All the species of fauna (even flora) have a defense mechanism against their predator species and non-friendly stimulus. It doesn’t matter if it is the marine, terrestrial or aerial species, every organism ranging from the smallest organisms like amoeba to the blue whale, the biggest living species on earth has these mechanisms to reduce the chances of getting attacked and is used to even find food.
One of the quickest defense mechanism is found in the phylum Cnidaria, constituting the species like jellyfishes, Portuguese man-o-war, sea anemones, and corals. It consists of Cnidocyte and Nematocyst.
Cnidocyte vs Nematocyst
The main difference between a cnidocyte and a nematocyst is that the cnidocyte assists the thread, cnidocil to eject with the help of nematocyst that propels it out and in the waters.
Cnidocyte is a capsule present on a cnidarian’s body that ejects thread-like projections as a defense mechanism. It not only allows the organism to escape danger but also assists in hunting for food and adherence to the substratum during locomotion.
As soon as the species senses danger, the capsule makes the cell explode and release the hair-like cnidocil, which not only is a mechanical but also a chemoreceptor. This action occurs in milliseconds injecting an enzyme to stun or injure the predator.
A Nematocyst on the other hand is the explosion cell (a subcellular structure or organelle) that ejects the cnidocil, the hair-like projection is released when the trigger is activated containing venom that also helps in locomotion. However, this function is enabled after a certain time. It only works when the organism has matured. The cells that produce nematocysts are called nematoblasts.
Ewer, in 1947 defined the functional categories of nematocysts as prey capture (penetrants), defense (volvents), and adherence during locomotion (glutinants). The nematocysts contain the barbed or venomous coiled thread that is released at times of emergency. the sting contains a toxin called hypnotoxin which is a mixture of phenols and proteins.
Main Differences Between Cnidocyte and Nematocyst
| Parameters of Comparison | Cnidocyte | Nematocyst |
| Structure | It is an inverted cell. | It is a bag like globular structure. |
| Presence | It is present in the skin of the species. | It is present inside the cnidocyte. |
| Further Divison | It consists of cnida, cnidocyst, nematocyst, ptychocyst, and spirocyst. | It is a cell present in the capsule containing the thread. |
| Function | To assist the nematocyst to eject the sting/hair. | To eject the sting/hair like projection. |
| Composition | Composed of novel Cnidaria-specific genes. | Composed of mini-collagens and the glycoprotein NOWA. |
What is a Cnidocyte?
It is an explosive cell resulting in the ejection of a thread-like projection or a ting used as a defense mechanism. It consists of many organelles present in it- cnida, cnidocnseyst, nematocyst, ptychocyst, and spirocyst. It is a well-differentiated cell and the response span is in milliseconds.
Whenever a predator approaches and the trigger is activated, the organism or the animal, the cnidocyte reacts in a few milliseconds and assists the nematocyst to release the hair-like projection/hair/sting to stun or kill the enemy depending upon the species. It not only acts as a penetrant but also as volvents and glutinants. Cnidocytes are modified epidermal interstitial cells.
Other than defending the being, it can also be used to attack for food and for locomotion. It takes time for the creature to be mature enough to use the cnidocyte as a defense mechanism. These defense mechanisms are found in the phylum Cnidaria that constitutes the species like sea anemones, corals, jellyfish, Portuguese man-o-war, and other hydroids and sea pens.
What is a Nematocyst?
It is a subcellular structure or organelle containing extrusive filaments found in a group of unicellular eukaryotes and Cnidarians. It is present inside the cnidocyte and helps it to eject the hair/thread projection as a penetrant, volvent, and glutinant. When the trigger is activated, the cnidocyte makes the nematocyst to release the sting/hair, called the cnidocil.
When properly stimulated, the thread is ejected and, depending on the type of nematocyst, may either adhere to surrounding objects, penetrate prey or offending organisms. It is composed of mini-collagens and the glycoprotein NOWA, consisting of a globular or egg-shaped capsule.
Main Differences Between a Cnidocyte and Nematocyst
- A cnidocyte is present on the skin of the creature (on the tentacles in the case of jellyfishes) whereas a nematocyst is present inside the cnidocyte.
- A cnidocyte is like an inverted cell whereas a nematocyst is a bag-like and globular structured subcell.
- A cnidocyte assists the nematocyst when triggered whereas a nematocyst releases the cnidocil.
- A cnidocyte is composed of cnidaria-specific genes whereas a nematocyst is composed of mini-collagens and glycoprotein NOWA, a novel protein with mini collagen Cys-rich domains.
- A cnidocyte consists of cnida, cnidocyst, nematocyst, ptychocyst, and spirocyst, whereas a nematocyst consists of a cell present in the capsule containing thread.
Conclusion
Both the terms, cnidocyte, and nematocyst are confusing but are very different biological terms. Cnidocyte is the cell responsible for the defense mechanism in cnidarians and nematocyst is the subcell that releases the pincer/thread.
The cnidocyte is present on the skin of species in the phylum, Cnidaria. On the other hand, the nematocysts are present inside the cnidocytes. Cnidoblast is another term used for a cnidocyte.
References
- http://www.ijdb.ehu.es/web/paper.php?doi=10.1387/ijdb.113472ab
- https://science.sciencemag.org/content/243/4898/1589.abstract