What are those mysterious “blue jellyfish” washing ashore in California?

In the spring of 2023, beachgoers from Central to Southern California were presented with the sight of massive numbers of blue and bleached “jellyfish” washed up onto the sand.

So extensive were the little plastic-bag-looking heaps in some places that whole swaths of strand looked blue: “like somebody put grape jelly all over the whole beach,” as College of Marin biology professor Joe Mueller described to Business Insider.

To many eyes, the creatures producing such remarkable beach-wrack appear to be jellyfish indeed—maybe ones capable of delivering nasty stings.

But, in fact, the countless little bodies littering the California beachfront in April 2023—not really an unusual event, but one rather frequently observed along the U.S. West Coast and in other areas such as the Mediterranean—were not true jellyfish at all.

The tiny beastie in question was, in fact, an only-very-distantly-related organism known as the by-the-wind sailor.

Jymm, Public domain, via Wikimedia Commons

What exactly is this jellyfish lookalike? And if you see such a display on the beach, should you be concerned about painful, maybe even life-threatening, stings?

Let’s dig into the details! (Including some you didn’t even know you wanted to know!)

Velella: The By-the-Wind Sailor

The by-the-wind sailor, Velella velella, is the only known species within its genus, and is widely referred to simply as the velella.

Other nicknames include “little sail” and “sea raft.”

Despite superficial resemblance, the by-the-wind sailor is (as we’ve established) not a true jellyfish.

(A common jellyfish myth!)

It does, however, belong to the same great taxonomic phylum as true sea jellies: Cnidaria.

Other cnidarians include everything from corals and sea anemones to box jellyfish—the cubozoans, belonging to a separate class from true jellyfish, scyphozoans.

Velellas are cnidarians in Class Hydrozoa, which also includes freshwater “jellyfish” and, most famously, the Portuguese man-o’-war.

Indeed, by-the-wind sailors and Portuguese man-o’-wars share a lot of physical characteristics and may be mistaken for one another, though the man-o’-war brandishes much longer tentacles (and, as we’ll get to, packs a heck of a lot more venomous punch).

Like the Portuguese man-o’-war, the velella in its most familiar life stage encompasses a gas-filled float topped by a transparent sail-like crest and, dangling below, short tentacles armed with stinging cells called nematocysts.

All About the Velella: Individual Organism or Collective?

By Zhengan – Own work, CC BY-SA 4.0

The float allows the by-the-wind sailor to, well, float: right at the surface, protruding out of the water, with the stiff sail catching the wind and thus propelling the organism around.

(This on-the-surface lifestyle makes the velella, ecologically speaking, part of the neuston guild, other examples of which include everything from Sargassum seaweed to freshwater creatures such as water striders and fishing spiders.)

Velellas are commonly thought of not as single, individual organisms but collectives composed of a colony of polyps, one of the life stages of the creature.

These individual polyps can also be called zooids, and several different categories of zooids with different roles—from feeding to self-defense—compose these sorts of colonial organisms.

The digestive tract links them into one superunit. That’s also the colonial blueprint of the Portuguese man-o’-war.

Some authorities, however, contend that recent research leans more toward the by-the-wind sailor as a complex individual organism after all, not a colonial one like the man-o’-war.

Regardless, by-the-wind sailors are carnivorous, using their stinging tentacles to passively prey on zooplankton and larval fish as they drift about.

While they aren’t widely preyed upon themselves, they certainly have a few notable predators.

Those include the amazing-looking pelagic nudibranch or sea slug known as the blue dragon, which floats along in the neuston ecosystem and hunts not only velellas but also Portuguese man-o’-wars, from which the sea slug accumulates potent stinging cells useful in subduing prey and warding off predators. (Word to the wise: Don’t touch a blue dragon!).

Another remarkable velella-eater is the violet sea snail, which achieves its buoyancy using trapped air bubbles.

Blue dragons and violet sea snails are diminutive hunters, but the by-the-wind sailor is also vulnerable to much heftier organisms. Indeed, it’s a favored snack of the very biggest bony fish in the sea: the ocean sunfish or mola, a bizarre-looking giant with a vertically flattened form that may weigh over a ton.

Sea turtles such as the enormous leatherback will also munch by-the-wind sailors, and those that are driven into shore may be gobbled up by gulls, waterfowl, and various other undiscerning critters.

