Selpies

Selpies are fascinating aquatic creatures found in most permanent bodies of water on Mars, of which there are not many. They are thus solely restricted to glacial lakes or underground caverns.

“Selpies” is the common name given to most members of the phylum Mollizoa, which is likely one of the basal clades in the Martian animal kingdom. In many aspects they can be regarded as this planet’s equivalent of “coelenterates”, meaning cnidarians (jellyfish and anemones) and ctenophores (comb jellies). Like them, selpies are radially symmetric, have tentacles, grow out of two germ layers and lack a central nervous system, as well as an anus. The gut is instead a simple coelenteron, meaning a stomach connected to a single opening that acts as both a mouth and an anus. Given how basic this bauplan is, it does not seem surprising to find it again on another planet. The same principles apply to fractarians, whose earliest members have more than a passing resemblance to the simple life of Earth’s Ediacaran, as well as spongisporians, which broadly resemble poriferans. However, in all three, evolution has taken divergent paths that make them alien to us. If such differences between Martian and Terran life, despite similar beginnings, are merely due to the differences of the two planets or thanks to some truly Gouldian contingency, is one of the great philosophical debates of our time. It is up to the reader to decide that for themselves.

Thankfully for our explorers, one of the main differences that sets selpies apart from jellyfish is that they never evolved their characteristic stinging cells. It would have actually been surprising if they did, as those organs are highly derived and their origin remains mysterious. Instead of killing their prey through toxins, selpies can extend their circular maws into a large gape and simply swallow their still-living prey whole, much in the manner of comb jellies.

Distinguishing them from comb jellies, which are the largest organisms on Earth that still move with cilia, is their locomotion. Almost all selpies swim through the water using a peculiar form of jet propulsion. The coelenteron is surrounded by multiple tubes running across nearly the whole length of the body. These are muscular and open on both ends. Through wave-like peristaltic pumping, water is ingested at the front and pumped out at the back. Behind the exhausts, the body forms a cone-like tail-fluke with concave surfaces. The animals can quickly change directions by bending this fluke and therefore redirecting the jetstreams.

We may have some clue as to how these structures evolved. Earliest possible fossils of these creatures from Mars’ Late Neonoachian show a morphology very much like that of actual jellyfish, where instead of jet-tubes, the body was propelled by a medusoid bell (Sivgin 2345). Possibly, the tubes evolved through the bell attaching itself more firmly to the coelenteron through walls and becoming sectioned that way. The caveat with this speculation is that no such medusoid mollizoans survive until today and it has been argued that these fossils are not actually related to Mollizoa at all and instead represent a completely extinct phylum (Bomhoff 2343).

A few distinct body types have evolved among the selpies. Most have a hexaradial or pentaradial symmetry, such as the vurux on the left. Despite having no brain or higher sense organs, it swims through the Antarctic sub-glacial waters with surprising elegance and coordination. Its tentacles are used both as feelers and to grapple smaller prey. More lethargic relatives of the vurux live through filter-feeding, by using many sticky tentacles to sift the water for microorganisms and then lick the catch off the arms. Some living in the deepest lakes and caverns close to geothermal vents subsist almost completely on chemotrophy and have strongly reduced their feeding apparatus and propulsion organs. Some living in luminous sub-glacial lakes live in endosymbiosis with photosynthetic organisms living in their tissues.

More peculiar are quadradial selpies such as the tauin at the back. While following the same basic bauplan as its more circular cousins, the tentacles in these creatures are interestingly concentrated on only two sides, making them more “bi-radial”. All these forms are exclusively predators, often of smaller selpies. Cave-dwelling forms are known to make almost excessive use of bioluminescence.

Most peculiar are forms like the lamia in the foreground. This creature was originally classified as a “brachiostoman” due to its worm-like shape, segmentation and seemingly bilateral symmetry. However, the presence of two jet-tubes on the side of the body, tentacles surrounding the mouth and the lack of an anus make it far more likely that this organism is allied in some form with the selpies. What exact form this relationship takes remains to be investigated and has some interesting bearing on the evolution of Arezoans. If lamia turns out to be within Mollizoa, but also to be part of the Laterazoa (the bilaterally symmetric Martian animals), then Mollizoa is paraphyletic and ancestral to the rest of the arezoans (sans perhaps the trichordates). This might give some interesting insight on various laterazoan organs, such as the lungs of onychognaths, which are theorized to have evolved from jet propulsion organs in aquatic ancestors (though jet propulsion has evolved multiple times independently on Earth and could have done so just as well on Mars), and the feeding organs of spirifers, antitrematans and wadjets. But it is also possible that the lamia is just an oddity and bilateral symmetry evolved in some selpies independently of laterazoans, making the more classic hypothesis of the two clades just sharing a common ancestor more viable. A third option proposed by Krätschmer (2117) can be safely dismissed. His hypothesis that Arezoa is polyphyletic, with the Mollizoa being unrelated to the Laterazoa and the latter actually evolving out of the fractarian pseudarticulates, is not viable for obvious reasons.

References:

• Bomhoff, Nils: Discovery of a tuboid fossil from the Late Insolituzoic casts doubt on medusoid origin of Mollizoa, in: Astropaleontology, 569, 2343, p. 65 – 89.
• Krätschmer, Simon: Description of Eocephalus vermiformis and a possible alternative origin for Laterazoa, in: Strate Station Geological Journal, 451, 2117, p. 1589 – 1621.
• Sivgin, T.K.: Life on a Dead Planet. The first 3 billion years of Evolution on Mars, Zürich 2345.