 10 years. Over the last month, I’ve seen millions of sand “dots” grow into “dimes” off the Shores. This healthy deposit will hopefully net a hefty return ” and with interest ” as ocean alchemy turns the dimes into dollars.
” 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? Email [email protected].