The group Cnidaria, which includes jellyfish, anemones, corals and the familiar freshwater Hydra, is not generally known for its speed. In fact, it was only in the mid 18th century that they were first accepted as animals by biologists, having been categorized alongside plants for centuries. However, in a twist of historical irony, a recent paper by Nüchter and colleagues has demonstrated that these animals carry out one of the fastest mechanical processes in the animal kingdom.
One of the keys to the cnidarians' diversity and longevity (11 000 extant species, 600 million years on Earth) is undoubtedly their invention of the nematocyst. Nematocysts are explosive cellular structures that fire a tiny barbed harpoon into predators and prey and inject a painful and paralyzing cocktail of chemicals. While contact with nematocysts by humans is more often uncomfortable than fatal, small crustaceans, worms and even fish often do not survive the encounter.
Discharge of the harpoon tip or `stylet' was known to be fast, but no one knew precisely how fast. Using even the fastest high-speed cameras, the transition of the nematocyst from the undischarged to the discharged state appeared instantaneous, with no intermediates in between. However, high-speed camera technology has improved enormously in the past few years, and in this study, Nüchter and colleagues used a state-of-the art camera to finally resolve the kinetics of nematocyst discharge. With exposure times as short as 200 ns, they were just able to capture nematocysts in motion for the first time.
What they found was truly astounding. Discharge of the barbed stylet was complete in about 700 ns (0.0000007 s), which makes it one of the fastest biological events ever measured. The average velocity of the stylet was about 14 m s-1, with a final velocity of about 25 m s-1. This means that nematocysts are faster than the fastest human sprinter and somewhere between lions and pronghorn antelopes in terms of maximum speed. Of course, nematocysts don't keep this speed up for very long. In fact, the researchers found that the average distance travelled by the stylet was about 13 μm, which is perfectly adequate for a weapon that is discharged by direct contact with its target. While the speed of discharge is impressive,the acceleration is even more so. The team found accelerations that were 1-5 million times the acceleration due to gravity (g). This kind of acceleration is difficult for humans to imagine, especially if you consider that we tend to black out at sustained accelerations greater than only 6 g. In 2004, Patek and coworkers demonstrated in a Nature paper that mantis shrimps are capable of accelerating their striking limbs at more than 10 000 g, which at the time was the highest acceleration recorded from any organism. Nematocysts appear to achieve accelerations that are 500 times greater.
The upshot of all this is that the discharged stylet strikes its target with enough energy to pierce even the tough armour of exoskeleton-sporting prey (and sometimes human skin). Once the armour is breached, a long, coiled tubule everts from the nematocyst capsule through the stylet and into the victim. Further contraction of the capsule forces neurotoxins through the tubule, paralyzing and often killing the victim. The researchers found that tubule eversion was considerably slower than stylet discharge, with kinetics in the leisurely millisecond range.
With the kinetics of discharge now much clearer, researchers will undoubtedly turn their attention to other unresolved questions such as how energy is stored in the capsule and how that energy is released and converted into the kinetic energy of the stylet and tubule. Stay tuned for more studies of these fascinating microscopic weapons.
