Several years ago, scuba divers were exploring coral reefs around Kumejima Island in Japan's Ryukyu archipelago when they saw what looked like a graveyard of tiny panda skeletons. Well, not exactly skeletons – more like skeletons with the living heads of pandas still attached.
These things are, at most, around 2cm (3–4in) long – about the length of a fingernail. At one end, there’s a white ‘head’ with a black nose spot and two black panda-like eye patches.
Visible through the transparent body is a stack of white horizontal lines, which look like bony ribs. There’s even a black spot lower down that could be a tail. All very strange.
The divers recognised these skeleton creatures were a type of ascidian, also known as sea squirts. When photographs appeared on social media, people gave them the nickname gaikotsu-panda-hoya, Japanese for ‘panda skeleton sea squirt’.
The online hype brought them to the attention of sea squirt expert, Dr Naohiro Hasegawa at Hokkaido University in Japan. Looking at the photographs, Hasegawa realised these are unlike other known sea squirts and set out to study this new and rare species.
Fans of panda skeleton sea squirts donated to a crowdfunding campaign to support a dive trip to Kumejima Island.
Assisted by local fishers, divers found and collected four clusters of the sea squirts from 10–20m (approx 30–65ft) underwater.
Back in the lab, Hasegawa saw that the panda skeletons are unusual enough to be assigned their own species, Clavelina ossipandae.
Clavelina is a genus first described more than 200 years ago. The name means ‘little bottle’ and is a neat description for their transparent bottle-shaped bodies, or zooids.
A related species, found on rocky shores around Europe, is the lightbulb sea squirt (Clavelina lepadiformis), which indeed resemble tiny light bulbs.
The new species name, ossipandae, combines the word ‘panda’ and ‘ossis’, the Latin word for bone.
Characteristics that set them apart from other Clavelina species include those white ‘ribs’, which are actually blood vessels and the black ‘eye’ markings, which serve some as yet unknown purpose.
Like other sea squirts, panda skeleton sea squirts are simple colonial animals. They’re filter feeders, drawing water into their bodies through a siphon tube and passing it over their mucus-covered gills to pick up particles of food.
Then another siphon squirts the water out again, which is where their common name comes from; when taken out of the water, some sea squirts shoot out jets of water.
Sea squirts don’t spend their whole lives stuck to rocks, however. They start out as larvae that look like tadpoles and swim around for a while, before settling onto the seabed.
Their larval stage identifies sea squirts as chordates, the same animal group that includes mammals and other vertebrates. Sea squirt larvae have a nerve cord running alongside a rod-like structure called a notochord, which exists in vertebrate embryos, but breaks down as they develop.
So, although they may not be big, furry or partial to a bit of bamboo, C. ossipandae do have something in common with their black and white namesakes.
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