“Left-handed” and “Right-handed” By-the-Wind Sailors

Two main body designs of by-the-wind sailors are known, distinguished by the orientation of the sail.

The sail doesn’t run directly down the lengthwise centerline of the velella; it’s either angled slightly right-of-center or slightly left-of-center, resulting in what are sometimes called “right-handed” and “left-handed” by-the-wind sailors.

As with the sail of a sailboat, these different orientations cause the velella to tack to one side or the other of the prevailing wind.

Left-handed velellas, with the sail angled from the upper left to the lower right of the velella, appear to be more common than right-handed ones, and while some have speculated that the dominant sail orientation in a given population depends on which side of an ocean basin it’s found, the evidence isn’t clear.

Velella Washups & Blooms Explained

By-the-wind sailors are designed for offshore, open-ocean life: as, for example, within the great midocean gyres (closed-loop marine circulations).

But by nature they’re at the mercy of the winds, and the right weather patterns can steer them inshore.

Seasonal shifts in wind and wave patterns, for example, often bring at least some velellas onto beaches of the U.S. West Coast in the springtime.

And big storms (such as California had experienced before its 2023 velella event) may transport larger-than-usual numbers in.

Population fluctuations in local populations of by-the-wind sailors may also mean there are simply many more of these “little sails” out at sea, with consequently greater potential for major blooms to wash ashore.

Such blooms can see unbelievably staggering numbers of little beached velella carcasses.

How staggering? During a single two-week period within the 2015-2019 bloom along the West Coast, dead velellas piled up in a grisly wrack along better than 620 miles of continuous beachfront, according to the University of Washington.

Big masses of beached velellas are either reclaimed by the waves and tides or destined mainly to rot (though a few scavengers, such as gulls, might find some repast among them).

And the vivid blue of a freshly washed-up velella doesn’t last: The bodies dry out and bleach into transparency, their look often likened to cellophane.

While, as mentioned, the more-or-less yearly washups of by-the-wind sailors on the West Coast in the spring—typically of much lesser magnitude—are explained by seasonal shifts in winds that drive the hydrozoans inshore, there’s plenty of interest in why the epic blooms of millions, billions, even estimated trillions of “little sails” happen.

Namely, scientists are interested in what sort of larger marine triggers—and maybe what broad marine trends—they reflect.

Some two decades of data on West Coast velella washups—much of it coming from the general public in a clear demonstration of the value of citizen science, and analyzed by the University of Washington—suggests that large blooms are commonly associated with warmer winters. The existence of an abnormal large and persistent zone of warm water off the West Coast nicknamed “the Blob” overlapped with the major velella washups of the 2015-2019 bloom.

Along with toastier waters apparently being conducive to velellas, researchers have speculated that warmer winters also make for calmer ocean conditions, favoring more persistent, packed-together rafts of little sails.

Warmer winter waters may allow for population explosions of velellas out in the ocean off the West Coast, with consequently bigger numbers than normal being washed inshore during the typical spring period.

Are By-the-Wind Sailors Dangerous? Do They Sting?

Huge pileups of by-the-wind sailors can be pretty intimidating-looking, given it looks like a whole beachfront clogged with jellyfish.

But are these distant cnidarian cousins of sea jellies as hazardous?

The short answer: no. The little tentacles of by-the-wind sailors don’t deliver anything close to potent stings, and in fact many people likely wouldn’t notice them.

That doesn’t mean you should willy-nilly handle beached velellas.

Some people do seem to have a more pronounced reaction to their super-mild stings, with minor rashes a possible result. Picking one up by the sail is the recommended method, if you do want to take a closer look.

And you definitely want to avoid touching your eyes or your mouth if you’ve handled one of these little guys, just to be on the safe side.

Really, perhaps the greater risk from large-scale velella washups is the slipperiness of their bodies, if somebody’s taking a beachfront jog or something.

Bear in mind that Portuguese man-o’-wars also sometimes wash up on beaches—though usually tropical and subtropical ones, and not as frequently in large blooms—and they’ve got a much more powerful sting.

Wrapping Up

So—those blue-stained beaches that result when massive blooms of by-the-wind sailors are driven into shore aren’t something to stress about.

In fact, they’re pretty remarkable natural spectacles well worth taking in firsthand.

And heads up: Amid the beachwrack of these cnidarian corpses, you might well spot a variety of other, more rarely seen marine organisms similarly shoved onto the beach face by the surf!

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Hope this helps!