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Why are there so many sand dollars on Ocean Beach?

Sand dollars washed up along Ocean Beach / Photo: Angela Johnston

Listener Katie Taylor asked us why there are so many sand dollars on Ocean Beach, and if our sand dollar population is healthy. 

Sometimes if you walk along Ocean Beach, it will be littered with sand dollars. They’re are actually flat sea urchins, and here they’re white — because they’re skeletons.

“All the sand dollars that wash up here are dead, or very nearly so,” says Rich Mooi, a curator of invertebrate zoology at the California Academy of Sciences. Mooi says that there are millions of sand dollars living along Ocean Beach.

But why? As it turns out, the conditions at Ocean Beach provide a “perfect storm” for sand dollar populations to thrive. Rich local currents bring in loads of nutrients for sand dollars offshore, where they live for around a decade.

When sand dollars die, they dislodge from the bottom of the ocean. The strong surf pulls them inland, and our smooth shoreline helps keep them fully intact as they wash ashore.

"It's a gentle place for dead sand dollars to go find their final resting place," Mooi says.

Sand dollars can tell us a lot about the state of the ocean. Since the insides of their bodies are exposed to seawater, they’re sensitive to small changes in ocean chemistry. But Mooi says that for now, Ocean Beach’s sand dollars seem to be healthy.

So, when you see all those beautiful sand dollars, you can enjoy them, knowing it’s all part of a healthy life cycle.

Crosscurrents San FranciscoHey Area