Skolex

The variety of reproduction methods among the Antitremata can and has filled entire textbooks (such as Schröckert 2299). Ancestrally, all adults of this phylum must have been sessile aquatic animals which released their haploid gametes into the water, which then combined into eggs out of which hatched planktonic or mobile larvae, which eventually settle down onto the seafloor to grow into sessile adults. The archaic Cimmerozoa of the subglacial habitats still live like this. Some lineages, like the Ortholitha, Periostraca and wanderstalks have abandoned this metamorphosis by remaining mobile into their adult life, often practicing internal fertilization followed by laying shelled eggs or giving live birth. Shellubim on the other hand have retained the larval stage and adapted it to life on land. Instead of floating plankton, their nymphs are little winged animals which float or, rarely, actively fly through the air. As the sheer amount of their fossils in ancient strata attests, this was once a highly successful strategy, back when the denser atmosphere of Mars combined with its low gravity to make flying more akin to swimming. Today, however, the thin air has made the existence of aeroplankton unfeasible in large parts of this world, which has severely restricted the range of classic shellubim to the low-lying parts of the planet, such as the Hellas Basin.

And yet, in the highlands and tundras, can be found some creatures that look an awful lot like shellubim. Their stalks are clearly immobile, making them unrelated to the wanderstalks they share their habitats with. However, these shellubim have never been observed releasing flying nymphs into the air, which has for the longest time made it a great mystery how they reproduce. Especially so considering that without the assistance of airborne travel, reproduction for sessile creatures becomes a rather daunting challenge on land. It makes it nearly impossible for their gametes to find each other and for the eggs to be laid down in a suitable spot, at least with methods commonly available to nonmotile waterdwellers. It is perhaps for this reason why truly sessile terrestrial animals never evolved on Earth. The first suggestion was that these strange highland-shellubim reproduce asexually. Perhaps by budding, perhaps by “spitting out” eggs or larvae onto the surrounding soil and onto the hides of animals or maybe even by having seed-like eggs that are eaten by their predators and then dispersed through dung. But such behaviour and organs have never been observed and an exclusively asexual reproduction would make their populations greatly susceptible to diseases. Next up was the idea that, mimicking flowering plants, they do reproduce sexually by attaching their gametes onto other animals, such as wadjets or ballousaurs, which then travel between the shells and fertilize them. Methods of attracting wadjets (though for predatory purposes) are known from the distantly related wanderstalks, so it might not be far-fetched to think that some shellubim could have evolved similar pheromones. But alas, such “pollinators” have also never been observed, nor have gametes of these shellubim been found on the hides of other animals. How the eggs and larvae travel is also not explained by this approach.

An answer to this mystery and a rather momentous discovery was then found once my later colleagues analysed the four-eyed, armoured, slug-like critters that were sometimes found slithering in the dust and dirt close to shellubim colonies. Called skolex, these were for a long time classified in the waste-basket taxon of the Brachiostoma. After Brachiostoma was broken up, they were then allied with the Spiriferia, as the lip-like tentacles sheating their mandibles bore some resemblance to the lobostomian corona. But on closer inspection, various details did not fit with that classification. The mandibles of skolex were unlike any mouthparts seen in spirifers and their muscular foot was unsegmented. Their armour was also wholly non-spiriferian. Instead of single, large dorsal plates, they were covered in a multitude of biopolymerized scales called sclerites, which all grew from individual follicles and overlapped each other. This prompted further examination using molecular tests on their genome. To everyone’s surprise, all of the skolex turned out to genetically be antitrematans, rather deeply nested within the shellubim family tree. How was this possible? The first suggestion was that the skolex are in fact the long-sought larval stages of the highland-shellubim. This seemed plausible at first, but skolex have almost nothing in common with classic shellubim nymphs. Often, they also showed a growth pattern as if they already were fully adult and also seem far too derived to develop into the sessile shellubim stage, as they have no beginnings of a stalk or bivalve shells. Their classification as larvae also left the question open as to how the gametes of the sessile stage could be sexually transmitted without an aeroplanktonic system. Nonetheless, more analyses were made to test the larva-hypothesis and see if specific skolex corresponded with specific shellubim. This was indeed the case, but as it turned out, every skolex always had only half the number of chromosomes of the shellubim it corresponded with.

Life cycle of the Nosferatu skolex (Chiropterotorris kinskii). The individual stages are not shown to scale.

