Heavy metal: How this arachnid uses iron, zinc and manganese to fortify their stingers and pincers

Despite their small size, for many people scorpions are fearsome creatures thanks to their characteristic pincers and upward curling tail sporting a stinger filled with venom, which in some cases can be deadly. This weaponry was known to be fortified with metals in some of these members of the arachnid family, but little examination into this trait had been performed. Tha is, until now.

A team of researchers at the Smithsonian’s National Museum of Natural History and the Smithsonian’s Museum Conservation Institute analyzed 18 scorpion species and found a correlation between how they use their hunting arsenal and the concentration of metal. They published their findings recently in the Journal of the Royal Society Interface.

“Nature skillfully engineered these metals in the scorpion’s weapons”

Using microanalytical techniques, including high‑resolution electron microscopy and X‑ray analysis, they “identified clear and highly localized patterns of metal enrichment,” of three metals; iron, manganese, and iron. Manganese appeared only in the stinger, while iron was found only in the claws.

On the other hand, zinc was present in both, but in different amounts in each depending on the specie. “Scorpions that invest heavily in zinc for their stinger generally have lower zinc levels in their claws,” and vice versa explained Edward Vicenzi, a research scientist at the Museum Conservation Institute and a co-author on the study.

This “evolutionary trade-off” corelates with which weapon the specie of scorpion tends to use preferentially when it hunts. Those species which rely more on their pincers to crush prey had higher zinc concentrations in their claws, while those that use their stinger more had more zinc in the spur. “Nature skillfully engineered these metals in the scorpion’s weapons,” said the scientist in a statement.

“This is an interesting finding because it suggests an evolutionary relationship between how a weapon is used and the specific properties of the metal that reinforces it,” said biologist Sam Campbell, who was a graduate student fellow at the National Museum of Natural History at the time the research was completed.

Interestingly, those scorpions with smaller, weaker pincers had higher enrichment levels of zinc and iron in theirs than those with bigger, stronger pincers. This is most likely to reinforce these weapons to prevent breakage, as adult scorpions cannot repair damaged body parts. “That’s just what they’re stuck with for life,” noted Lauren Esposito, a scorpion biologist who was not involved in the study.

“If you have really big pincers, you’re more likely to use a lot of force but be kind of slow about how you use that force, whereas little things are just really fast; they’re really reactive. So that would, in my mind, mean that they probably also have a tendency to break much easier, because they’re using this really fast force on a pretty delicately constructed body part,” Esposito explained.

Studying metal enrichment across the tree of life

Scorpions aren’t the only heavy metal arthropods. Metal enrichment of body parts is found in ants, centipedes, and wasps. The researchers hope that their method of analyzing the metal fortified parts of scorpions will help other studies.

“We were able to efficiently and drastically expand our understanding of the evolution of metal enrichment in scorpions. Not only does our work illustrate the material properties of scorpion’s weapons, but it establishes a new approach towards analyzing the role of metal enrichment across the tree of life,” said senior author of the study Hannah Wood, a research entomologist and curator of arachnids at the National Museum of Natural History.

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