Jellyfish

Jellyfish illustration

Unlike sea anemones, the adult form of jellyfish is a floating disc with stinging tentacles around the rim and a mouth in the middle – very much like a sea anemone turned upside down. Jellyfish are most common around Irish coasts in the summer and autumn months, but are becoming increasingly common later in the year, possibly due to changes in ocean currents.

The tentacles of some jellyfish can reach several metres away from their bodies and are almost invisible to the naked eye. So give jellyfish a wide berth when swimming and never touch one in the water or on the shore, even if it appears dead, since the stinging cells in the tentacles can go on stinging for a long time after the jellyfish has been washed ashore.

The most common species around the Irish coasts are:

• The Common Jellyfish (Aurelia aureta) which looks like a transparent disc of jelly up to 25 cm in diameter with four purple horseshoe-shaped reproductive organs around the centre.

• The Compass Jellyfish (Chrysaora hysocella) which looks like a white disc about 30 cm in diameter with brown stripes radiating from the centre, brown marks around the rim and long, flowing tentacles hanging down below it. Normally this jellyfish is found floating offshore, but occasionally washes up on beaches in the summer months.

• The Portuguese Man-of-War (Physalia physalis) is actually a colony of four types of connected polyps working together – an inflated polyp to give it floatation, long stinging polpys to sting and capture food, a third polyp to digest the prey and a fourth to act as its reproductive organs. The Portuguese Man-of-War gets its name from the inflated bladder on its back that looks somewhat like an old sailing ship in full sail. The long, thin tentacles can extend on average around 30 metres in length, although 50 metres is not unknown. These jellyfish can occur in large swarms of up to 1,000 or more and have no means of independent propulsion.

• The Lion’s Mane Jellyfish (Cyanea capillata) has been seen in Irish waters during recent years and is the largest known species of jellyfish in the world. They are a coldwater species capable of growing up to 2.5 metres in diameter across the bell (although in Irish waters sizes of 50 cms are much more common) and have sticky tentacles that trail behind the body for up to 30 metres or more. These tentacles can deliver a very painful sting. Once washed up on the shore, the bell of the Lion’s Mane looks like a purply-brown floppy hat with eight lobes around the edges. It must not be touched, even if it appears dead.