I am quite familiar with the nudibranchs along the Northern California coast, but am unfamiliar with this Glaucus species (other than the beautiful pictures I've seen posted online). I decided some research was in order...
First, where is this guy found? Many of the beautiful nudibranch species are found in Indonesian or Philippine waters, but not this flamboyant blue sea slug. Its distribution is quite large and they can be found in three oceans, but unfortunately nowhere near me. This slug can be found around Europe, Eastern and Southern South Africa, and along the East Coast of Australia and Mozambique; distributed in the Pacific, Atlantic, and Indian Oceans.
One of the first things I read while researching this cute little animal is that it spends the majority of its time floating upside down on the surface of the water. What?! That is awesome! This species moves at the whim of the currents, behaving like plankton. According to Encyclopedia of Life, this little critter maintains its surface position by swallowing an air bubble. The majority of species I'm familiar with exhibit very different behavior. They have the ability to swim but tend to hang out in the benthos of more shallow regions, near their prey items of sponges, anemones, and hydroids.
One thing Glaucus atlanticus has in common with other nudibrachs is what it eats. Many nubdibranchs feed on cnidarians (anemones, jellyfish, and hydroids), not only for nutrition but also to steal their defense mechanisms. Cnidarians have stinging cells that help them capture their own food. These stinging cells are partly what cause the pain from certain jellyfish species and why it feels like a sea anemone is sucking your fingers. Normally, these cells are triggered open upon touch and act like miniature harpoons for defense or food capture. Nudibranchs are able to digest these cells untriggered and pass them to their cerata, or gills, that are exposed on their back. ("Nudibranch" literally means naked gills because their gills are exposed on their backs.) A favorite prey item of Glaucus atlanticus is the Portuguese Man-O-War. These jellyfish are highly toxic to humans because they have both stinging cells, or nematocysts, and a poisonous toxin. This makes anything that can eat them a badass! And Glaucus atlanticus is only 3cm long!
I'm getting ahead of myself. I was more interested in where Glaucus atlanticus was located ("atlanticus" was a hint!) than the common name. It turns out there are many common names: Blue Glaucus, Blue Ocean Slug, Blue Sea Slug, Sea Swallow, and at least one person has called it the Blue Dragon Sea Slug. I like name Blue Dragon Sea Slug best but it isn't an official name of the species.
What a beautiful and interesting creature! Here are a couple photos from google images:
A great link: Glaucus Atlanticus on Encyclopedia of Life
And, a link to the necklace if you would like to purchase one: http://ilovesciencestore.com/glaucus-atlanticus-nudibranch-pendant.html
