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Impurest's Guide to Animals #32 - Sea Swallow

Wow, who knew 12 hour days in a 12 day week would be so hard. Well really I should because I’ve done them before so…anyway we have Issue 32 of Impurest’s Guide to Animals. Last week the Neon Cuckoo Bee showed nature’s malicious and underhanded side. Continuing the trend of beautiful with a side of sinister is the animal chosen by @knightsofdarkness2. Hope you enjoy.


Issue #32 Sea Swallow


Kingdom – Animalia

Phylum – Mollusca

Class – Gastropoda

[Clade*] – Neudibranchia

Family – Glaucidae

Genus – Glacus

Species – atlanticus

* At the moment the Class Gastropoda is going through a taxanomic shake-up with no easily defined Orders. As such the Sea Swallow could be in the following orders; Aeolidia, Cladobranchia, Dexiarchia, Euthyneura, Hetrobrancha, Neudibranchia or Neudipleura

Related Species - There is one other member of the genus Glacus, a dwarf Neudibranch known as Galcus marginatus (1)

Range - Sea Swallows are found throught the worlds Temperate and Tropical Oceans

The Wings that Sting

The Sea Swallow is a miniscule neudibranch, with an adult size of only 3cm when it’s ‘wings’ are fully outstretched. Unlike it’s (mostly) sea bed or benthic dwelling cousins the Sea Swallow is a free floating animal that uses a gas sac on it’s belly to keep it afloat on the surface of the ocean. Because of this arrangement the animal actually floats upside down. As such the counter shading on the Sea Swallow is reversed when compare to other marine animals, with the darker colours on the belly rather than the surface.


Unlike most terrestrial molluscs all nudibranch are carnivorous and the Sea Swallow is no exception to the rule. Its prey consist of other free floating animals such as the ‘By the Wind Sailor’ (Vellea vellea) and Violet Sea Snail (Janthina janthina), which is rips apart using its toothed radula (2). The Sea Swallow will even take on much larger prey such as the Portuguese Man of War (Physialla physilis)) thanks to it’s immunity to its preys venom. In fact after ripping off the Man O War’s tentacles, the Sea Swallow will store and then super concentrate, the venom in the wing like cereta that run down the flanks, using it as a defensive weapon.


Sea Swallows are hermaphrodites like other gastropods but are unable to self fertilize, thus requiring a partner to mate with. Both animals will lay strings of eggs in the carcass of their prey until they hatch. Larval Sea Swallows have tiny shells which they outgrow when their gas bladders finally develop (3).

Five Fun Sea Swallow Facts

Despite being labelled as ‘Sea Slugs’ the Nudibranchs are their own separate family, completely unrelated to Marine or Terrestrial Slugs

The name Nudibranch comes from the Latin words Nudus (Naked) and the Greek word Brankhia (Gill) referring to the groups exposed respiratory apparatus

The Sea Swallow’s penis is particularly strange as it features a hook on the end. This has probably evolved to bypass the venomous rows of cereta along its partner’s flanks

Misidentification has followed the Sea Swallow since its discovery. For a long time it was thought to be a marine insect and then the larval form of the Angel Shark (Squatina sp)

Despite being un-aggressive the number of stings from Sea Swallows is on the increase due to the popularity of ‘Bluebottle Fights’ in Australia, where the ‘combatants’ throw stranded Sea Swallows at each other, only to receive a painful (potentially life threatening sting) (4)


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2 - Thompson, T. E.; McFarlane, I. D. (2008). "Observations on a collection of Glaucus from the Gulf of Aden with a critical review of published records of Glaucidae (Gastropoda, Opisthobranchia)". Proceedings of the Linnean Society of London 178 (2): 107–123

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Picture References

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And that’s the Sinister Sea Swallow, not to mention its stinging wings of pain. Next week we get back to scheduled issues staring with something slippery down in the dark waters of Slovenia. Until then comment, critique and check out the Bestiary of Past Issues.

Many Thanks

Impurest Cheese