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Jane Sheehan
Jun 17, 2022

An Introduction to Jellyfish


Jellyfish might not be the most charismatic ocean animals, but they are a natural and important part of life in the Atlantic and in the Celtic and Irish Sea. Drifting through the water like translucent aliens, they can be mesmerizingly beautiful. We are familiar with the characteristic little blobs adorning both Iveragh and Llŷn shorelines. We see them on our strolls and often retreat at the site of one. But do we ever stop to wonder, what exactly is a jellyfish?


The term “jellyfish” refers to a group of diverse animals, which are some of the oldest in existence. [1] A “true” jellyfish refers to the scyphozoans - the larger, umbrella or bell-shaped animals, which we are familiar with. They are 95% water and do not have a heart, brains or blood. The “jelly” in jellyfish refers to their mesoglea, a gelatinous tissue which provides support between the outer skin and inner organs, almost like an elastic skeleton.[2]


The margin of their bell is lined with tentacles, and in the centre of their bell is a mouth lined with four oral arms which help them feed. Sometimes these oral arms will be solid, other times they will look like a veil (as seen on the lion’s mane jellyfish).


Figure 1: Anatomy of a jellyfish


Though they have a mouth, jellyfish do not have jaws like most other predators, and this makes it hard for them to catch and eat prey. This is where their tentacles come into play. Their tentacles are lined with stinging cells, known as “nematocysts”. [3]


These enable jellyfish to inject their venom into passing prey to paralyse them in preparation for digestion. Jellyfish are classified into the Phylum Cnidaria. Cnidae is the Greek for “nettle”. Anyone who’s been stung by a jellyfish can attest to this apt naming!


Their mouth opening, which also functions as their anus, leads to a simple digestive cavity, which is both its stomach and intestine. The radial canals then help distribute the digested food to other parts of the jellyfish’s body.


Figure 2: Anatomy of a jellyfish (bell view)


Lacking a brain, jellyfish instead use their central nervous system paired with sensory receptors to alert them of nearby organisms, chemical changes, and help give them balance in the water. These sensory receptors (rhophalia) allow them respond to predators by triggering their stinging mechanisms. In the box jellyfish, an exotic species not found in Irish waters, this rhophalium is developed into a rudimentary eye which allows them to sea and actively hunt prey.[4]


Another extraordinary ability of jellyfish is that around 50% of different gelatinous species are bioluminescent. This is the ability to create flashes of light. When their sensory receptor is activated, jellyfish bioluminesce as a defence mechanism against predators. Some species can even produce a trail of glowing slime to confuse predators. [5]


The true jellyfish live for many months growing in the open water. Given the correct conditions, true jellyfish can form dense blooms or aggregations. True jellyfish also have an unusual life cycle. The first person to ever observe and detail the jellyfish life cycle is Valentia’s own pioneer marine biologist, Maude Delap. Maude Delap reared jellyfish in her own tank which allowed her to fully observe how jellyfish reproduce. [6]


This two-part life cycle consists of a free-swimming phase, known as the “medusa” stage, and a sessile phase, known as the “polyp” stage (Figure.2). The “Medusa”, named after the Greek monster who had venomous snakes for hair, refers to a fully mature jellyfish; the bell-shaped animal we described earlier which floats along the water column.


Most jellyfish are either male or female, meaning that an adult medusae will either produce sperm or eggs. Once an egg has been fertilized internally within the medusa, it produces a “planula”. A planula is a small free-swimming larvae shaped like a Tic-Tac. This floats about the waters in its free-swimming stage, and eventually settles onto a hard surface, like the sea floor or the underside of a pier. [7]




This is when it develops into a “polyp”. A polyp, or hydroid, is a stalked structure a few millimetres tall which has a ring of tentacles much like a sea anemone (another cnidarian and relative of the jellyfish). The polyp cannot swim but feeds using its tentacles to trap and ensnare food. Finally, the polyp grows and becomes the “budding” polyp and produces a baby jellyfish, referred to as an “ephyra”.


Polyps in Ireland typically produce ephyrae between December and March. These float in the water, feeding on plankton, until they eventually grow to become the dome shaped jellyfish we know so well. The jellyfish life cycle is complex and bizarre, even more so when you realise that after a polyp has finished releasing baby jellies, it turns back into the feeding polyp stage! Each winter it will turn back into a polyp, produce more baby jellyfish, and then metamorphose again and again for up to 10 years! This means one single hydroid can produce 100’s of baby jellies. To think an Iveragh woman was one of the first people in the world to see this strange life cycle in action 100 years ago is astounding! [8]


Of course, this life cycle can vary between species. In oceanic species, there is no hydroid phase. Instead, they reproduce by direct development. This refers to ability of the adult jellyfish medusa to produce a larval form known as a planula, which will grow in size and eventually develop tentacles and transform into a free-swimming medusa without any hydroid phase.


You’ve probably been wondering why they are referred to as “true” jellyfish. This is because some species are jellyfish-like, while not actually being within the same classification as our classic jellyfish. Sometimes we see species in our waters or washed up on our shores which are strange and unusual, and don’t quite look like the classic umbrella shaped jellyfish. Examples of these are the Portuguese Man-O’War and the By-the-wind-sailor. These fascinating cnidarians are not one single organism, like a true jellyfish, but they are colonies of animals. All individuals in the colony are identical, but each one carries out different functions such as feeding and reproducing. Together each of these individuals work to operate as a single colony.[9]



Next time you are out and about within your marine environment stop to contemplate the absolute wonder and marvel that are jellyfish. The best place to find and observe jellyfish is by strolling along your local beach. Piers and raised platforms give a good view of jellyfish from above.


It’s always best to observe jellyfish with caution, as many species can provide a very painful and dangerous sting. The HSE recommended advice for jellyfish stings can be found here: https://www.hse.ie/eng/health/hl/water/bathing/jellyfish-in-irish-coastal-waters.html.


Recent studies into the efficiency of various remedies for stings from the Portuguese man o’ war have recommended differing results. The study concluded that sting management should include pouring vinegar onto the wound for 30 seconds and applying a 45°C hotpack for roughly 40 minutes. Read further into this research here: https://www.mdpi.com/2072-6651/9/5/149


Do you have a sighting of a jellyfish? Please consider logging it to the National Biodiversity Database and sending your photo and description to The Big Jellyfish Hunt. Follow our social media pages, where we will be giving detailed descriptions of the jellyfish found in Irish waters.



Figure 4: The mauve stinger (Pelagia noctliuca)


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