Phylum Cnidaria: jellyfish, corals, and anemnem… amenome… we give up.


Phylum Cnidaria contains all your favourite ocean stingers from jellyfish to corals to anenomes, and we can’t forget our favourite Hydrozoan, the Portuguese man o’ war.



The body wall is composed of three layers:

  1. Epidermis tissue – outer layer.
  2. Mesoglea – “jelly in the middle” composed of mucopolysaccharides & collagen; is not true tissue but provides support, buoyancy, and locomotion.
  3. Gastrodermis tissue – inner layer which lines the gastrovascular cavity.

A distinguishing feature of cnidaria is their simple gastrovascular cavity, present in only one other primitive phylum. It is a two-way system, where food enters through an opening that serves as a mouth and an anus and is extracellularly digested within the gastrovascular cavity, then waste exits back through the same hole (Shostak, 2001).

Medusa & polyp form

Cnidaria can exist in two forms: medusa or polyp (Piraino, Boero, Aeschbach, & Schmid, 1996; Seipel & Schmid, 2004). Some cnidarians only exhibit a polyp or medusa form, or have one predominantly over the other. Some may pass through both forms throughout their life histories.

Polyp form:

  • Sessile
  • Cylindrical tubes point upwards
  • The oral end is on top
  • The aboral end usually attached to the substrate
  • Tentacles point upward
  • E.g., sea anemone
  • Some polyps form colonies (e.g., coral)

Medusa form:

  • Swims
  • Is the inverse of the polyp
  • The oral end is below a bell-shaped body
  • Th aboral end is to top of the umbrella-like structure (velum)
  • Tentacles point downward
  • E.g., jellyfish
Difference between polyp and medusa. ( Copyright 2001 by Sinauer Associates Inc.

Nematocyte and nematocyst

Yikes. Let’s get this clear:

NematoCYTE is the CELL.

NematoCYST is the ORGANELLE.

The nematocyte is a specialised, ectodermal stinging cell involved in defence and prey capture. It contains the nematocyst (Beckmann & Özbek, 2012; David et al., 2008; Östman, 2000).

The nematocyst is an organelle inside the nematocyte consisting of an ejectable thread that causes a sting and injects toxins into predators or prey. The nematocyst capsule is firm and made from a type of collagen, and it holds a coiled thread that can be barbed, smooth, or hold toxins. A chemical or physical stimulant stimulates the ejection of this thread, and once stimulated, will uncoil and extrude out, penetrating or wrapping around the prey. Once paralysed, captured, or killed (who knew sea anemones are so gruesome??), the tentacles will move the prey to the oral cavity to be digested by the gastrovascular cavity.

File:Nematocyst discharge.png - Wikimedia Commons
Showing the discharge of a barbed nematocyst from a nematocyte.

There are three types of nematocysts:

  1. Penetrant – barbed thread with open tip; when discharged, it pierces the skin/exoskeleton and injects venom to paralyse or kill.
  2. Glutenant – smooth or bristled thread with an open tip that is sticky and has toxins.
  3. Volvent – smooth, lasso-like thread with closed tip entangles prey.


Cnidaria are capable of both sexual and asexual reproduction (Shostak, 2001). Sexual reproduction involves gametes, usually produced in separate individuals, and are fertilised in the gut, ovary, or water after being released by the mouth, tentacles, or breaks in the epidermal layer. Female gametes may produce a substance that attracts male gametes. Asexual reproduction usually happens in warmer months, where a bud develops via evagination from the adult body wall and contains an extension of the gastrovascular cavity. Once fully developed, it detaches from the parent.


Coral have a unique symbiotic relationship with zooxanthellae which gives the coral a range of different colours (Shostak, 2001). Coral produce carbon dioxide (CO2) and ammonium (NH4+) as a by-product of cellular respiration, and zooxanthellae use the CO2 and NH4+ to conduct photosynthesis which, in turn, supplies the coral with sugars, lipids, and oxygen.

