This, is a Glaucus atlanticus.
Also known as the blue dragon, this small slug goes by many other names including sea
swallow, blue angel, blue glaucus, and blue sea slug.
These sea slugs are pelagic: they float upside down by using the surface tension of the water
to stay up, where they are carried along by the winds and ocean currents.
Glaucus atlanticus makes use of countershading: where the blue side of their body faces upwards,
blending in with the blue of the water, while the silver side faces downwards, blending
in with the sunlight reflecting on the ocean's surface when viewed upwards underwater.
Growing up to 3cm (1.2 in) in length, this slug occurs throughout the world's oceans,
in temperate and tropical waters.
Even though they live on the open ocean, they sometimes accidentally wash up onto the shore,
and therefore they may be found on beaches.
Glaucus atlanticus feed on other pelagic creatures, including the Portuguese man o' war and other
venomous siphonophores.
They can move toward prey or mates by using their cerata to make slow swimming movements.
Occasionally, individuals attack and eat other individuals in captivity.
Glaucus atlanticus is able to feed on the Portuguese man o' war due to its immunity
to the venomous nematocysts.
It consumes the entire organism and appears to select and store the most venomous nematocysts
for its own use.
The nematocysts are collected in specialized sacs at the tip of its cerata, the thin feather-like
"fingers" on its body.
Because Glaucus concentrates the venom, it can produce a more powerful and deadly sting
than the Man o' War on which it feeds.
In fact, humans handling them may receive a very painful and potentially dangerous sting.
Like almost all heterobranchs, Glaucus atlanticus is a hermaphrodite, having both male and female
reproductive organs.
Unlike most nudibranchs, which mate with their right sides facing, these slugs mate with
ventral sides facing.
After mating, both animals produce egg strings.
Studies have indicated that the Glaucus atlanticus is not globally panmictic but is localized
within ocean basins.
Gene flow among Afro-Eurasian and American populations is thus hindered by physical obstructions
and water temperatures in the Arctic and Southern Oceans.