The sea swallow, Glaucus atlanticus, is a type of pelagic nudibranch. Pelagic means it lives in the open ocean, and being a nudibranch makes it a member of the phylum Mollusca. They are also called blue dragons, blue sea slugs, and a few other similar names. Because of where they live, these striking creatures are not frequently seen, so we were lucky to encounter a few of them after a dive at Batsata Maze in the south western part of False Bay, just south of Smitswinkel Bay.
The blue patterned side of this nudibranch that is visible when viewed from above is actually its underside. The top surface of the animal, which points down, is counter-shaded (like a great white shark). It is a greyish silver colour to blend in with the surface of the sea when viewed from underwater.
Sea swallows suck air into a gas-filled sack inside their bodies, for buoyancy. They prey on blue bottles (also called Portuguese man o’war) and retain and concentrate the blue bottles’ venom in their bodies for use against their own enemies. This makes them extremely venomous with the potential to sting badly.
Luckily the intrepid Carel leaped into the water to scoop one into a cup and we could all take a closer look (don’t touch!) on the boat and get some photographs. Afterwards, our visitor was returned safely to the ocean.
They are widely distributed through many of the world’s oceans, and sometimes wash up on the beaches in False Bay. They are unusual, but not earth-shatteringly rare. If we were more social media savvy we would have managed to use this sighting to manufacture the kind of hysteria generated by that facebook page whose title expresses an intense and profane love for “science“, or a few other media channels. But we’re not, so you get this blog post!
If you are looking for a marine life reference, first prize for Capetonians is A Field Guide to the Marine Animals of the Cape Peninsula, otherwise the Two Oceans guide.