Beachgoers on the west coast of the US have noticed an influx of a mysterious blue sea creatures stranded on the sand.

Velella - also known as sea raft, by-the-wind sailor, purple sail, little sail - are gelatinous, oval-shaped critters that live on the surface of the ocean.

The by-the-wind sailor began washing up on beaches in Washington and Oregon more than a month ago.

Since then the mass stranding has taken on the form of an alien-like invasion.

The critters have been showing up as far south as Southern California, and as far north as British Columbia, according to GrindTV.

Velella sting to stun their prey, but are not considered harmful to humans. Mass strandings are sometimes associated with strong warm-water events, such as an El Niño.

David Bader, director of education at the Aquarium of the Pacific in Long Beach, California, told CNN: “Every now and then, the currents and winds will change and these guys will, instead of being pushed out to sea, they actually wind up on the beach.”

By-the-wind sailors feed mostly on plankton and fish eggs, which they catch with their tentacles.

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