Entomology of Star Wars. Episode II: Metallic snails and vent mussels

As Episode I proved so popular, I've been thinking about extending the 'Entomology of Star Wars' into an occasional series - not easy as I don't go in for a lot of 'speculative biology', but I'm up for a challenge and do have a penchant for Science Fiction. So, time for a bit of a nerd-fest...

One beast that I have wondered about is the big asteroid-dwelling worm that the Millenium Falcon encounters in 'The Empire Strikes Back' - what it's made of, what it lives on, how it survives in space and so on. A little online digging soon told me that it's called a 'space slug' or 'exogorth' and that it's a silicon-based gastropod (I'm pleading poetic license and sticking with 'entomology' rather than 'conchology' though), feeding mainly by metabolising asteroid minerals through its root-like tail and absorbing stellar energy, though not averse to eating the occasional spacecraft or other unwary space-dwelling creatures. Also, it apparently reproduces by fission, moulting as it grows, then simply splitting in two when large enough.

The exogorth - in space, no-one can hear you roar!

Now, I don't intend to get into the whole hypothetical silicon-based life/alternative biochemistry thing - there are plenty of people doing that already, and there's even a Wikipedia page about it here. Instead I'd like to see if there are any parallels between this fictional beastie and real-world organisms.

Firstly, the metabolising of rocks and their constituent minerals. It's well known that some bacteria (lithoautotrophs) can metabolise a variety of minerals such as sulphur, iron and manganese and are involved in both the creation of limestone cave systems (speleogenesis), and the phenomenon of acid mine drainage (AMD), the outflow of acidic water from metal and coal mines. The biochemistry of such acidophilic bacteria can be complex, but in the case of cave systems such as Carlsbad and Lechuguilla in the US, rocks have been attacked microbially in three ways:

• Oil-metabolisers - their biochemistry produces hydrogen sulphide which in turn produces sulphuric acid.
• Rock-eating bacteria - the lithoautotrophs directly metabolising minerals.
• 'Snotites' - large bacterial colonies which are primary producers in such ecosystems and drip sulphuric acid.

So, maybe the exogorth has lithoautotrophic gut flora - it has a mouth and dentition, so a gut seems likely. Thinking of a worm-like creature with symbiotic bactera does lead neatly onto the annelid tube worms such as Riftia associated with hydrothermal vents. These have become familiar creatures over the last decade or so through various TV programmes, and although they do not have a mouth or gut, they do contain symbiotic bacteria. These live within a specialised organ (the trophosome) in the worm and metabolise compounds such as carbon dioxide and hydrogen sulphide which are absorbed by the worm's plume. In turn they provide carbon compounds which nourish the worms. Similarly - and providing a handy molluscan parallel with the exogorth - vent mussels (Bathymodiolus thermophilus) are almost entirely dependent on symbiotic bacteria in their gills. Not only that, but in 2001, the Scaly-foot Gastropod (Crysomallon squamiferum) was discovered associated with hydrothermal vents in the Indian Ocean. What is unusual about this species is that its foot is armoured with 'scales' (technically 'sclerites') of iron sulphides (greigite and pyrite) and that its shell has a third layer also containing iron sulphide - it is the only animal known to use this mineral skeletally in this way. This means we do have molluscs with mineral-metabolising symbiotic bacteria, and that at least one can incorporate what we consider 'unusual' compounds into its anatomy. But, what about reproduction?

A heap of chemosynthetic vent mussels.

This seems fairly straighforward - all molluscs reproduce sexually (most gastropods are hermaphroditic), so definitely no fission. Still, although asexual reproduction is rare in higher animals, they are not unknown. For example, in Mexico, there are Topminnows of the genus Poeciliopsis where it is seen. Two species, P. monacha and P. lucida, reproduce sexually with their own species members when separate, but where they co-exist, they hybridise to produce all-female forms that reproduce by cloning (hybridogenesis and gynogenesis). In hybridogenesis, the female mates with a male, producing female offspring with both the maternal and paternal genomes. When that female produces eggs, the male genome is discarded, leading to the all-female form. In gynogenesis, females are triploid and also mate with males, but the male genome does not contribute to the offspring. There's also a good diagram explaining the process of hybridogenesis in water frogs here and there's plenty of research into gynogenesis in the African Clawed Frog (Xenopus laevis) - certainly, amphibians do seem to have a flexible approach to reproduction...

So, parallels between exogorths and real-world organisms? For molluscs, nutritionally yes but reproductively no - but what about the whole 'surviving in a vacuum' issue? Well, I think I'll leave that for Episode III...