Thomas Holstein and his colleagues at the University of Heidelberg have used an ultra-high speed camera to study in detail the mechanism of the jellyfish sting, and report their findings in today’s issue of Current Biology.
Jellyfish belong to the phlyum Cnidaria, a diverse group of aquatic organisms which includes corals and sea anemones. The name Cnidaria comes from the Greek word cnidos, meaning ‘stinging nettle’; all cnidaria possess stinging cells called nematocysts (left), which are believed to have been inheritied from one common ancestor.
On their tentacles, jellyfish have high densities of nematocysts. These specialized stinging cells contain neurotoxic and haemolytic (red blood cell-destroying) chemicals and are discharged when the tentacles are stimulated by the appropriate stimuli. The toxins contained in the nematocysts act as a sedative on prey captured in the tentacles.
It was known that nematocysts are discharged under extremely high pressure and that they can penetrate the shells of crustaceans, but the process was, until now, too quick to be resolved by the best high-speed cameras.
Holstein’s team has used an ultra-high-speed electronic framing-streak camera to measure the kinetics of nematocyst discharge. Because the camera records over 1.4 million frames per second, its resolution was high enough to capture the discharge of Hydra nematocysts, and to determine that the process takes 700 nanoseconds (billionths of a second).
During discharge, Hydra nematocysts studied create an acceleration of up to 5.4 million g, and exert a pressure of more than 7 giga-Pascals, on a par with some types of bullets. Holstein and his team believe that the high speed and pressure generated during nematocyst discharge are created by thesudden release of energy stored in the stretched collagen-polymer wall of the nematocyst.
Axial patterning in the cnidaria
Hox genes control axial patterning (the development of the anterior-posterior axis) and are highly conserved in evolution, being found in clusters across the animal kingdom, from fruit flies to humans. A mutation in the fruit fly antennapedia gene, for example, results in the development of a leg in place of an antenna, because the patterning of the antero-posterior axis has been disrupted.
A paper by Kamm, et al, in the same issue of Current Biology, shows that although Hox-like genes are found in the sea anemone Nematostella and the hydromedusa Eleutheria, they are not organized in clusters. This led Kamm, et al to conclude that the diversification of the cnidaria predates the evolution of homeobox (Hox) genes.