9 May 2006

Jellyfish Quickest On The Draw?

by Kate Melville

Using a super-fast camera, researchers from the University of Heidelberg have captured the explosive discharge of tiny jellyfish stingers (nematocytes), revealing what could be the fastest cellular process in nature. Working with Hydra magnpapillata and Hydra oligactis, the researchers believe they are the first to quantify the kinetics and forces involved in the stinging action. This was impossible in the past with conventional high-speed imaging as the discharge was simply too fast.

Hydras are small jellyfish-like animals that can be found in bodies of freshwater. They anchor themselves to surfaces using a simple adhesive foot. At the other end of the body is a mouth surrounded by a ring of thin mobile tentacles. Each tentacle is covered with specialized stinging cells called cnidocytes. These cnidocytes contain specialized structures called nematocysts which look like miniature light bulbs with a coiled thread inside. At the narrow outer edge of the cnidocyte is a short trigger hair. Upon contact with prey, the contents of the nematocyst are explosively discharged; firing a dart-like thread containing a cocktail of hemolytic and neurotoxic poisons into whatever triggered the release.

The injection of the toxins requires the physical barrier of the prey's outer-surface to be broken - not an easy job if the prey is a crustacean. But Hydra stingers are up for the job, as the camera, running at 1,430,000 frames per second, showed. It established the discharge kinetics of nematocysts in Hydra to be as short as 700 nanoseconds; creating an acceleration of up to 5,410,000 g. The researchers, writing in Current Biology, calculate that although the accelerated mass is very small (~1 nanogram), pressure generated at the site of impact is more than 7 GPa, which is in the range of that generated by some bullets, and sufficient to penetrate the cuticle of crustacean prey.

The researchers hypothesize that the high speed of discharge is caused by the release of energy stored in the stretched configuration of the collagen-polymer of the nematocyst wall. This inspired piece of evolution allows the cellular process of vesicle exocytosis to release kinetic energy in the nanosecond range.

Source: Current Biology