Watch that stinger, if you can...
One of the fastest biological
processes known to man — that of the jellyfish sting — now has been
captured on camera. Thomas W. Holstein and his colleagues at the University of
Heidelberg in Germany recently recorded the phenomenon.
Organisms in the phylum Cnidaria have nematocysts,
or stingers, that contain a mixture of hemolytic and neurotoxic poisons. The toxins
can be discharged with a pressure of about 15 MPa, enough to penetrate even the
thick outer shells of a crustacean and to cause a great deal of pain.
The tentacles of a polyp Hydra contain hundreds of painful stinging
nematocysts. Researchers studying the sting process found that the nematocystscan
accelerate millions of times faster than gravity and that they generate 7 GPa of
pressure upon impact, roughly equivalent to that produced by some bullets. All of
this occurs in 700 ns.
The researchers studied the cnidarian
Hydra oligactis and H. vulgaris, whose tentacles have hundreds of
nematocysts each. They cultured the tentacles at a temperature of 18 ±1 °C
and placed them on a slide attached to two gold or aluminum electrodes spaced 1
cm apart. The electrodes produced an electronic pulse of 0.4 kV/m, 50 to 100 μs
long. This pulse, for the experiment, triggered the nematocyst discharge.
A schematic of a nematocyst shows the steps involved when a jellyfish
stings its prey (A). Nematocytes comprise a cyst (pink) that contains a minicollagen
wall, a tubule and a hatchlike opening from which stylets (black) eject, piercing
the prey (A, left). A two-photon micrograph (A, right) shows microtubules anchoring
the capsule (tubulin antibody, green; DAPI-stained nucleus, red). A high-speed camera
in framing mode captured sequential images at 1.43 million fps; arrows indicate
progress of discharge (B). The same camera in streak mode shows, at 3-μs resolution,
the ejection of the stylets (arrow), followed by wall contraction (red circle),
a short arrest phase (red arrowhead) and tubule eversion (C). Micrograph on left
shows slit position (red brackets) during discharge. Courtesy of Cell Press.
They used a Hamamatsu framing/streak
camera, which can capture up to 1.43 million frames per second, and a cooled Hamamatsu
CCD camera. The instruments produced reliable information on the kinetics and forces
involved in the stinging process.
The investigators found that the tip
of the nematocyst travels about 13 μm in roughly 700 ns, yielding an average
velocity between 9.3 and 18.6 m/s, a final velocity of approximately 18.5 to 37.1
m/s, and a kinetic energy of approximately 0.17 to 0.7 μJ.
Via a photocathode, the camera used to capture jellyfish stings converted
photons into electrons that were accelerated by the electrodes, deflected by either
the streak or the framing deflection plates, amplified thousands of times by a multichannel
plate, and moved to a phosphor screen and converted into an optical image. Courtesy
of Cell Press.
The acceleration is more than 5 million
times greater than gravity, and the researchers believe that it is produced by vesicle
exocytosis, which releases stored energy from the contraction of the minicollagen
polymer nematocyst walls. The tip has a mass of about 1 ng and an accelerating force
of between 13.2 and 53.1 μN. It can reach its target with a pressure at the
penetration site roughly equivalent to that produced by some bullets, or more than
The results, published in the May 9
issue of Current Biology, also show that this process seems to slow as the
level of calcium ions present in the culturing medium drops.
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