Jellyfish Anatomy – Stinging Cells and Sticky Cells

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Jellyfish Anatomy – Stinging Cells and Sticky Cells

The organisms of the Cnidaria and Ctenophora, the 2 main phyla containing jellyfish, the structures used to capture food and in defence are completely different from each other. In the Cnidaria stinging cells, cellular organelles, known as nematocysts, are used, while ctenophores use colloblasts, in food capturing and defence.

Nematocysts – stinging cells

All cnidarians possess nematocysts (stinging cells), their phylum name Cnidaria came from the Greek word for “nettle.” The possession of nematocysts is the primary characteristic that unites such disparate organisms as stony corals, soft corals, sea anemones, sea pansies, sea fans, hydra, medusan jellyfish, and siphonophores.

Each nematocyst is essentially a keratinised capsule with double walls with a coiled up harpoon inside and a trapdoor with a hair Trigger at one end. Even slight mechanical stimulation may cause the harpoon to discharge because of the hair trigger. The harpoon is discharged with an explosive power of 40,000 Gs (40,000 times the force of gravity) which makes the discharging nematocyst one of the fastest biological processes. Eversion, or turning inside out, is the mechanism by which the nematocyst discharges. The harpoon shaft is hollow and often has perforations. As the harpoon penetrates the skin it may deliver venom in 3 different ways: by hypodermic injection through the tip of the harpoon, through the perforations along the shaft of the harpoon and by the residue on the outside of the shaft. These 3 delivery mechanisms result from the venom being contained in the capsule, inside the harpoon as well as on its outer surface. There are strong spines on the shaft of the harpoon, particularly near its base, that help anchor the harpoon in the prey as it penetrates. There may be 3 rows of smaller spines on the remainder of the shaft, or possibly none at all.

Colloblasts – sticky cells

Colloblasts that are present in ctenophores have no harpoon, being more similar to a rope covered in an adhesive substance. There is no venom in colloblasts, and they are only used in the capture of food, not in defence.

The colloblast consists of a bouquet-shaped structure, a collosphere, which has adhesive granules, and is supported by an axial (central) thread around which is wrapped a spiral filament. When the colloblast is stimulated the spiral thread straightens, which activates the colloblast, the granules bursting to release their glue. There are large numbers of colloblasts on tentacles and lobes present on Ctenophores, and present on the fine tentacles around the lips of some species. Haeckelia rubra is one species that completely lacks colloblasts, though it co-opts nematocysts for its own protection from the jellyfish it preys upon.

Jellyfish stings

Nematocysts have received a lot of attention from researchers because of their venom, but not so much research has been carried out on colloblasts. It has been found that generally different areas of jellyfish have different types of nematocysts, such as the bell, tentacles, lips and stomach. Many dozens of different types of nematocysts have been identified. Typically the structures discovered have been spherical, ovoid, lemon-shaped and banana-shaped. For many cnidarian groups the identification of species is helped by the number and forms of nematophytes that are present. They are often the only means by which a species can be distinguished from another species, especially after stings, fragments of tentacles or nematocysts remaining on the skin may be the only objective evidence that is available.

There is a very simple method for retrieving nematocysts from the skin following a sting: adhesive tape is placed on dry skin area sticky side down and pressed to the skin and peeled up. The tape is then examined under a microscope. This provides a method of identifying the species of jellyfish responsible for the sting that is non-invasive, safe and effective.

The treatment for a jellyfish sting is largely species specific.

Sources & Further reading