Tuesday, May 23, 2017

Arthropods 101: Spiders, mites, and horseshoe crabs, oh my! Get to know Chelicerata!

Greetings arthropod lovers of all stripes! Today Mike Skvarla and I sit down with Ray Fisher of the University of Arkansas to discuss Chelicerata. Now you may be saying to yourself, "What is a Chelicerata? It sounds like an exotic sports car." and you would be wrong. Chelicerates are a diverse group of animals that we interact with on a regular basis and includes things like spiders, ticks, mites, scorpions and many more. 

In this episode, we will unpack this large and diverse group by discussing their appendages, evolution, and interactions with humans. Ray is an expert in his field and has provided us extensive show notes if you care to check them out! (Please note that some of the links are to papers posted on Research Gate. If you wish to access these papers, you will need to create a profile, which is free).

Here are few sources for chelicerate phylogeny highlighting Lamsdell's work (the first one has a nice summary tree).

Here are two major works that discuss the new system for arthropods and chelicerates. The first highlights some review material from another titan in this field, Javier Ortega-Hernández, and discusses some interesting terminological problems with words like "Arthropoda". He introduces a new term, Deuteropoda, which emphasizes the shift from stem euarthropods to those that have the first appendage being the deuterosomal one. The second paper is Lamsdell's beast where he demoloshes Xiphosura, and in turn, Merostomata.

I had to pass this gem along. I didn't mention it, but it's cool to see such ecological diversity in horseshoe crabs, a mere relic group today. Check it out:

I mentioned that the timing of terrestrialization for most major terrestrial lineages (insects, arachnids, myriapods) has been supported by both molecular and fossil data. So, both lines of evidence supports an Ordovician or Silurian terrestrialization event for Arachnids and an Ordovician terrestrialization event for Hexapods. However, a recent study found that millipedes are different. For them, molecular data is in stark contrast to fossil data; in other words, millipedes likely colonized land long before there were even trace fossils of them... so we're looking at a terrestrialization event at the mid-to-late Cambrian! So Myriapoda likely colonized land twice! Apparently this is supported by millipede biogeography and also by differing structure of the tracheal systems between millipedes and centipedes.

Here is the paper I mentioned that supports, albeit very weakly, the idea that arachnids colonized via the interstitial as small-bodied creatures, vs. the popular conception of larger-bodied scorpions.

It's hard to find a compelling figure illustrating the evolution of the chelicera, but Figure 3 of the below article describing Yohoia seems to be best. The point is that chelicerae (Chelicerates) evolved from raptorial deuterosomal limbs used for prey-capture, whereas mandibles evolved from 4th-segment biramous legs with endites that were already acting as gnathobases, but then by reducing the telopodite and simultaneously enhancing the protopodite. There's a great figure for this in the below dissertation (Fig. 1.3.)

A great write-up of the aquatic scorpion issue is found in Dunlop and Penney's wonderful new book on Fossil Arachnids . However, the below paper is also a good place to go. Both places discuss the weird gills of Waerengoscorpio and note that this could have been a primitive condition for all scorpions, or a secondary invasion into water.

Finally, here is yet another topic I forgot to discuss during the podcast. Remember the controversy over the placement of scorpions? That they are either derived eurypterids, or basal arachnids? Well... I forgot to mention the most interesting part of that whole story! Namely, we already know the answer... and the answer is neither! 

Also remember that there is really only one robustly-supported clade within all Arachnida---Tetrapulmonata. That clade is united with an obvious morphological character---book lungs. The problem is that scorpions also possess book lungs, but these have been interpreted to be derived from the gills of eurypterids, hence the entire controversy. Well, even in the early studies looking at arthropod phylogeny with phylogenomics (e.g., Regier et al. 2010), a strange relationship was recovered... Tetrapulmonata + Scorpiones. That's ridiculous! Scorpions are so primitive! That must be just an aberrant quirk of phylogenomic data, right? Well, that relationship keeps popping up in study after study. Finally, arachnologists started taking it seriously and looked into it. Turns out... it's a real thing. This means that book lungs, one of the most iconic adaptations in Arachnida, evolved only once in a derived clade known as the "pulmonate arachnids", or Arachnopulmonata (Scorpiones + Tetrapulmonata). Here are some cool papers about it:

Even more interesting is that morphology supports this relationship. Comparative histological examinations fail to find differences in their structure (Scholtz et al (2006) - The book lungs of Scorpiones and Tetrapulmonata (Chelicerata, Arachnida): Evidence for homology and a single terrestrialisation event of a common arachnid ancestor) and even cooler is that their elaborate ciculatory systems support the relationship as well (Klußmann-Fricke & Wirkner 2016 - Comparative morphology of the hemolymph vascular system in Uropygi and Amblypygi ).

So what's up with those Silurian aquatic scorpions anyway? Well, it turns out they may have secondarily invaded water... because even earlier scorpions were probably terrestrial: Waddington et al. (2015) - A new mid-Silurian aquatic scorpion—one step closer to land?  

Questions? Comments? 

Email: unl.jonathanlarson@gmail.com
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This episode is freely available on archive.org and is licensed under Creative Commons: By Attribution 3.0. http://creativecommons.org/licenses/by/3.0/

Beginning/ending theme: "There It Is" by Kevin MacLeod (incompetech.com) Licensed under Creative Commons: By Attribution 3.0