Careful now! This is your easy Jellyfish Identification Guide

It’s that time of the year when the warmer waters around our coast see a huge rise in jellyfish activity.

Moon, Barrel and Compass jellyfish are among the more common species that are have been spotted in large numbers. Some more newsworthy jellyfish, like the dangerous Lion’s mane, will be making their annual summer time appearances along the coast as well, and with them a mild hysteria typically follows.

Let’s take a look at these and other free-floating fellows, that for the most part mean you no harm, so you can know which ones are best to avoid.

If you do see any jellyfish why not become a citizen scientist and log your sightings with Explore Your Shore. It’s easy to do and helps scientists keep track of the wildlife around our coast. Check out Explore your Shore for more info.

Here’s your Easy Jellyfish Identification Guide:

lions mane jellyfish Identification Guide

*The great thing about Jellyfish is lots of them are named by how they look. Let’s investigate.

Lion’s Mane Jellyfish (Cyanea capillata)

For sure the highly venomous Lion’s Mane jellyfish – a species which in recent years has become more common in Irish waters – is a red flag for swimming. There have been lots of reported sightings of these along the east coast. They have a nasty sting and in some people can cause anaphylactic shock.

These jellyfish got their name because they literally look like a floating lion’s mane

  • Can grow up to 2 meters in diameter
  • Tentacles divided into eight clusters, each made of 150 long sting covered tentacles
  • Oldest tentacles turn a dark red
  • Sting can produce blisters, irritation and anaphylactic shock

Risk factor: Very dangerous, severe sting

jellyfish Identification Guide

Common or Moon Jellyfish (Aurelia aurita)

As their names suggest these are the most common type of jellyfish found in Irish waters and can usually be found from April to September.

Also called the Moon Jellyfish because their round shape and appear like a moon in the sky

  • Have four pinky purple rings on its back
  • Body is transparent with short tentacles
  • Measure around 25 -40cm in diameter

Risk factor: Very mild sting, you can actually pick them up from their back without being stung

jellyfish Identification Guide

By-the-wind-sailor (Velella velella).

While not a true jellyfish they are a close relative. These beautiful creatures are blue in colour and shaped like an oval disk with a small sail. This ‘sail’ allows them to catch the wind and glides them across the surface of the water. They can arrive any time of year sometimes gathering in large numbers on beaches

These little jellyfish have an impressive sail which makes them easy to identify

  • They float on the water
  • Deep blue in colour
  • Oval disc shaped
  • A thin semicircular fin acts as a sail.
  • Short tentacles hang down into the water from the float.
  • Up to 10 cm in length

Risk factor: mild sting

jellyfish Identification Guide

Barrel jellyfish (Rhizostoma octopus)

These jellyfish have a massive dome shape, is a ghost white colour with purple lobes around the edge.

Barrel shaped, though Cauliflower Brocolli Jellyfish would have been a better name.

  • Grow up to 1 m in diameter
  • Have eight dangling cauliflower like mouth arms.
  • Have no tentacles to sting but prolonged exposure can cause an allergic reaction
  • Most commonly spotted from July to September

Risk factor: Can cause allergic reaction

jellyfish Identification Guide

Compass Jellyfish (Chrysaora hysoscella)

They are yellowy white colour and can be found from July to September.

Their head markings are like a compass

  • Measure about 30cm to 50cm in diameter
  • Have reddish brown V shaped markings around the bell.
  • Have 24 long dangling tentacle and four frilly mouth arms.

Risk factor: Painful Sting

jellyfish Identification Guide

Blue jellyfish (Cyanea lamarckii)

Not as common as other types can be identified by translucent body and blue purple rings inside.

They’re blue!

  • Can grow to about 30cm
  • Have a lot of tentacles around the margin of the dome.

Risk factor: Painful sting

jellyfish Identification Guide

Pelagia jellyfish or Mauve stinger (Pelagia noctiluca)

These guys are becoming more and more common

Yep, they are mauve in colour and have a stinger

  • Very small jellyfish only up to around 10cm in diameter
  • Often found in large groups
  • Occur in autumn and winter

Risk factor: Painful sting

jellyfish Identification Guide

Portuguese Man O’War (Physalia physalis)

Not exactly a jellyfish but closely related.

Ok, not all named as they appear, but this is one you should really be aware of!

  • Large balloon like float that rises up above the water line
  • Can be blue, pink or purple
  • Grow up to 30cm long and 10cm wide

Risk factor: Severe sting and may still retain sting weeks after having washed ashore

What should you do if you get stung?

If you or someone you know gets stung the best option is to go to a lifeguard for treatment. But if that’s not an option here are a few tips:

  • Rinse the affected area with sea-water (do not use fresh water, vinegar, alcohol or urine)
  • If any tentacles are still attached to the skin remove them with a gloved hand, stick, or towel (none of these available use the tips of your fingers)
  • Make sure you do not rub the affected area as this may result in further venom release
  • Apply a ‘dry cold pack’ to the area (i.e. place a cold pack or ice inside a plastic bag and then wrap this package in a t-shirt or other piece of cloth)
  • Seek medical attention if there is anything other than minor discomfort
  • If the patient is suffering from swelling, breathing difficulties, palpitation or chest tightness then transfer to the nearest emergency department urgently
  • For more information on these jellyfish as well as other water safety advice visit: Irish Water Safety

Ultimately if it looks like a Lion’s Mane or a Portuguese man of war (whose image we hope you have now memorised!), swim time should be cancelled.

Intrigued by seashells? Check out our Shell Identification Guide right here!

Curious about Seabirds? This is our Seabird Identification Guide

Want to know about Garden Birds? Here’s our easy Garden Bird Identification Guide

About the Author

Ann Robinson
Has a passion for coastal heritage and maritime history. Loves sharing the best of the Irish coast online. Contact me or follow me on Twitter @AnnRobinson22