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.

The Polar Ice Caps

 

Mars may very well have had seas at some point, but those days are long gone. Toxic, salt-covered deserts are now where the seafloor once was and the lack of liquid precipitation makes permanent bodies of open water exceedingly rare. Most water is now either underground or bound in ice.

Like Earth, Mars has quite prominent ice caps on both its poles. Those of the Red Planet must be considerably older, however, as it has been cold throughout its history, while Earth’s poles only froze over in the last few million years. Mars’ ice caps are quite different from each other in size. The northern polar cap is smaller and does not extend past the 90 degrees latitude, while the southern cap is massive and can in some places extend as far north as 50 degrees S. In Aonia Terra it even connects with the massive ice shields of the Tharsis Plateau, creating a continuous tongue of ice all the way into the northern hemisphere. That so much ice is found in the South seems odd at first, for it is the North which is thought to have once housed an ocean, but it makes sense when looking at the topography and climate. As previously discussed, the southern hemisphere is much taller than the northern one and experiences more intense winters. Any water evaporating on the northern hemisphere is therefore more likely to adiabatically precipitate in the form of snow or ice in the southern hemisphere than to rain down again in the North. By this process, the southern ice shield stole away most of the North’s moisture over the eons as the planet cooled down.

 Fig. 2: Extent of the southern and Tharsis ice caps. The small northern ice cap is not shown

The surface of most of the ice caps is nigh uninhabitable, especially in their centres. Only the hardiest of microorganisms tend to survive inside the glacier crevasses, exposed rocks and wind-eroded valleys. Among them are algal scum and a few interesting types of flechtoids, a type of proteroareozoan that resembles a mix between Antarctic lichen and slime molds. Thanks to highly melanized cells they are able to survive even the worst days full of UV-radiation. A series of images captured in 2001 by the ‘98 Mars Polar Lander potentially shows flechtoids slowly creeping across the ice, similarly to slime molds, perhaps a coordinated escape in response to deteriorating conditions. This behaviour has never been observed by human explorers, however. 

 Fig. 3: Starbursts on the southern ice cap.

As one travels farther away from the poles, one comes to the “tidal zone” where seasonal changes finally start affecting the ice shields. A fascinating phenomenon observed here every spring thaw are the so-called “starbursts”, a form of geyser that erupts from the ice and paints it in radial patterns in reddish and brownish hues. These are a product of complex interactions between the atmosphere, the ice and the life inside. When it is summer in the South, it is dust storm season in the North and, as you already know, these storms can often engulf the whole planet. Thus, the ice caps are covered in layers of dust every year, which are consequently covered by layers of ice and snow in the freezing winter. Various organisms, such as phytoareonts, macroareonts and sub-glacial flechtoids, are buried alongside the dust and stay dormant during autumn and winter, but once spring arrives, the dark particles heat up and reawaken the thawed-out organisms. Using the dust as nutrition, they engage in photosynthesis and generate heat through their metabolic processes, creating pockets of liquid water and air inside the ice sheaths. Through rifts and cracks formed by the shifting glaciers, these may connect with each other, creating more complex, though ephemeral ecosystems. Once these pockets connect with the surface, a geyser forms due to the significant difference in air-pressure between the atmosphere inside and outside the ice. In quick bursts, dust, water and microbes are spread across the glacial surface, creating dark coverings that in summer might heat up enough to create small pools. Most organisms, however, either do not survive the ejection or enter a dormant state. All of them enter dormancy once the days grow shorter, for autumn and winter last 370 sols, longer than a year on Earth.

Most fascinating from a zoological perspective are of course the many sub-glacial lakes dotted beneath both ice caps. Most of them are inhospitable. Created by pressure-melting at the very sole of the ice, they are kept liquid at sub-zero temperatures only by their extremely salty perchlorate-mixture. In some places, however, topographical or geothermal features lead to the creation of lakes with amenable conditions for macroscopic life. The water is still cold, but liveable, and not much saltier than what one might find in the Arctic oceans of Earth. In some places they even consist of freshwater. The most productive lakes are those with only a thin ice sheet protecting them, as the flora here can engage in photosynthesis for part of the year, being fed by the nutritious dust that continuously erodes out of the overlying frost. In summer these may even thaw open, allowing for interactions between the surface and sub-glacial life. Many other lakes are under ice sheets too thick and layered with dust to let in much light. Here life makes do with chemotrophy and is adapted to the darkness, much like deep sea life on Earth. It may not sound like heaven, but at least it is not hell.

Most of the lifeforms so far discovered in the lakes resemble the phyla also known from the surface, though a few are entirely unique to this habitat. There is also a considerable difference in the faunal composition between the northern and southern ice cap. The debate is still ongoing as to whether the sub-glacial lifeforms represent the last remnants of marine ecosystems from Mars’ long-gone aqueous phase or if they are merely former surface or sub-terranean creatures that have invaded the ice lakes from above or below in more recent times. The former situation seems plausible for the northern cap, but not for the southern one.

Image Sources:

• Fig. 3: Jet Propulsion Laboratory

Caraxor

The salty wastelands of the perchlorate deserts are not completely devoid of macroscopic life. The process by which the perchloareont microbes create oxygen also turns toxic perchlorate back into regular sodium chloride. While the brine oases inside these deserts are thus still extremely salty, they are far less toxic than the perchlorate flats that surround them. A few halophilic extremophiles can thus carve out a life inside the brine ponds. They largely consist of simple plant-life, various worms, and some antitrematans and pseudarticulates. The majority of which are barely visible with the naked eye. These salt-dwarves may tremble when the shadow of a caraxor glides over their home. It is the only large animal thriving in the northern wastes.

The Caraxor is a periostracan distantly related to the nothornithes. Whereas Nothornitha are bird-like bipeds, the ancestors of the Caraxor and its relatives, also known as the Pedicambulata, have gone down a different route. In the ancestral periostracans, the tail was exoskeletal, constructed of hardened tunicine rings, making it similar to the chitinous tails of crustaceans. In some of their descendants, many of these rings seem to have fused with each other into elongated, solid elements, until they formed a third appendage comparable to the hindlegs of some insects. The evolution of a third leg has given the older limbs of these tripods more freedom than in the bennus, allowing them to adapt to more specialized uses. Evidently, some took to flying.

With its bat-like wings, the Caraxor soars through the deserts in search of brine pools. In these, the aerial tripod wades and feeds much like an Andean flamingo. Pedicambulates have a much higher tendency towards polydonty than their nothornithe cousins, meaning that their scolecophores can bear multiple tooth-shafts. The four scolecophores of its jaw thus can thus form comb-like tight slits. With this derived jaw apparatus, it sifts the brine for small organisms, of which it is the only predator. Strong glands underneath the armpits help with excreting much of the salt, allowing it also to drink the brine. During flight and feeding, a dense pelt of white fur protects it from the harsh solar rays and the cold desert nights. Possibly, the Caraxor also has some form of tolerance to the toxic effect of perchlorates, perhaps through endosymbiosis with the same organisms that make the desert liveable.

Caraxors do not sleep or nest beside the brine pools in which they feed. Instead, they fly out towards rocky outcrops in the middle of the perchlorate wastes, where they can rest and hatch their eggs in safety above the toxic ground. The many remains of dead animals along the desert outskirts attest to the success of this strategy. Any would-be predator venturing here to feed on the Caraxor or its eggs is likely to die beforehand from the aridity and especially the toxicity.

Thus, the caraxors rule alone over the white wastes, their only company being the dry, bleached bones of fools. But to be a king in the desolation is still better than to be a pawn in death. It is a largely peaceful life, with their only enemies being disease or themselves.