Far from the shore, out in the vast expanses of the open seas lives a group of unusual animals, collectively known as ‘neuston’.
Their home is the giant, two-dimensional skin of the sea, which forms a link between the atmosphere and the ocean.
Within this family, one of the most curious creatures is a type of sea slug, or nudibranch, more commonly known as blue dragons, sea swallows or blue glaucus, thanks to their scientific name Glaucus atlanticus.
Blue dragons spend their lives hanging down from the surface, buoyed by a bubble of air that they swallow. To keep hidden from predators, they use a two-sided biological trick known as countershading.
Hanging upside-down, the underside of their body is vivid blue, blending into the ocean below and hiding from winged predators above.
Their other side, the side that hangs down from the surface, has silvery stripes like the shimmering surface of the sea, helping them disappear from swimming predators looking up.
Overall, blue dragons are odd-looking for sea slugs. The main part of their 3cm (0.4in)-long body is sluggy enough, but poking out sideways are three pairs of splayed appendages that look like long fingers of different lengths.
These are not for waving or swimming. They’re a part of the sea slug anatomy known as cerata, which essentially act as second gills, extending the gut and respiratory system outside of their bodies to help them breathe.
Blue dragons, like many sea slug species, also use their cerata as weapons. They’re vicious hunters, with their main prey being other blue-tinted members of the neuston, including Portuguese man-o-wars (Physalia physalis) and jellyfish-like stinging creatures such as blue buttons (Porpita porpita) and by-the-wind sailors (Velella velella).
Blue dragons can bite off chunks of these creatures without getting stung.
Remarkably, the sea slugs can then repurpose their prey’s stingers, keeping them intact and pushing them to the surface of their cerata.
Then, when they’re threatened by predators, they can launch the stingers at them as a dangerous deterrent.
A modern threat puts the blue dragons and other members of the neuston family at risk. Surveys between Hawaii and California have shown that they occupy the same remote parts of the Pacific Ocean as the notorious Pacific Garbage Patch, where floating plastic debris is swept together by a swirling oceanic current.
One method of dealing with this plastic pollution is to sling a net between two ships to skim debris from the sea surface. This process could also be catching large numbers of neustons, however.
The full ecological impact of this process is not yet fully understood, but it could have wider implications for the oceanic food web – these animals are an important food source for many other marine species, including sea turtles and seabirds.
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