By-the-wind sailors on the sand near Haystack Rock.

They look a bit like if blue mussels were to shed an outer layer of shell.

The iridescent, disc-shaped creatures that have lately been washing onto nearby shores are called by-the-wind sailors, also known by their scientific name, velella velella.

On a recent walk along Cannon Beach, they dotted the shoreline for miles, their translucent “sails” catching the sunlight and attracting the attention of nearby gulls.

Farther onto the dry sand, they piled up by the thousands, as has been reported over the past few weeks on beaches across the state and as far south as the San Francisco Bay Area.

According to the Oregon Coast Aquarium, by-the-wind sailors are hydrozoans, groups of tiny predatory animals that are distant relatives of corals, sea anemones and jellyfish.

Unable to swim, they drift along the ocean surface, helped along by the “sails” that give them their name and by stinging tentacles that allow them to catch plankton. They’re moved along by winds and currents. Strong westerly gusts can cause them to get stranded on the shore.

Archive photos from The Astorian show that they often appear in late spring. At first, they retain brilliant hues of blue and purple, before losing their color as they dry out in the sun.

Unlike some jellyfish, by-the-wind sailors’ tentacles are unlikely to cause a harmful sting for humans. Still, the aquarium warns beachgoers to keep their distance.