Another Life: Jellyfish join the ecological anarchists of the western lakes

If the warmth of this summer were to become the norm, explosions of freshwater jellies could add to competition for zooplankton food

Minuscule: a Craspedacusta sowerbyi medusa is the size of a €1 coin. Illustration: Michael Viney

Minuscule: a Craspedacusta sowerbyi medusa is the size of a €1 coin. Illustration: Michael Viney


The great oceanic trout lakes of the west of Ireland, with their rich limestone feeding and plump specimen fish, have no counterpart in continental Europe: the summers there simply aren’t windy or cool enough. Waves that rock the anglers’ boats on Corrib, Mask, Derg or Erne keep the water mixed and full of oxygen from top to bottom. Water temperature rarely reaches 20 degrees, the point at which trout and salmon begin to feel uncomfortable.

In the generally hotter summers of Europe, lakes stratify more readily and severely: they split into layers, warm water above and cold below. The lower layer can lose oxygen as summer progresses and no longer offer fish a cool retreat. In such warm, weedy lakes, salmonids give way to cyprinids, notably carp. Bred to obese sizes by European fish farmers, carp are often sluggishly easy to catch with rod and line.

It is in this context that the recent discovery of minuscule freshwater jellyfish swimming happily in Lough Derg and Lough Erne could become a troubling omen. Their existence as free-swimming little bells, or medusae, was made possible by the preceding weeks of hot sunshine, in which local water temperatures rose, for long periods, above 25 degrees. In various waterways across the world, including those of Europe and most US states, this alien vagrant from China has bloomed only sporadically, when the weather was right.

Craspedacusta sowerbyi is not, strictly speaking, a jellyfish – an extra flap of tissue sorts it from those of the sea – but it looks, acts and reproduces like a jellyfish, and a shoal of their medusae, each the size of a €1 coin, makes an eye-catching glint under the water. The species has travelled widely in its resting forms, as hard-covered microscopic cysts or tiny polyps that grow from them, stuck to transported fish, aquatic plants or even the feet of migrant waterbirds. The polyps then stick to underwater rocks as a cylindrical trunk with a mouth at the top, surrounded by food-catching tentacles.

The polyps can tolerate cold and may live in a static state for years. But if the temperature rises above 25 degrees it may enter a process typical of jellyfish life cycles and bud off a plume of the little free-swimming bells. These medusae have their own tentacles around the fringe, some very long to help in swimming, some toxic-tipped to capture zooplankton food.

The explosion of jellies is often single- sexed (those of Derg and Erne are all female), and their bloom disappears within a few weeks or months, not to be repeated, perhaps, for years. The polyps are insignificantly small and live on unnoticed (they may have been here for decades), which is why Irish scientists are hungry for more sightings of the medusae (tell, the records centre at or

As aliens go – or come, rather – this one, in itself, though reckoned invasive, may be relatively harmless, so long as it sticks to the rare explosions of the past. It brings the number of aliens now in Lough Derg to almost 20, however, and so intensifies the ecological anarchy described as invasional meltdown by Dr Dan Minchin of the Lough Derg Science Group, now a leading expert on such threats.

Smothering, plankton-hungry zebra mussels have already wiped out the lake’s big native species. They were followed by the equally invasive Asian clam. The alien bloody-red shrimp is preying on native species that feed the lake’s last few pollan, an Arctic fish unique to Ireland in western Europe. All these have been introduced by human activity, stuck to the hulls of pleasure craft or pumped out in their bilge or ballast water, imported in anglers’ bait boxes or emptied from aquariums stocked with imported fish or decorative aquatic plants. Discarded alien waterweeds are proliferating in many Irish lakes, Derg among them.

The new mussels and clams are clearing the water of plankton and manuring the floor of the lake so that even native plants are growing in the better-lit depths and crowding into long-established fish spawning areas. If the warmth of this summer were to become the norm, explosions of freshwater jellies could add to competition for zooplankton food.

Lough Derg got its name, so one folk tale asserts, from the bloody slaughter of a fairy badger chased into its depths. Some of the proliferating aliens are beginning to seem almost as unlikely, bringing chaos to aquatic ecosystems long shaped and balanced by Ireland’s native species. Even without the invaders, rising water temperatures and summer droughts could be bad news for the salmon and trout of the western lakes and rivers, affecting them adversely at every stage of their lives. (The salmon, so a recent report says, have an extra three degrees of thermal tolerance.)

Be careful what you wish for.

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