Meet the By-the-Wind Sailor

Mini Neptune's Note XIII

Welcome to the Mini Neptunes Notes, a slightly smaller species spotlight for the quirky and the cute creatures who might not be a cetacean, but still deserve to be celebrated on Beached.

Enjoy! 🐙

Imagine tiny blue sailboats, no bigger than the palm of your hand, drifting across the ocean's surface. They aren't toy boats, or a deflated plastic bag, but living beings known as by-the-wind sailors.

Vibrant blue oval discs with sails as delicate as glass, these fascinating marine creatures might look like jellyfish, but they’re actually colonial hydroids, meaning that rather than being one single animal they are a colony of tiny individuals hanging out together and sharing a digestive system. Closely related to the Portuguese man o’ war, by-the-wind sailors are formed of a unique structure of polyps. Some (gonozooids) specialise in feeding and reproduction, and others (dactylozooids) provide protection. 

A creature so intriguing inspires many names, including their official name velella velella, and a few nicknames like sea raft, purple sail, or little sail. Whatever the name, they’re a true mini of the big blue, measuring up to 10cm in length. Their most distinctive feature is the thin, semi-circular fin that rises vertically from their oval-shaped body. This translucent 'sail' is a sophisticated piece of natural engineering that allows the by-the-wind sailor to catch a breeze and propel themselves across the open ocean.

Beneath the surface, tiny tentacles dangle from the underside of their body. These short tentacles are armed with nematocysts – microscopic stinging cells used for capturing prey and defence¹. Their bodies, known as a float, serve as both a buoyancy device and a home for the colony. And its deep blue hue isn't just aesthetically pleasing either — in the open ocean, this colour provides excellent camouflage.

By-the-wind sailors are found in all of the world's temperate and tropical waters, and always at the surface. But their delicate frame and inability to actively swim means they’re at the mercy of winds and currents. Fascinatingly, the diagonal angle of their sail along their body determines whether they will travel left or right of the wind direction. Their presence often indicates the edge of ocean currents, making them valuable indicators of sea conditions for researchers and sailors. Their diet consists primarily of plankton and small fish, which they encounter during their endless drift. Using their tentacles, they paralyse their tiny prey with a mild neurotoxin before drawing it up into their central mouth for digestion, just like jellyfish do. 

However, it’s no surprise to learn that these little mariners aren't at the top of the food chain. Aside from being a source of food for seabirds when they wash ashore, they also fall prey to a variety of other small sea creatures from nudibranchs to sea snails. Interestingly, some of their predators, like the blue sea dragon², are immune to the sting of by-the-wind sailor's and can even absorb the toxin for their own defence. It seems unfair to repurpose the ‘little sail’’s one defence mechanism like that, but such is the circle of life. 

Stranded velella velella.

Speaking of which, the life-cycle of by-the-wind sailors is arguably their most impressive feat with a complex reproductive strategy that involves both asexual and sexual stages and results in colonies being either all male or all female. 

The process begins with the gonozooid polyps (specialised for reproduction) producing thousands of tiny jellyfish, called medusae, through asexual budding. Each medusa is minuscule, measuring just a couple of millimetres tall. Once sexually mature, these medusae release their eggs and sperm into the water. The fertilised eggs develop into a planktonic larva (essentially the tiny individuals needed to form a new colony) called a conaria. This conaria then grows into another by-the-wind sailor, completing the life cycle and allowing vast numbers of offspring to be produced.

This incredible reproductive strategy is crucial for the species' survival, as while each colony can produce thousands of medusae, fully grown colonies are also, very sadly, prone to mass strandings. Due to their dependence on wind patterns, they often wash ashore in staggering numbers, lining hundreds of kilometres of beaches. However, this isn’t the worst thing to happen to by-the-wind sailors as they are facing increasing challenges from changing ocean temperatures and acidification. They are also prone to ingesting microplastics, mistaking them for a tiny meal – this is extremely harmful as plastics bioaccumulate as they move up the food chain. 

By-the-wind sailors are no different to any ocean animal in need of a clean and healthy ocean to thrive.

Photo by Tara Silber, Oregon Bureau of Land Management.

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Amie 🐋

1. The sting of a by-the-wind sailor is generally harmless to humans.

2. aka Glaucus atlanticus.