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Origins of a Mysterious Arachnid Revealed

I won’t soon forget a collecting excursion nearly seven years ago on Guam with Prashant Sharma. He and I had been crisscrossing Micronesia and the Philippines looking for various daddy-long-legs and ants, and although there were some species of these animals collected only from Guam and the nearby Mariana Islands, it had been several decades...

A female short-tailed whip-scorpion (schizomid) from Panama. (Photo by Gonzalo Giribet)

I won’t soon forget a collecting excursion nearly seven years ago on Guam with Prashant Sharma.

He and I had been crisscrossing Micronesia and the Philippines looking for various daddy-long-legs and ants, and although there were some species of these animals collected only from Guam and the nearby Mariana Islands, it had been several decades since they were last seen. Perhaps the Marianas’ extensive development had pushed these species to remote scraps of forest or caused them to disappear altogether. We were there to find them again and obtain fresh specimens for DNA sequencing.

Guam was heavily “bombed, burnt, and bulldozed” during World War II (to quote a 1976 ecology text produced by the Guam Department of Education), then reforested with a non-native legume tree to prevent erosion (the first of many exotic reforestations). It gets hit by the most devastating typhoons on the planet (leading to an eye-popping wind speed measurement in 1997 of 381 km/h or 236 mph). It’s flat enough to be developed across its whole area, and is overrun by feral pigs and other invasive animals.

Oh, and it also has earthquakes. If you’re looking for “disturbed” habitat, go to Guam.

Views of Guam (top) and Pohnpei (below) in Micronesia, with Prashant Sharma (foreground) and Ron Clouse sifting leaf litter to inspect for schizomids and other little arthropods. (Top and lower left photos by Ron Clouse, lower right photo by Josh Fuder)


An Unexpected Find

This was the last day of a six-week collecting trip that had taken us to seven Pacific islands, and we were encouraged by an invitation to search the leaf litter on someone’s rural property. Previously on Guam, Prashant and I had labored to crawl under thick vegetation alongside roads and then either scrape sharp limestone rocks for bits of leaf litter or look for piles of it between where pigs had rooted. Now we were walking to what looked like a large forest along a stream, away from the paved roads. What we found, however, was a patch of tall bamboo that had crowded out nearly every other plant and sheltered swarms of hungry mosquitoes. As we expected, our sifted leaf litter turned up almost nothing of interest.

With one exception: short-tailed whip-scorpions—and lots of them. These little arachnids were known to prefer somewhat disturbed habitats, but they had a reputation for being uncommon. However, we were seeing schizomids (as they are called by arachnologists, pronounced “shi-ZO-mids”) in large numbers not only on Guam but all over Micronesia. A collecting team normally might see one schizomid in a day, not one or more in every pan of leaf litter, like we saw on the islands.

At this last locality on Guam we collected 15 schizomids, mostly out of boredom. We did not embark on this trip with plans to study these animals, but finding few of our target species, we decided to investigate some obvious questions about schizomids when we got home, such as whether this was all one species spread among the islands or unique species on each one. When life gives you lemons …

What IS a Schizomid?

A schizomid has the usual arachnid appendages: little pinching mouthparts flanked by grasping “pedipalps,” and four pairs of legs. However the first pair of legs is highly elongate and used like antennae, not for walking. On their posteriors schizomids have a single, small, tail-like appendage, which in males is enlarged into a club. To the non-expert, they are very similar to each other in size, shape, and color, and at a glance they could be confused for juvenile insects or spiders. However, their rapid backward scooting when disturbed, plus the combination of appendages above, makes them easy to recognize after a little practice.

Schizomids from Guam (top left), Palau (top right), and Mexico (bottom left); a uropygid found in the southern U.S., Florida, and Mexico. (Palau schizomid photo by Jesse Czekanski-Moir; all other photos by Gonzalo Giribet)

Back at the American Museum of Natural History in New York, schizomid DNA sequences revealed that the various Pacific populations (like those on Guam) were nearly always highly genetically distinct, not only between islands, but also among different localities on the same island. This made us wonder how an animal could be so adept at traveling across open ocean but so inept at traversing a few kilometers of unbroken forest. Reconstructing schizomid history in Micronesia required us to compare them to specimens in our collections from other regions, which led us to tackle the most fundamental questions about these animals, namely, what are they, where did they come from, and when did they arise?

