Nematocysts are stinging cells characteristic of the phylum Cnidaria (corals, anemones, and jellyfish). Each one is built with a coiled filament that is stored inside out within the cell. When prey activates the trigger (probably a combination of touch and chemical stimuli), calcium is released rapidly, which creates an osmotic gradient and causes a rapid influx of water, which in turn causes an explosive eversion of the filament. Some nematocysts contain long threads that slowly elongate to a millimeter or more, and trap the prey, while others contain barbed needles that inject venom into the prey. This image is constructed from a high-speed movie sequence (also embedded below), and shows several nematocysts everting from a tentacle pulled from the foot of the sea anemone Metridium senile, a large species, which grows abundantly on marine docks throughout the Pacific Northwest.
The pedal tentacles of the sea anemone can be induced to fire their nematocysts in the laboratory in response to a flood of potassium chloride—an artificial stimulus, which likely works by mimicking the firing of a nerve cell that would normally control the nematocysts.
The movie was recorded with a high-speed video camera mounted on a light microscope at 1000 frames per second. It is played back at 30 frames per second, so the total time covered by the movie sequence is just two seconds.
For the image above, three frames were extracted from the movie sequence, and the individual frames were colored in red, green and blue, and merged into a single image using a computer so that color represents time elapsed—less than 2 seconds.
The field of view is about a half a millimeter wide.
George von Dassow PhD, Oregon Institute for Marine Biology, Charleston, OR