Cladistics: Skysquirts

Some of the most characteristic creatures of Eryobis can be found in the sky. The lower gravity and somewhat denser and wetter atmosphere allow for a much more productive aerial environment than we see on Earth. Like we could see in the oceans on Earth, the skies of Eryobis can host algal blooms and even have baitball events. The skies of the red and blue marble are home to a great variety of organisms, from tiny aeroplankton to massive griffins.

But few creatures are as diverse and widespread as the skysquirts.

Skysquirts are a group, or rather two groups, of tunicates that took to life on the wind some time after the devastating mass extinction known as the World Scarring. The leading theory of how they came to leave the water is that they were (partially) nectonic tunicates that got swept up by the heavy winds in the early Thyellian age. They would not have had a hard time adapting to life in sky due to the greater air density and water content, just as many other aquatic organisms did before and after them.

Genetic analysis has revealed that skysquirts are not actually a natural group, but rather two different clades of tunicates that independently became aerial. One of these, the blooded skysquirts (order Aerascidea), has been placed within Styelidae and the other, the bloodless skysquirts (order Phyterastia) within Aplousobranchia. Where the terms “blooded” and “bloodless” came from is a bit unclear. It might have been because if you were to cut them open, an Aerascidean would bleed a yellow liquid while a Phytasterian would bleed a mostly colorless fluid.

Both blooded and bloodless skysquirts would have appeared around the same time under the similar conditions, but would end up looking quite distinct.

While the phylogeny of the bloodless skysquirts is still a bit unclear, the blooded skysquirts can be clearly divided into two main branches: the Peravolantes and the Urophytoidea.

The Urophytoidea are the most basal of the two and all its members exhibit neoteny to some degree, keeping their larval tails into adulthood. Curiously though, they use their tails for more than just drifting on the wind. All members of Urophytoidea are kleptoplasts that steal the chloro- and rhodoplasts from the algae they consume and store them in special pockets in their tails. This often gives them a blotchy mix of red and green pigments in their tails and no two Urophytoid individuals look the same. By far the most members of this group are small and planktonic, using their tails to generate lift and stay afloat. But there are some that have rediscovered the water, descending into the water as they mature and living their adult lives like sailboats of sort.

The most basal branch of Urophytoidea are the Urophytes themselves.

They spend most of their lives in the air, but as their live enters its last stage, they sink to the ground and become sedentary much like the ancestral aquatic tunicates. While on the ground, they can no longer replenish their stolen plastids, so they spend the next week after settlement growing exponentially until they exhaust their plastids. They then release their gametes and die. Such Urophyte blooms usually happen after these blooded skysquirts have lived between 2 months and 4 years in the air.

While Urophytoidea clearly demonstrate the Styelid ancestry of the blooded skysquirts, the other group, the Peravolantes look more akin to salps or pyrosomes. Unlike the Urophytoidea, the Peravolantes do mature out of the larval stage into creatures that look more like tunicates. The lose their larval tails and become elongated with a mouth on one end and anus on the other, like salps. By far the majority of Peravolantes exist as simple “air-salps”, floating like trashbags in the wind.

But a more advanced group, the Plotorexioidea have evolved specialised compartments of their digestive tract to store methane gas. This makes them lighter and more buoyant, allowing them to grow much larger than most skysquirts that are typically only about 10 cm long. The Plotorexioids are also the only skysquirts to evolve eyes. They typically have one large eye simple eye on the front of their body. The eye is mirror like and similar in structure to those of scallops and helps them detect food and threats.

They’re among the only skysquirts to be able to accurately control their movements in the sky and are not entirely bound to the air currents. Unlike the Urophytoids and some more basal Peravolantes, the Plotorexioids are not kleptoplasts. However, a family of Plotorexioids, the green soarers, have developed a symbiotic relationship with certain species of chlorophyte algae. They store them in special chambers in their “wings” and thus provide them a safe and nutritious environment, while getting energy in return. Green soarers also have a chambered methane bladder that extends into their “wings”. Because of this, green soarers are capable of becoming the very largest of all skysquirts, with some reaching wingspans of over 3 meters wide.

Another family of Plotorexioids, the Phoneascidids (also known as Hrouthra by by the locals), are perhaps some of the most alien looking creatures in all of Eryobis. With a massive highly developed eye, “toothed” wing extensions around the mouth, multiple wing tendrils and a three-way-split atrial siphon, they are without a doubt the most advanced of all skysquirts. As their anatomy might suggest, they are carnivores and prey on other skysquirts. Hrouthra’s have shown to be quite intelligent as well, possibly being comparable to the intelligence of creatures like sharks and frogs.

Then there are the bloodless skysquirts, the Phytasteria. Being Aplousobranchians, they often have quite long abdomens, which in many species look like stalks. Bloodless skysquirts are a very diverse order whose internal relationships are a bit vague. Like Urophytoids and green soarers, most bloodless skysquirts also rely on photosynthesis to get their energy. But they do it in a different way. It seems that very early in their evolution they began a symbiotic relationship with zooxanthellae. Dinoflagellates that are also often in symbiosis with corals and certain bivalves. The bloodless skysquirts often house their zooxanthellae within their (semi)circular “wings” and these little algae give the bloodless skysquirts a distinct yellowish color.

The most successful of all skysquirts are a group of small disc shaped bloodless skysquirts that have adapted a life as aeroplankton for themselves. Some of their closest relatives have kept their stalks and live sedentary on the ground like plants.

There are also bloodless skysquirts that have evolved balloon-like air pockets that, like those of the Plotorexioids, are filled with methane. Some have multiple such balloons on their stalk like abdomens while others have a single balloon organ above the wings.

Curiously, there also appear to be bloodless skysquirts that are not symbiotic with algae, but instead lost this symbiosis in favor of a more active lifestyle.

Skysquirts are among the most characteristic creatures of Eryobis and form the backbone of many aerial ecosystems around the globe. They often congregate in gigantic swarms that attract predators from all around. These swarms similarly form a huge obstacle for our aerial transports, think birdstrike, but far worse.

Sometimes the wind will suddenly drop and huge portions of skysquirt swarms will just fall out of the sky. This creates a feast for many ground dwelling creatures and can give rise to sudden booms of biodiversity when such an event occurs in a desert.

Our current understanding of skysquirts is still limited and there is still so much to be learned about these wonderful floating trash bags.