At first, this mite (family Erythraeidae) did not catch my attention. These large, predatory mites are often abundant on the alpine tundra, where they could easily be mistaken for spiders or daddy long-legs as they actively scurry over the ground. We had collected this one from a nunatak (a bare peak surrounded by snow and ice) on the Harding Icefield.
I quickly became confused when I tried to identify this specimen because it had an extra pair of legs. In grade school we learn that arachnids (spiders, scorpions, mites, etc.) have eight legs, but this one clearly had ten! In this case, the fourth pair of legs (the pair farthest from the mouthparts) had been duplicated, making a fifth pair.
I posted images of this freak on an acarology e-mail list, where it spawned a lively discussion. I eventually mailed the specimen to Joanna Makol (Wroclaw University of Environmental and Life Sciences, Wroclaw, Poland), an expert on mite deformities, who later published an article in the International Journal of Acarology describing this specimen.
This monstrosity was an example of polymelia, the condition of an animal having more than the usual number of limbs. While polymelia is a relatively rare developmental disorder, this mite was not the first known example from the Kenai Peninsula.
In a study of deformities in wood frogs from five Alaska national wildlife refuges, Mari Reeves (U.S. Fish & Wildlife Service, Anchorage) and her colleagues examined 9,269 frogs. Of these, only one polymelous individual was found, a frog from the Kenai National Wildlife Refuge. These two examples of polymelia on the Kenai do not appear to be linked since the conditions and mechanisms leading to polymelia in frogs and arachnids are quite different.
In frogs, extra limbs can be caused by diverse stressors including man-made contaminants, naturally occurring compounds, parasites, and even failed predation attempts. Unlike arthropods and most terrestrial vertebrates in which limbs are formed in early development of the embryo, the legs of frogs develop in the tadpole stage, much later than embryony.
Anything that damages a growing limb on a tadpole can potentially lead to deformities such as polymelia. For example, flatworm parasites that form cysts on growing legs have been shown to greatly increase the risk of limb deformities. When dragonfly nymphs or other predators attack tadpoles, they can mar new limb buds, causing similar damage. Reeves and her colleagues determined that rates of abnormalities in wood frogs on Alaska refuges increased with proximity to roads and that at least a portion of the deformities were caused by predators, but they were not able to explain why deformities were more frequent near roads.
Polymelia in arachnids can be brought about by exposing embryos to extreme environmental conditions, such as unusual variation in temperature and humidity. Extreme cold is known to cause development errors in mites in particular, especially for species that overwinter as eggs. An observed realization of this tendency is a pattern where rates of deformities in some mite species appear to be higher in the northern parts of their ranges in Scandinavia than in more southerly portions of their ranges in Poland. Consistent with this pattern, our 10-legged Kenai monstrosity came from an island of exposed rock amidst the Harding Icefield, a place where it would have been very likely for a mite egg to have experienced extreme cold during development.
What is most exceptional about this specimen is not that it has extra limbs; it is that its extra limbs were well-formed and apparently fully functional. Most cases of polymelia are at best mildly deleterious to the affected individuals and are more often fatal, the majority not surviving through development. Even when polymelous animals do live, the additional limbs are often asymmetrical and are usually not attached in such a way as to be useful to their owners.
In contrast, this mite from the Harding Icefield had reached adulthood with an extra pair of nearly symmetrical, apparently useful legs. She might have lived on to reproduce and grow to a ripe old age (for a mite) had we not collected her, but then we and the readers of the International Journal of Acarology would not have had the opportunity to learn about this exceptional case of polymelia.
Matt Bowser serves as Entomologist at the Kenai National Wildlife Refuge. For more detailed information about the Refuge, you can check the refuge website at http://kenai.fws.gov or Facebook page at http://www.facebook.com/kenainationalwildliferefuge.
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