Coral bleaching is a phenomenon where the coral consumes or expels their symbiotic inhabitants to ensure short-term survival when exposed to stressful conditions such as rising water temperature, leading to a white “bleached” appearance (Hoegh-Guldberg, 1999; Lesser, 2011; Nir, Gruber, Shemesh, Glasser, & Tchernov, 2014). The coral continues to live after bleaching, but under a prolonged, stressful environment, they will die from starvation.


Class Anthozoa

This group includes sea anemones, stony corals, and soft corals.

Apple anemone (Stomphia didemon).
Rare, long-lived, deep-sea Hawaiian gold coral (Kulamanamana haumeaae).

Class Scyphozoa

These are the true jellyfish.

Chrysaora melanaster, commonly known as the northern sea nettle or brown jellyfish, is a species of jellyfish native to the northern Pacific Ocean and adjacent parts of the Arctic Ocean.
A species of jellyfish from the genus Cephea, or the cauliflower jellyfish.

Class Cubozoan

Box jellyfish are distinguishable by their cube-shaped medusae. Some species have potent venom that can be extremely painful and potentially fatal to humans.

Chironex fleckeri, or the sea wasp, is thought to be the most lethal jellyfish in the world and is responsible for sixty-four deaths in Australia from 1884-2021 (Fenner & Williamson, 1996). C. fleckeri is said to contain enough venom to kill sixty adult humans, and stings are typically excruciatingly painful and, if left untreated, can kill within two to five minutes.

Chironex fleckeri.

Malo kingi, or the common kingslayer, is a species of Irukandji jellyfish named after one of its victims, Robert King (Gershwin, 2007). The Irukandji jellyfish are any of several box jellies that cause Irukandji syndrome after stinging their victims; Irukandji syndrome is characterised by severe pain, vomiting, and rapid rise in blood pressure. M. kingi are very small and inconspicuous in the water, making it hard for victims to see them.

Malo kingi.

Class Hydrozoa

To end on a fun note, let’s finish with Hydrozoa. They are a group of very small, predatory individuals that can live in solitude or within a colony. The colonial species can be large and sometimes the specialised individuals cannot survive outside of their colony. So, the next time you hear someone calling a Portuguese man o’ war (Physalia physalis) a jellyfish, feel free to roll your eyes and let them know they are actually a Hydrozoan.

Portuguese man o’ war (Physalia physalis) washed up on a beach.


Beckmann, A., & Özbek, S. (2012). The nematocyst: a molecular map of the cnidarian stinging organelle. International Journal of Developmental Biology, 56(6–7–8), 577–582.

David, C. N., Özbek, S., Adamczyk, P., Meier, S., Pauly, B., Chapman, J., … & Holstein, T. W. (2008). Evolution of complex structures: minicollagens shape the cnidarian nematocyst. Trends in genetics, 24(9), 431–438.

Difference between polyp and medusa. From Difference Copyright 2001 by Sinauer Associates Inc.

Hoegh-Guldberg, O. (1999). Climate change, coral bleaching and the future of the world’s coral reefs. Marine and freshwater research, 50(8), 839–866.

Lesser, M. P. (2011). Coral bleaching: causes and mechanisms. In Coral reefs: an ecosystem in transition (pp. 405–419). Springer, Dordrecht.

Nir, O., Gruber, D. F., Shemesh, E., Glasser, E., & Tchernov, D. (2014). Seasonal mesophotic coral bleaching of Stylophora pistillata in the Northern Red Sea. PLoS One, 9(1), e84968.

Östman, C. (2000). A guideline to nematocyst nomenclature and classification, and some notes on the systematic value of nematocysts. Scientia Marina, 64(S1), 31–46.

Piraino, S., Boero, F., Aeschbach, B., & Schmid, V. (1996). Reversing the life cycle: medusae transforming into polyps and cell transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa). The Biological Bulletin, 190(3), 302–312.

Seipel, K., & Schmid, V. (2004). Mesodermal anatomies in cnidarian polyps and medusae. International Journal of Developmental Biology, 50(7), 589–599.

Shostak, S. (2001). Cnidaria (Coelenterates). e LS.


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Marine Biologist and Geospatial Scientist. Obsessed with sharks; enamoured with the ocean.

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