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
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 fisheriesireland.ie, the records centre at nbdc.ie or jellyfish.ie).