Dust Slugs

One of the larger phyla on Mars are the Spiriferia (not to be confused with Spiriferida, a brachiopod order from Earth). These can be generally described as “armored slugs”, where the soft body is protected by a series of dorsal plates. Unlike molluscs, the muscular foot by which they move across the ground is segmented and split up into multiple pseudopods. The defining characteristic that gives this phylum its name is their unique mouth. While early researchers often compared it to the radula of molluscs, in reality it resembles more a larger version of the “corona” seen in rotifers. Two major classes can generally be identified in the Spiriferia, which have evolved two structurally different versions of such a corona. In the class Lobostomia, the corona is built into either a hood above or two large arms on the side of the mouth, which are beset with hundreds of hair-like setae. Through swishing-motions, these bristles transport particulate food to their mouths. In the class Verticutia, the corona is split up into a bristly “lower lip” and an “aggressive upper lip”, where the setae are replaced by teeth that are each moved in an alternating motion that can only be described as the biological equivalent to a scarifier (like the ones I used to work with in my father’s garden). One presumes that this originally evolved to effectively plough through nutritious ooze at the bottom of the sea (with the bristle-lip acting as a filter for food particles as in the Lobostomia), though in many derived forms it has become a tool for predation as well.

A polyphyletic sub-group of the Spiriferia are the so-called dust slugs, which is a general term applied to both lobostomian and verticutian spirifers which have, amazingly and independently, adapted towards lithotrophy, whereby Martian dust and sometimes even ground-up rock is digested with the help of lithotrophic areonts living in the animals’ guts. Almost all of Mars’ dust is made of iron oxides, which these microbes can turn into energy through iron reduction. The sifting also helps with taking up water in form of the dew laid down onto the sands in the morning. It should be stressed however that most of the iron-reducing reactions are inefficient, which is why dust is not the main diet of these organisms. Lobostomian dust slugs mainly sift the desert sands for smaller plants and for spores, aeroplankton and other organic debris that has sunk down, while the verticutian ones are mostly predators of smaller animals such as onychognathans and trichordates. Herbivorous verticutians also exist, which mainly shred up the water-bearing bases of desert-spongisporians. Another interesting trait which separates dust slugs from other spirifers is that their skin is not soft and wet, but instead dry and scaly, like in a reptile, allowing them to slide across the sands with the ease of a desert cobra. They also lay hard-shelled eggs.