Cnidaria is a group of animals that includes – among other things – jellyfish, corals and sea anemones. They take their name from the greek word for nettles (knide) because of to the sting and rash that a close encounter with them can cause. They elicit this response with a particular type of stinging cell that only they possess, the cnidocyte, which is arguably the most complex cell possessed by any animal. When triggered, a cnidocyte releases a hollow harpoon that penetrates prey organisms – or a swimmer’s skin- and injects toxins. These harpoons are microscopic, and there are many types of cnidocytes each with a different type of harpoon. Some create a painless sticky sensation, others are so powerful that a sting from just one cell can cause considerable burning.
The siphonophores, a group of colonial cnidarians, have multiple polyps and medusae that are specialized for tasks such as locomotion, feeding or reproduction. The picture on the left shows a feeding polyp (the prominent white structure in the center) of the siphonophore Nanomia bijuga. This feeding polyp is attached to the stem of the colony, which stretches across the top of this photo. Each feeding polyp has a single tentacle, and this tentacle has side branches with dense batteries of cnidocytes. Most of the cnidocytes are densely packed into a fascinating complex structure – the cnidoband. These are the orange spirals in the photos. The cnidoband ends in a filament (lower part of the picture) which contains sticky cnidocytes. The terminal filament makes first contact to the prey and sticks to it, which then tugs the cnidoband as the prey struggles. The cnidoband then stretches out and its cnidocytes fire as a unit, deploying their deadly power. These Nanomia bijuga were collected using the ROV Ventana with the friendly support of MBARI. Photos by Stefan Siebert.