A long-term study (Pajitnov 2332) finally affirmed the conclusion that has been drawn from this find: Skolex are not larvae but organisms with alternating generations, a diploid one followed by a haploid one, which live as completely different animals. It is no wonder then that we had trouble recognizing this for so long, as there is nothing comparable on Earth. While some of our animals are known to occasionally dabble with varying ploidy (male worker ants and bees are all haploid for example while the queens are diploid; a similar system can also be observed in other arthropods and rotifers), using such variations to have alternating generations is on our own planet only known from plants, fungi and some unicellular organisms. Animal life with such a life cycle is so far truly unique to Mars. And it is quite a clever system that solves most of the aforementioned problems faced by a sessile land animal. The sessile skolex is the sporozoon (comparable to the sporophyte in plants). It is diploid, meaning it has two sets of chromosomes. Inside the gonads, the reproductive cells of the sporozoon undergo meiosis, splitting up into soft, egg-like spores which are haploid, only having one set of chromosomes. These spores then undergo regular mitosis, growing into juvenile skolex-worms, which are the gametozoon (comparable to the gametophyte in plants). The worms crawl out of the sporozoon’s shell and live a life of their own as motile animals, solving the problem of sexual reproduction and egg-dispersal for the preceding generation. The gametozoa can have widely differing lifestyles and appearances, being detrivores, carnivores or herbivores. The gametozoa of most species have mandibles which can be used for masticating all sorts of food. During the dissection of a deceased Syncarpus, even endoparasitic skolex were found, with sucker-like mouths attached to the skin beneath the animal’s plumage. Many skolex have also been observed around carcasses, dissolving bone with acid secreted from mouth-glands, likely to gather up enough calcium to give the next generation a head-start. When two gametozoa of adult age meet, mating occurs, fusing their haploid gametes back into diploid zygotes in the process. The skolex then seeks out a suitable spot for its young and lays the zygotes in the form of eggs into the soil. Said eggs actually have a calcitic shell and yolk, not too dissimilar from those of large snails. From these eggs the next generation of sessile sporozoa develop, eating up the yolk and dissolving the eggshell to build up their young stalk and valves. After sprouting out of the soil, these sessile forms largely live through photosynthetic endosymbiosis and hydrogenotrophy.

A few questions still remain unanswered. For example, how do the sporozoa acquire the endosymbionts they require for their lifestyle? In classic shellubim, the airborne larvae absorb them through their aeroplanktonic diet, but this is obviously not an option for skolex. At least in herbivorous skolex, the possibility exists that they are also absorbed through the diet but then infused into the egg-cell of the sporozoon, which is how some corals on Earth pass down their zooxanthellae. Then there is the interesting observation that some sporozoa produce gametozoa of varying morphotypes. Some have interpreted this as being sexual dimorphism among the gametozoa, which, if true, would make the skolex one of the very few Martian animals with true differentiated sexes. Research on this is lacking, however. Of course there is also the question of how this system of alternating generations evolved in the first place. If we take the aforementioned speculation as fact and insects such as ants as an analogue, the differing ploidy may have originally evolved as a form of sexual dimorphism (perhaps even in swarm organisms) before turning into true alternating generations to cope with the loss of air pressure on Mars. But again, research in this field is lacking. What we can say is that this adaptation seems to be working out quite well for the skolex, as they are far more widespread than their more archaic relatives and seem to quite capably compete with the wanderstalks.

Lastly, there is the question of the gametozoon’s appearance. Its anatomy looks nothing like that of other known antitrematans, living or extinct, which is why they were originally believed to hail from a wholly different phylum. Though some commonalities do exist. The sclerite armour has the same mineral composition as the classic antitrematan shell, the eyes are identical to those found at the tips of shellubim tentacles, the scolecodont mandibles resemble the mouth apparatus found in periostracans or wanderstalks and the mouth-tentacles have a muscle structure akin to boneless lophophores. The same underlying genetics seem to be at work, just used in very different ways or in odd arrangements, like building a car out of airplane parts. From the Lyotian period of Mars’ very deep prehistory, fossils looking similar to the skolex are known. Called Sklerotaria, they are believed to be completely extinct and nobody had previously made the connection that they could be related to the Antitremata. But with this discovery, some have now speculated that the Antitremata as a whole may actually descend from or at least share a common ancestor with the ancient Sklerotaria, perhaps by fusing the sclerite-armour into the classic bivalves and adapting other parts of their anatomy into the sessile filter-feeding morphology we know today (Sivgin 2345). If this is true, the haploidy in skolex may uncover and co-opt an ancient set of genes which otherwise lie buried and dormant in diploid antitrematans. Without surviving Sklerotaria to test this hypothesis on, we may never know, but recent news from Phobos may show promise.

References:

• Pajitnov, Anton: Alternating generations in a Martian arezoan. Explaining the anatomy and life history of the skolex through differing ploidy, in: Soviet Journal of Astrobiology, 89, 2332, p. 98 – 115.
• Schröckert, Daniel: Fortpflanzungsweisen der marsianischen Antitrematen. Der Komödie erster Teil, Bochum 2299.
• Sivgin, T.K.: Life on a Dead Planet. The first 3 billion years of Evolution on Mars, Zürich 2345.