Six years ago, I wrote the very first post on this blog about some strange little ocean travellers that had blown into Emily Bay. There were bluebottles on the beach and, underwater, more than a hundred violet sea snails, Janthina janthina, scattered across the sand. I had seen the occasional violet snail before, but never anything like that.
While trying to find out what on earth was going on, I discovered the ‘Blue Fleet’, a name used by marine biologist Sir Alister Hardy in The Open Sea in 1956 for the extraordinary collection of blue and violet creatures that spend their lives at the ocean surface. In that first post I rechristened the characters I was meeting on Norfolk Island my ‘Elite Fleet’.
Southern bluebottle, Physalia megalista
Since then I’ve seen plenty more bluebottles and occasionally another violet sea snail, but one of the better-known members of the fleet had always eluded me. Until this morning.
Among the bluebottles washed ashore were these beautiful little blue creatures with a transparent, almost glass-like sail rising from their backs – by-the-wind sailors, Velella velella.
Life at the ocean’s skin
Velella belongs to the pleuston, the remarkable community of organisms that live right at the surface of the ocean, where water meets air. Some float on or straddle that boundary, while others hang immediately beneath it. You will also see the broader term ‘neuston’ used for this surface community – the terminology has varied over the years – but the important bit is that these animals have evolved to make the ocean’s surface their home.
The ‘fleet’ is not a fixed little flotilla that cruises around the ocean together. Its familiar members include bluebottles (Physalia – ours here seem to be the southern bluebottle, Physalia megalista), by-the-wind sailors (Velella), blue buttons (Porpita), violet snails (Janthina) and blue dragons such as Glaucus. They come from quite different branches of the animal kingdom and may appear together or separately, depending on currents, winds and where their drifting lives have taken them.
They also eat each other, which rather ruins any notion of a happy little fleet sailing the seas in company. Violet snails and blue dragons are among the predators of Velella, so when I found all those Janthina back in 2020 I had actually met one of the hunters before I ever saw its prey.
Above and right, violet sea snails, Janthina janthina, Norfolk Island
So what exactly is a by-the-wind sailor?
Despite appearances, the familiar Velella we see floating at the surface is not a true jellyfish. It is a colonial hydrozoan, a relative of jellyfish and of corals within the Cnidaria. Velella is a genus containing just one recognised species, Velella velella. Like other cnidarians, Velella has stinging cells, or nematocysts, which it uses to capture tiny prey. Unlike a bluebottle, however, its sting is generally too weak to cause any significant reaction in people.
While it looks like one animal it is actually a highly integrated colony. Beneath its oval float are specialised polyps, or zooids, with different jobs. Some feed, some are involved in reproduction, and others carry the stinging cells used to capture tiny prey. The colony has a gas-filled float to keep it at the surface and that unmistakable stiff, transparent sail above it.
The name ‘by-the-wind sailor’ is quite literal. Wind catching the sail helps carry Velella across the ocean, although it does not simply travel straight downwind. The shape and angle of the sail cause it to move at an angle to the wind, and there are mirror-image forms whose sails are angled in opposite directions. What I find fascinating is that scientists have even put Velella models in wind tunnels to work out how they sail – exactly the sort of research rabbit hole I love!
Velella velella. Emily Bay, Norfolk Island
When winds and currents bring large numbers towards land, strandings can be spectacular. Recent research on the US west coast has documented repeated pulses of Velella, with wind patterns playing an important role in transporting offshore populations towards shore.
There is another twist too. Although the floating colony is not itself a jellyfish, Velella has a surprisingly complicated life cycle and produces enormous numbers of minute medusae – tiny free-swimming jellyfish-like stages only about a millimetre across. In one study, colonies released hundreds to more than a thousand of these tiny medusae during short laboratory observations. There is still plenty we do not know about what happens during parts of their life at sea.
That is perhaps what I like most about the Elite Fleet. These creatures are easy to dismiss as bits of blue stuff blown onto the beach, but they belong to an entire ecosystem sitting on one of the thinnest and strangest habitats on Earth – the skin of the ocean.
Back in December 2020 I finished my first blog post by saying I would look out for the fleet the next time the wind and currents conspired to blow them into Norfolk Island. It took nearly six years, but this morning a new member finally sailed in.
References
Francis, L. (1991). Sailing downwind: Aerodynamic performance of the Velella sail. Journal of Experimental Biology, 158, 117–132. https://doi.org/10.1242/jeb.158.1.117
Hardy, A. C. (1956). The open sea: Its natural history. Part I: The world of plankton. Collins.
Helm, R. R. (2021). Natural history of neustonic animals in the Sargasso Sea: Reproduction, predation, and behavior of Glaucus atlanticus, Velella velella, and Janthina spp. Marine Biodiversity, 51, 99. https://doi.org/10.1007/s12526-021-01233-5
Helm, R. R. (2024). Animal evolution at the ocean’s water-air interface. Current Biology, 34, R54–R58. https://doi.org/10.1016/j.cub.2023.11.013
Purcell, J. E., Milisenda, G., Rizzo, A., Carrion, S. A., Zampardi, S., Airoldi, S., Zagami, G., Guglielmo, L., & Boero, F. (2015). Digestion and predation rates of zooplankton by the pleustonic hydrozoan Velella velella and widespread blooms in 2013 and 2014. Journal of Plankton Research, 37(5), 1056–1067. https://doi.org/10.1093/plankt/fbv031