Anja Murray: Jellyfish to watch out for and ones you don't need to be afraid of

Jellyfish are often present too, swimming gently through the water with their graceful pulsing movements, beautiful and menacing. Like most people, I do not welcome the sight of jellyfish in waters where I swim, even though I realise that most jellyfish are not a threat to us. Any time I’ve been stung, it feels like a nettle sting, a little uncomfortable though the hurt is short-lived. A sting from a jellyfish, in my experience, is never as bad as the fear of being stung.

Even though jellyfish are simple creatures, without a heart, a liver, a brain, or lungs, their stinging cells have the most complex structure of any cells found in any animal. Without jaws or teeth, jellyfish have evolved to rely on their stinging to stun their prey and feed themselves. Each of their stinging cells, known as a nematocyst, is like an inflated balloon containing a tiny harpoon inside. When the cell brushes off something, perhaps a fish or a human leg, the balloon bursts, shooting the tiny harpoon outwards. Chemical receptors can also trigger the sting to release. In most jellyfish, this tiny harpoon contains a small dose of venom at its tip, so that if it pierces the skin of another animal, the venom is injected.

Stinging is a tactic that jellyfish use both for defence and attack. Because jellyfish are easy prey, the knowledge that they sting can deter some creatures from trying to gobble them. But as well as being prey, jellyfish are predators too — using their sting to stun smaller prey animals such as tiny shrimp like crustaceans and crab larvae.

Moon jellies, also known as common jellyfish, are the ones we often see washed ashore, with four purple rings visible in their round, transparent bodies. These jellies change colour depending on what they eat. Moon jellies don’t have especially strong stingers and more often than not, the tiny harpoons aren’t strong enough to pierce the surface of human skin, which means that they don’t really sting at all.

Compass jellyfish are also abundant in Irish waters, recognisable by their distinctive pattern of strong brown lines radiating from their centre out toward their edge. They have long tentacles and a moderate sting, though the discomfort of being stung by a compass jellyfish fades quickly.

One jellyfish that’s always a joy to see is the sea gooseberry, only slightly bigger than an actual gooseberry. They don’t have any stinging cells at all. If you’re lucky enough to see sea gooseberry, look closely, for these mesmerising creatures have bioluminescent lines running down the length of their transparent little bodies, emitting light in pulsing, glimmering flashes as they move about in the water.

One jellyfish that is never a welcome sight is the Lion’s mane. These huge jellyfish have a mass of very long tentacles and a severe sting. Fortunately, they are not so common in Irish waters.

The Portuguese Man O’War is the only one here that is considered properly dangerous to humans. It is not a true jellyfish but rather a colony of many different polyps attached to a floating gas-filled sac that acts like a float. Rather than actively swimming, the Portuguese Man O’ War moves about by using this ‘float’ to catch the wind like a sail. Normally a tropical species, they are only found in Irish waters after a long spell of southwesterly winds are they blown this far north.

Jellyfish are resilient and abundant, predators and prey, which makes them a key constituent in coastal marine environments. Endangered Leatherback turtles travel all the way here from the Caribbean and the coast of west Africa to feed up on jellyfish in Irish waters in the summer months. Because jelly fish are not especially nutritious, the enormous turtles have to eat as much as 100 kg of jellyfish a day. Sunfish are the largest bony fish in the world, and also depend on jellyfish for sustenance.

500 million years ago, before most of the lifeforms we know today had evolved, Gelatinous jellyfish ancestors dominated the oceans. The jellyfish that exist today are members of the oldest family of animals in the sea, the cnidarians, a group that includes sea anemones and corals.

In the 1800s, pioneering biologists were investigating how evolution guided the development of plants and animals, especially how they developed sense organs, mental abilities and even consciousness. Studying jellyfish physiology yielded valuable insights in to evolution. Back in the days when many were still dubious about the theory of evolution, biologists were able to demonstrate that nerve cells initially developed in cnidarians are replicated in other animals that evolved later.

Although jellyfish don’t have a central nervous system, nor a brain as command centre, they possess a complex network of decentralised neurons throughout their bodies that allows them to touch, feel and interact with the world around them. The basic structure of nerve bundles that receive and send messages is essentially the same as ours, though far less complex. We could even go so far as to say that it is thanks to jellyfish that we humans a nervous system.

Jellyfish seem to be on the increase, although in the absence of monitoring and data about jellyfish distribution and abundance, it is hard to know for sure. We do know however, that warming waters resulting from climate change are impacting the abundance of jellyfish. Widespread overfishing has messed up marine food chains too. It is perhaps worth asking whether jellyfish are the problem or the human activities that are so damaging to marine habitats.