Amstiel and Rhoson

In some of the subglacial lakes surrounding the ice caps, life is teeming. Peculiar residents are the amstiel and its tribe. This is a type of basal antitrematan, related to the terrestrial shellubim, which has evolved a unique method of locomotion. Most non-periostracan antitrematans begin life as a mobile, albeit planktonic, larva, which grows up into a sessile adult. The amstiel on the other hand stays mobile throughout its entire life. It achieved this by retaining the muscles inside the tunicine stalk, which the shellubim lose in their adult life, and expanding them into what used to be holdfasts. While the stalk still develops into a hardened, wood-like stem in the adult, said holdfasts have developed joints, becoming arthropodous legs.

This lifelong mobility gives the amstiel a great advantage in an everchanging environment, for it lives by walking across the ice-ceiling of the frozen-over lakes. Often the ice is thin enough to let through precious light, which photosynthetic organisms use to live, including relatives of the amstiel. The amstiel itself is a pure filter-feeder dining on the plankton and detritus produced in these upper lake-levels. But the ice is everchanging, always growing and shrinking with the temperatures above surface. Sessile organisms are entombed by the ice in winter, while in summer they may lose their footing and can even float to the surface through temporary holes in the ice, where they can die from the exposure or be picked up by above-ground predators. The amstiel can simply walk away from these problems, migrating to suitable spots and anchoring itself there to filter in peace.

These newfound legs have also opened up a new way of reproduction for these organisms. Instead of broadcasting free-floating propagules and larvae into the water, two amstiel simply meet up, impregnate each other and walk away from the affair. The young gestate inside the body and are birthed as miniature adults already capable of walking, though the shell and stalk still need to solidify.

Of course, the amstiel cannot walk away from every problem, for speedy predators may lurk in the water. Though thankfully, for our individual here, the passing rhoson is seemingly just telling it to get out of the way, as it hunts a darting selpie. The rhoson is a periostracan of a kind even more basal than the kratox. Like it, the valves have fused into a skin-covered carapace, the main feeding organ has become a toothed, periscopic proboscis and the stalk has turned into a tail, but there are still reminders of an earlier form of life. The scolecodonts which make up the jaw are not fused into a bird-like beak, but articulate independently of each other as vertical mandibles, much like the mouth apparatus of a polychaete worm. Viewable through the thin skin of the fins is also the arm-skeleton, which is still organised much as in the exposed lophophores of the amstiel, just more robust and muscular. Though seemingly primitive, animals such as this are among the top-predators in the few remaining aquatic habitats of Mars and, as the fossil records indicate, have been for a very long time. In some ways they were the closest thing Mars ever brought forth approaching fish and possibly they even suppressed the early evolution of the aquatic onychognaths and aspiderms. But whereas some distant fossil relatives of the rhoson could grow to sizes rivalling the largest of Earth’s sharks, each of the animals you see here pictured could easily fit into a human hand. Actually holding them is not recommended, for the rhoson’s jaws could easily cut through a spacesuit, while for the amstiel it would just be demeaning to be held like a popsicle.