Impurest's Guide to Animals #119 - By-the-Wind Sailor

And now we’re in May, and at last the weather is finally feeling like it’s something more than frigid. Last week we finally left the Amazon after a brief look at the swamp monster known as the Mata-Mata. This week we head out to the open ocean to go sailing on the waves with the week’s animal. Hope you guys enjoy.


Issue #119 - By-the-Wind Sailor


Kingdom – Animalia

Phylum – Cnidaria

Class – Hydrozoa

Order – Anthomedusa

Family – Porpitidae

Genus – Velella

Species – Velella

Related Species – By-the-Wind Sailors are closely related to the Blue Button (Porpita porpita) (1)

Range By-the-Wind Sailors are found on the surface of the majority of the world’s temperate and tropical oceans


By-the-Wind Sailors are small marine animals, with a dimeter of up to 7cm, and despite looking like jellyfish are in fact members of the siphonophore family. Each seemingly individual By-the-Wind Sailor is formed of multiple polyps, which all are specialised for a certain role, which will be either entirely male, or entirely female. Regardless of gender, the creature’s most distinguishing feature is the dorsal sail that acts as an aerofoil (2) and is the species only mode of locomotion. As such By-the-Wind Sailors are completely at the mercy of the wind which results in mass strandings when the winds and tide drag swarms of these animals onto shore.


Like their larger relatives such as the Portugese Man ‘O’ War (Physalia physalis), By-the-Wind Sailors are carnivorous and capture and subdue plankton with stinging tentacles that hang under the float that makes up this body. While the venom is toxic enough to subdue prey, it has a minimal effect on larger animals, and results in a brief spell of itching and potentially numbness when it comes into contact with humans. By-the-Wind Sailors are preyed on by other floating animals such as the Sea Swallow (Glaucus atlanticus) and the Violet Sea Snail (Janthia janthia) as well as larger jellyfish species.

When the time comes to breed, By-the-Wind Sailors produces thousands of free swimming medusa which are about a millimetre in length and resemble tiny jellyfish. These medusa stay near the surface using photosynthetic bacteria in their bells to fuel their growth to sexual maturity, which usually takes three weeks to a month to occur. From their both male and female medusa release eggs and sperm into the water column, with fertilisation occurring through the movement of the water (3). Sometime later a larvae hatch from the fertilized eggs and drift among the plankton, eventually developing into the floating ‘adult form’ recognisable as By-the-Wind Sailors.

Keys to the Kingdom?

For this issue I decided to do something different, and instead of fun-facts, invasive species and long essays on ecology I have instead written a key, as well as found an obliging animal to describe, as seen below. Your task, should you choose to accept it, is to use the description to find out the animals true identity. Hope you guys enjoy, and be sure to leave some feedback on what you think of this segment.

Animal is has an orange bell with black markings radiating out from a central point. In addition to four mouth tentacles trailing there are numerous smaller feeding tentacles arranged around the bell.



1. Large bell or float with tentacles trailing either below or behind the main structure - 2

Compact, almost oval shape with no visible tentacles – 10

Long chain like structure that appears to be formed of many compartments - 14

2. Float is sail shaped and sticks above the water with tentacles hanging directly under the structure - 3

Large Bell is spherical with tentacles dragging behind the main structure – 4

3. Sail is supported by a large bulbous float with long tentacles hanging down beneath the main structure – Portuguese Man of War (Physalia physalis)

Sail is thin and floating on a small blue or purple disc with very short tentacles hanging below the main structure – By the Wind Sailor (Velella velella)

4. Bell has four or more long mouth tentacles hanging below or behind the main structure as it swims - 5

Bell is connected to two very thin tentacles with multiple fringes hanging off them – Sea Gooseberry (Pleurobrachia pileus)

5. Bell support multiple feeding tentacles that are far longer and more numerous than mouth tentacles - 6

Bell supports long mouth tentacles that are longer than the feeding tentacles - 7

6. Bell is very wide, often orange and supports numerous very long tentacles – Lion’s Mane Jellyfish (Cyanea capillata)

Bell is wide (around 30cm in width), dark blue and supports numerous feeding tentacles – Blue Jellyfish (Cyanea lakarki)

7. Bell features four mouth arms surrounded by numerous visible feeding tentacles – 8

Bell is blue or yellow and features eight mouth arms and no visible feeding tentacles – Barrel Jellyfish (Rhizostoma pulmo)

8. Bell is bulbous with long feeding tentacles surrounding the mouth tentacles – 9

Bell is umbrella like, with four purple horseshoe marking with short feeding tentacles hanging round the edge – Moon Jellyfish (Aurelia aurita)

9. Bell is yellow with dark markings with numerous feeding tentacles surrounding the long mouth tentacles – Compass Jellyfish (Chrysaora hysocella)

Bell is pale blue or white with numerous purple spots across the surface, feeding tentacles are short and hair like – Mauve Stinger (Pelagia nociluca)

10. Visible purple shell nesting under a small raft of bubbles – Violet Sea Snail (Janthina janthina)

Gelatinous body with no external hard structures – 11

11. Body is elongated, and almost torpedo shaped with a visible head and fins – 12

Body is oval shaped with rows of microscopic cilia arranged on ridges leading to an opening at one end of the animal – 13

12. Torpedo shaped body is supported by an internal rod and features a partially developed fish like tail as well as hook like structure on the head – Arrow Worm (Parasagitta setosa)

Body contains no hard parts and has two stubby tentacles on the head as well as two stubby wing like fins – Sea Butterfly (Clione limacina)

13. The body has two fleshy lobes at the end of the body that are used to catch prey – Comb-Jelly (Bolinopsis infundibulum)

Body has no fleshy lobes and instead simply has an opening at one end of its body – Comb-Jelly (Beroe cucmis)

14. Chain like body consists of identical transparent barrel like structures with visible internal organs – Salp (Salpa fusiformis)

Body consists of sting like substances attached to float like structures with no visible internal organs – Siphonophore (Apolemia uvaria)


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2 - McNeill Alexander, R (2002). Principles of Animal Locomotion. Princeton University Press.

3 - A. Brinckmann-Voss (1970). Anthomedusae/Athecatae (Hydrozoa, Cnidaria) of the Mediterranean. Part I. Capitata. Fauna e Flora del Golfo di Napoli 39. Stazione Zoologica. pp. 1–96, 11 pls.

Picture References

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Next week we are doing something a little different with an issue on Sunday instead of Monday to commemorate the 90th Birthday of Sir David Attenborough, with regular service resumed the week after. Until then make sure to critic, comment and suggest future issues as well as making sure you check past issues in Impurest’s Bestiary.

Many Thanks

Impurest Cheese

Want more IGTA? If sailing wasn’t your thing and your prefer sailing then click here to see the tide riding Plough Snail. Or if marine stingers are more your taste click here to face the wrath of the Immortal Jellyfish.