Cuticular fluorescence in sea scorpions, horseshoe crabs and other chelicerates

Exocuticular hyaline layer of sea scorpions and horseshoe crabs suggests cuticular fluorescence is plesiomorphic in chelicerates

M. Rubin, J. C. Lamsdell, L. Prendini, and M. J. Hopkins, 2017, Journal of Zoology, vol 303, pp. 245–253

I never thought that working in a tiny dinosaur museum in Wyoming would lead to research on glow in the dark arachnids, but here I am. At the Wyoming Dinosaur Center one of our favorite things to do was catch the small scorpions scuttling around our dig sites and keep them in the lab where we could turn out the lights and shine a UV flashlight on them, lighting them up a brilliant blue-green. Kids’ eyes would light up as bright as the scorpions and they all had the same question, “how do they do that?!” I dreaded that question, because I had no answer. As it turns out, scorpions glow under the UV light because of chemicals in their shells, but that leads to the question of why Why do they do that, what possible adaptive purpose could that serve? That is one of the questions we were trying to answer with this research.

Many reasons have been suggested for why scorpions have this trait, including identification of mates, luring prey, and protection from UV light due to the terrestrialization of formerly aquatic taxa, but experimentation has been unable to prove any of these theories. We chose instead to investigate the occurrence of fluorescence in all chelicerates due to the undocumented observation that horseshoe crabs (Xiphosurids) also fluoresce under UV light. Critically, horseshoe crabs represent the sister group to all arachnids. The fluorescence of scorpions is due to the chemical compounds beta-carboline and 7-hydroxyl-4-methylcoumarin in the hyaline layer of the exocuticle. If both fluoresce for the same reason, these compounds and layers in the exocuticle, that would suggest that a shared common ancestor also had this trait.

Fluorescence in extant and extinct chelicerates. (a) Scorpionid scorpion; (b) Buthid scorpion; (c) Thelyphonid whip scorpion; (d) Galeodid camel spider; (e) undetermined gonyleptid harvestman; (f) Phalangiid harvestman; (g) horseshoe crab; (h) sea scorpion; (i) undescribed sea scorpion. Scale bars = 10 mm.

In order to test this hypothesis, we made our way to the invertebrate collections of the American Museum of Natural History. A randomized sample of every arachnid clade was tested for fluorescence which basically meant wandering around the invertebrate collections in the dark with a UV flashlight, opening drawers and shining the light in to see if anything lights up. What we found was that in addition to scorpions and horseshoe crabs, solifuges and some harvestmen appeared to display the same fluorescent qualities. Further histological examination then took place, involving removing the exocuticle from fluorescing specimens and either mounting them on Scanning Electron Microscope stubs or impregnating them with resin and slicing them into thin sections, staining them so that the hyaline layer would be visible. Horseshoe crabs and scorpions both fluoresce because of the hyaline layer, but the solifuge and harvestmen fluoresce less brightly. This is because they do not have a hyaline layer; instead, the cuticle is extremely thin and it was actually the blood inside the animals that was fluorescing. Even more exciting, we found the hyaline layer in 450 million year old eurypterid (sea scorpion) fossils, which suggests that they would have fluoresced under UV as well! Moving forwards, it would be interesting to investigate the molecular structure of the fluorescent compounds found in these organisms.

I learned a lot from the experience, about the subject, research, and myself.  An understanding of the anatomy of chelicerates has been useful in further studies of invertebrate paleontology.  Learning how to use the microtome to make thin sections was struggling, especially given that manual dexterity is not my strong suit.  It showed me that I was capable of working through independent research as an undergrad but it also taught me how to work and communicate with a group.  I had the opportunity to work with an amazing group of people in the Invertebrate Zoology and Paleontology departments of the American Museum of Natural History. My co-authors welcomed me into their world and did not hesitate to show me the ropes and answer any questions that I had, both about the research and about pursuing a career in science research, specifically in a museum.  This summer of research proved to me both that I have the ability and the desire to pursue a museum career, and I could not have done it without the guidance of my mentors.

Margaret Rubin, Oberlin College

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