Beware jellyfish tentacles

How do jellyfish sting?

Jellyfish and their relatives, the sea anemones, all have special cells in the skin called cnidocytes. These are especially abundant on their tentacles. Inside the cnidocytes are harpoon-shaped structures called nematocysts, these are the stings. They are triggered when something such as a small fish or shrimp brushes past a hair-like structure, called a cnidocil, which acts like a trigger. When the cnidocil is triggered the lid or operculum of the cell opens, hydrostatic (water) pressure turns it inside out and the nematocyst explodes to the outside.

The nematocyst often remains attached to the animal by a threadlike tube, which may have spines on it. The animal uses the thread to reel its catch into its central mouth. There is a lot of variation in the spines on the thread and these can help jellyfish scientists work out what sort of jellyfish they are looking at.

Some nematocysts can deliver poison to the prey. The poisons can have an effect on the nerves, the blood, the muscles and some can even make tissues rot. Most of the poisons do not affect humans much, but some can be very painful, such as the portuguese man-o-war (Physalia sp) and some can be fatal such as the box jellyfish (Chironex sp). Fortunately New Zealand has very few of these.

The Portuguese man-o-war is our most common stinger along with the harmless moon jelly (Aurelia sp).

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