Schizomids are in their own order, at the same taxonomic level as spiders, ticks, scorpions, daddy-long-legs, and some other groups of arachnids. Schizomids are diverse in Asia and the Pacific, but is this a clue to their origin? They closely resemble the bigger “whip-scorpions” (uropygids, another order, pronounced “YUR-o-PID-gids”), so are they simply a miniaturized version of them?

Assembling the Evidence

We immediately started assembling a trove of recently collected specimens from around the world, and we were assisted by the top expert in schizomids, Mark Harvey from the Western Australian Museum in Perth. Jesse Czekanski-Moir of the State University of New York in Syracuse sent us his large collection of schizomids from Palau, we collected new specimens with our colleagues in the Philippines while looking for cyphos, and other friends and colleagues kindly loaned us specimens they had collected in such places as New Guinea, Colombia, Vanuatu, Equatorial Guinea, Indonesia, New Caledonia, and Cameroon.

Jack Longino and Michael Branstetter checking their collecting equipment in Costa Rica (above), and the final hanging of Winkler bags (below). A Winkler bag is a canvas bag with a funnel at the bottom leading to a container of preservative. Leaf litter is sifted to remove large leaves and pieces of wood, and the fine material is placed in a flat mesh bag, which is then hung inside the larger bag. As the litter dries out, small organisms leave to find more moisture, and they fall down the funnel into the preservative. (Photos courtesy of ADMAC)

A major collection of schizomids well outside of my normal study region was provided to us by the Leaf Litter Arthropods of MesoAmerica project (LLAMA, now continued as the Ant Diversity of the MesoAmerican Corridor project, ADMAC) organized by Jack Longino of the university of Utah, Bob Anderson of the Canada Museum of Nature, and Michael Branstetter of the Smithsonian Institution. This and other biodiveristy survey projects had been picking up schizomids and storing them for years, and now it was time to put them to use.

From Mexico With Love

Back in the laboratory of Ward Wheeler at the American Museum, with the help of Louise Crowley and others, we sequenced four genes from nearly 400 schizomids, scoured genetic databases and found sequences for almost 200 more, and from these built the first global, DNA-based tree for the schizomids. Our study has just been published in the Journal of Biogeography, and what we’ve learned is that schizomids and uropygids are each others’ closest relatives, but they are distinct groups, and that their oldest living lineages are now found in the Americas.

For schizomids, our analyses consistently recovered species from Mexico and Southern California as the earliest branches in our trees of relationships. Schizomids appear to have arisen over 330 million years ago, well before the dinosaurs. Then, about 125 million years ago, during the Cretaceous, they really diversified, including spreading to the Pacific.

If it seems rather odd that a whole order of arachnids arose in the middle of an arid landscape and now live primarily in the tropics, it is important to remember what these regions looked like when schizomids first evolved. At that time what is now Europe, North Africa, the eastern coast of North America, and Mesoamerica was a contiguous landmass on the equator. The only part of that area that has escaped rising oceans, complete desertification, or cooling climates has been Mesoamerica, which is where we find the earliest schizomids today. Indeed, fossils suggest that the ancestors of schizomids were once spread across this entire, ancient, tropical region.

Questions and (Some) Answers

As for their strange combination of long-distance dispersal, genetic differentiation at local scales, preference for disturbed habitats, and abundance on Pacific islands, these and other mysteries about schizomids are ripe for further study. At the moment, we’re just thrilled to have made some headway on understanding where and when these odd little arachnids originated.

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Meet the Author

Author Photo Ronald Clouse
Ronald Clouse investigates the histories of invertebrates found in the leaf litter. He works mostly with arthropods in the Indo-Pacific, especially harvestmen (daddy-long-legs) and ants. He also works with harvestmen in Brazil and the southern U.S., and he is interested in the processes by which we reconstruct history using DNA and morphology.