Thousands of the creatures, known as velella velella and commonly called "by-the-wind sailors," recently piled up at Baker Beach in San Francisco, forming a bluish, translucent layer across the sand.
This is not the first time the creatures have appeared along the coast, but the sight remains striking.
The name "by-the-wind sailors" refers to the strong winds that carry them ashore.
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Cousins to jellyfish, they have small sail-like structures that stick up and allow ocean currents to move them along.
Some years, large numbers of velella velella wash up on beaches, often sparking questions from beachgoers.
Experts say the appearance of the creatures is a natural occurrence.
This spring, they have been seen along the Pacific Coast from Santa Barbara to San Francisco, Stinson Beach and north to Oregon.
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In a few weeks, experts say the creatures will die off and when that happens, they can emit a fishy smell.
Marine experts say velella velella are harmless to humans, but they caution pet owners to be careful.
If dogs consume them, the creatures can cause gastrointestinal distress and may also irritate a dog's skin or eyes if pets roll in them.