The Biology of Star Wars: Are exogorths just really big caecilians?

     ResearchBlogging.orgDon’t be fooled, what you’re looking at is a recently discovered species of a limbless amphibian called a caecilian. They prefer tropical climates and can be found in South and Southeast Asia, East and West Africa, and parts of South America. These creatures are burrowers and have adapations that make them well-suited for life underground. Their heads are reinforced through the fusion of bones in the skull and they are capable of using their bodies like a piston to drive through earth. Caecilians also have “primitive” eyes that allow them to see light and dark. Some species are known to secret toxins from their skin like other amphibians do.

     As hinted by the picture, caecilian mothers tend to their offspring by building a nest and staying with her eggs until they hatch. Unlike other amphibians that have a larval stage (think tadpoles), caecilians emerge from their eggs as miniature adults. Some species of caecilians continue to care for their newly-hatched young through an interesting exfoliating behavior: (see video).

“What’s the secret to my youthful, radiant glow? I make my kids eat my skin.”

Evolution of Caecilians     The story of caecilian evolution is rather murky given their incomplete fossil record. In fact, the evolutionary history of the living amphibians (frogs, salamanders, & caecilians) remains  a hotly contested debate in the field of batrachology(study of amphibians). Who knew! While its accepted that frogs, salamanders, & caecilians all belong to the subclass Lissamphibia (a subclass of Amphibia), there is disagreement about the evolutionary relationship of these amphibians with now extinct subclasses of amphibians. Currently, there are three competing hypotheses:

  • current day amphibians are monophyletic, meaning they all share a common ancestor with either 1) Lepospondyli (a subclass of “newt-like, eel- or snake-like, and lizard-like” tetrapods) or 2) Temnospondyli (a subclass of primitive, amphibian tetrapods)
  • current day amphibians are polyphyletic, meaning that they were derived from different ancestors 3) with frogs and salamanders being more closely related to Temnospondyli and caecilians more closely related to Lepospondyli
     Now depending on which strategy is used to build the phylogenetic tree–it’s like a family tree that shows the degree of evolutionary relatedness–different conclusions are reached. Using morphology and the fossil record, Anderson et. al (1) place the caecilians among the Lepospondyli and frogs and salamanders with the Temnospondyli, which supports hypothesis 3:

     However, taking a molecular clock approach Diego San Mauro (2) estimates that caecilians split from the other amphibians around 315 million years ago which is a timeframe more in line with hypothesis 1 or 2:
     The molecular clock is a technique that is used to estimate when in geologic time two species diverged. When species diverge from each other, scientists noted that the DNA sequence between the two species will change over time at a fairly constant rate. Scientists thus can use the number of DNA sequence changes (percent difference) in combination with this rate to back calculate the time at which the divergence occurred. The problem is determining the rates at which these changes occur. Here is a great example/analogy from The Molecular Clock and Estimating Species Divergence:

“Assume, for example, that researchers have two DNA sequences that have a content difference of 5%. From this information alone, it is not possible to tell whether these sequences have diverged from each other at a rate of 1% per 1 million years over a period of 5 million years, or whether they have diverged at a fivefold higher rate over a period of just 1 million years…This is equivalent to trying to determine the average speed of a car merely by looking at its odometer. To deduce the average speed, one would also need to know the length of time for which the car has been travelling.”

     To circumvent this, molecular clocks need to be “calibrated” against the fossil record which can tell us when an evolutionary event occurred, such as the split between bird and lizards lineages. Armed with that date and DNA sequence data of modern day birds and lizards it is possible to calculate the rate at which DNA sequences change between birds and lizards. For amphibians, however, this is problematic because the fossil record is pockmarked with gaps, so San Mauro calibrated the molecular clock against the 3 evolutionary events: the split between reptiles and mammals, the split between the Archosauramorpha and Lepidosauromorpha reptiles, and the split between birds and reptiles. The molecular data he compared was the DNA sequence of 23 different genes from 18 representative species of current day amphibians. As more of a molecular biologist, I am biased toward San Mauro’s approach. However, both strategies could benefit from a more complete amphibian fossil record, which could not only move some of the branches of the morphologically-based phylogenetic tree but also provide for a better calibration of the molecular clock. Alas, the controversy lives on.

But I thought this post was about the biology of Star Wars…
     Sorry for the bait and switch. Now after all this talk about evolution and after watching the caecilian video (especially the yawning juvenile), let me come back to my original premise: Are exogorths just gigantic caecilians? You know, the giant space slug that tries to eat the Millennium Falcon:

According to Wookieepedia, exogorths are a silicon-based, “gigantic species of toothed gastropod” that generally reaches 10m in length. Thesespace slugs inhabit the caves and craters of asteroids where they feed on minerals, stellar energy fields, and mynocks. They also reproduce “asexually by fission. Once an adult slug reached a certain size, a chemical trigger would cause it to split apart into two identical slugs. The two new space slugs would immediately become self-reliant. Space slugs also molted as a result of their growth.”

Ok, so maybe I was wrong about that one, but you can’t tell me they don’t look a lot alike.

Next up in the Biology of Star Wars series, I’ll cover the ever divisive midichlorians.
Correction: In the original post I misspelled exogorth as exogarth.
Picture credit for amphibian taxonomy.tif  (edited by me)

1. Anderson, J., Reisz, R., Scott, D., Fröbisch, N., & Sumida, S. (2008). A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders Nature, 453 (7194), 515-518 DOI: 10.1038/nature06865

2. San Mauro, D. (2010). A multilocus timescale for the origin of extant amphibians Molecular Phylogenetics and Evolution, 56 (2), 554-561 DOI: 10.1016/j.ympev.2010.04.019
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One response to “The Biology of Star Wars: Are exogorths just really big caecilians?

  1. These things are gross. They’re like cannibals, but they don’t kill their mom when they eat her. It’s like this species couldn’t figure out milk-feeding, so it was like, “Eh, just eat what you can.”

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