Like all desert-dwellers, scorpions are masters of efficiency. A new study shows that their efforts to conserve energy even extend to their arsenal of venom. In one scorpion species, researchers have found that the arachnids use a relatively weak, but easily produced, prevenom for small jobs such as paralyzing insects and save their deadliest venom for serious work like fending off a larger animal.
Parabuthus transvaalicus is a South African scorpion that can grow to be more than 10 cm long and squirt its potent venom like a water gun. The venom, specialized for use against mammalian predators, can kill a barefoot human unlucky enough to step on it. South African researchers working on an antivenom asked Bruce Hammock, a protein chemist who studies insects at the University of California, Davis, to analyze the animal's venom. While milking a scorpion, one of Hammock's students noticed two different secretions coming out of the animal's stinger.
The first secretion to emerge, the clear prevenom, was potent enough to paralyze larval flies and moths and make mice lick themselves in pain, the team found. Mass spectroscopy revealed that the prevenom contains proteins that block potassium channels in nerve and muscle cells. Prevenom acts fast and is easy to make, Hammock says, requiring only a few proteins and lots of readily available potassium. Hammock speculates that scorpions may also use it when they mate--the male is known to sting the female repeatedly during courtship.
For large prey or persistent attackers, the scorpion gets out the big guns: a thick, opaque venom that wreaks havoc on more parts of the victim's cells. This venom affects insects and mice at lower doses than prevenom, the team reports online this week in the Proceedings of the National Academy of Sciences. It's not easy to make either, since there are at least 100 kinds of peptides in P. transvaalicus venom.
Venom glands take time to pump out venom, so it makes sense that they release another liquid first, says Philip Brownell, a scorpion biologist at Oregon State University in Corvallis. Scientists had noticed before that some scorpions put out two fluids, but no one had analyzed them separately. What's interesting here, Brownell says, is that each part of the venom has a separate function. "I think it's a significant advance on what we understand, the subtleties by which venoms have evolved to really deal with ... paralyzing an animal."