Where do jellyfish ellyfish come from??

MY first memory of jellyfish comes from a family excursion to west Cork when I was a child. I do not remember the exact location but it was somewhere near Bantry.

I remember arriving at the beach and looking out on a shoreline that was completely choked with jellyfish. Naturally we were disappointed at not being able to swim but even then we recognised that this was something quite normal that happens from time to time.

Little did I know then, that 30 odd years later I would be studying jellyfish and trying to understand what drives these enormous blooms that occur from time to time? Why does this not happen every year?

The group of jellyfish that is familiar to most people, like the lion’s mane (pictured right), are called scyphozoans and the lion’s mane is one of six species that occur annually around Ireland.

The scyphozoans are often referred to as the ‘ true jellyfish’ and what you can see in the picture is a lion’s mane medusa and is in fact the second stage in a two-stage life cycle.

The first stage is a polyp, similar to a tiny anemone, which is fixed to rocks or other hard objects including manmade structures like piers and pontoons. Every single polyp is a living animal that feeds on tiny zooplankton and has the ability to multiply asexually simply by growing a new polyp from itself in a process called budding.

Each polyp can in turn produce anything from one to 30 tiny medusa called ephyrae ( a process known as strobilation) depending on the species. The ephyrae quickly develop small, fullyformed medusa becoming voracious predators that consume a variety of zooplankton including other jellyfish.

Each medusa is either male or female and when fully mature they release sperm and eggs into the water where fertilisation can take place. Each fertilized egg develops into a planula larvae which will eventually settle on a hard surface and then develop once again into a small polyp.

It is this ability to multiply quickly that results in certain species of jellyfish washing up on our beaches during the summer and autumn in vast numbers.

By studying and quantifying these strandings, scientists have shown that each species has a different seasonality and distribution around the Irish coastline.

For example, while moon jellies are found all around Ireland and peak in June, lion’s mane jellies are only found north of Dublin in the Irish Sea and peak in late August.

These differences in the medusa populations can be partially explained by looking at the oceanography and prevailing wind patterns around Ireland, however the polyp populations which seeds the medusa populations remain virtually unknown. Are the polyps distributed evenly around the coastline, or perhaps in dense colonies in few small places? We simply do not know.

So the next time you see stranded jellyfish, you will know that what you are seeing is in fact part of a complex two-part life cycle that is the result of a subtle interplay between biology and physical oceanography. And just maybe in another 30 odd years we will have solved some of the mysteries remaining around these fascinating animals.