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A few weeks back a beach in California had lots of this animal (if it is an animal?). Can anyone identify what it is?

They had a distinct blue bottom and a see-through top.

Animal on beach

Animal on beach again

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  • $\begingroup$ Hey, funny seeing this question here. My wife and I just visited a beach in San Diego yesterday and also noticed these creatures. At the time I just Googled "flat animal on beach with fin" and found out it was by-the-wind sailors as someone already answered. They are very alien looking. Wonder what caused them to come to shore in such large numbers. $\endgroup$ Commented May 2, 2023 at 0:28
  • $\begingroup$ With billions of years to work with, evolution comes up with some truly amazing and beautiful creatures. $\endgroup$
    – IconDaemon
    Commented May 2, 2023 at 11:27

1 Answer 1

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By-the-wind sailors

This appears to be Velella (a monospecific genus), commonly referred to as by-the-wind sailor.

  • More specifically, this is Velella velella, and is also less commonly referred to as sea raft, purple sail, or little sail in addition to usually being called by-the-wind sailor or even sometimes simply Velella.

enter image description here

Source: Wikipedia

As can be seen from the above image, this 40-80 mm long [source] organism has a translucent sail with a lower bluish body that includes (not visible in your image or mine) small tentacles -- see below (as well as here):

enter image description here

Source: Wikipedia

For more detailed description, see Scripps Institution of Oceanography:

A clear, chitinous semicircular to triangular sail sticks up above the water with a blue float beneath made of concentric circles of gas-filled chambers. Small tentacles extend below the circular chambers. The sail is angled left or right.

Like the better-known man o' war (see below), this carnivorous hydrazoan (think hydra or other related cnidarians like jellyfish), floats on the water's surface, using wind to direct its small, rigid sail while dangling tentacles below armed with nematocysts catch and stun prey. From Wikipedia (my emphasis):

Having no means of locomotion other than its sail, V. velella is at the mercy of prevailing winds for moving around the seas, and are thereby also subject to mass-strandings on beaches throughout the world. For example, most years in the spring, there is a mass stranding that occurs along the West Coast of North America

  • You can also see a report about these guys washing up on California beaches in mid-April from the Orange County Register.

Educational video of these sailors available here: YouTube


Man o' war' is incorrect

Note: my initial gut reaction was that it was a small, beat-up Portuguese man o' war (Physalia physalis), often referred to as the Pacific man o' war (or even a "blue bottle") in the Pacific Ocean. A colonial marine siphonophore (within class Hydrazoa) made of many smaller "zooids". Like Velella, it also floats on the water surface with its "pneumatophore" (a gas-filled bladder that forms a dorsal "sail" that can be pushed around by wind above the ocean) while tentacles armed with nematocysts dangle below to capture/stun prey.

  • Like your specimen, the man o' war's sail (or "pneumatophore") appears to be somewhat translucent with the lower portion often taking on a blue coloration (or sometimes purple, pink, or mauve) [source] like your organism. It's also not irregular to have many of these wash ashore all at once, as seen here.

enter image description here

However, the size and morphology (especially small size of the "sail", lack of obviously large bladder, and the clear concentric-circle morphology) of your specimen all pointed to another organism (and clearly toward Velella as stated above).

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  • $\begingroup$ For what it's worth, NPR had a brief story on By-The-Wind-Sailors this week. $\endgroup$
    – keshlam
    Commented May 3, 2023 at 20:59

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