Blooming of the Tisza

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Every year from late spring to early summer, an incredible natural spectacle transforms Hungary's Tisza River. Millions of long-tailed mayflies, or Palingenia longicauda, rise in huge clouds from the river, flutter on the surface of the water, mate and perish, all in just a few hours. The unique event which lasts for three to four days, is frequently referred to as “blooming of the Tisza”, and is amongst the most fascinating natural phenomena in European rivers.

Palingenia longicauda also known as the Tisa or Tisza mayfly after the European Tisza river where it is found, is the largest mayfly species in Europe, measuring 12 cm from head to tail. The insect spends most of their life-time – 3 years – developing as a larvae in the mud at the bottom of the river. After hatching the male adults only have a few hours to find a female and mate with her before they die. To do this, the male will glide down the river, trying to impregnate one of the females on the surface of the river. After mating, the female mayflies will fly over the surface of the river several kilometers upstream to compensate for downstream larval drift and laying their eggs along the way. The eggs drift to the bottom and after 45 days hatch into larvae, which dig tunnels in the mud forming dense colonies up to 400 per square foot. After three years, the larvae break for the surface and the adult mayfly appears. After they have fully matured, mayflies have roughly three hours before they die.

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Photo credit: Reuters

With no time for courtship, reproduction is often a forcible act with up to 20 males simultaneously going after a lone female. An eager male might also lie in wait atop the skin of a female that has yet to shed.

The synchronized hatching of the Palingenia longicauda is one of nature’s great sights, as the surprisingly large insects seem to dance above the river’s surface, smothering the riverbank and any other available surface including cars, roads and people as they seek to find a mate in the short time available to them. The vast majority of mayflies die immediately after mating. At the end of the mayfly’s day-long dance, the surface of the Tisza is covered with large dead mayflies. In some cases the piles of dead mayflies block roads or bridges and need to be removed with a snow plow!

From the evolutionary point of view, the advantages of this huge, synchronized hatch are obvious: as insects can only mate in their adult stage, by synchronising the hatch,  the chance of finding a partner is maximized, which is very important for species with such a short adult life stage. Furthermore, predators like bats or birds are surprised by this sudden appearance of clouds of prey, and so can hunt only a small fraction of the population.

Today, Hungary's Tisza River and its tributaries are the only place in the world where you can find long-tailed mayflies, Palingenia longicauda. But a century ago the insects flourished in Europe's lowland rivers. According to experts, a combination of water pollution from heavy industries and riverbank re-engineering destroyed the mayfly larvae's natural habitats. After communist factories closed in the early 1990s, many rivers in Eastern Europe experienced a drop in pollution levels, and in some cases, the mayflies have returned, for instance, in River Danube some 30 km north from Budapest. Efforts are now being made to restore these once-tainted rivers, by harvesting hibernating eggs from the Tisza, then transplanting them into other European rivers.

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Photo credit: Attila Volgyi

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A car crossing a bridge illuminates the swarming mayflies along River Danube in Tahitotfalu, some 30 km north from Budapest, Hungary, on Aug. 24, 2013. The improvement of the water quality helped the mayfly species return to River Danube in recent years after four decades of absence. Unlike the more commonly known long-tailed mayfly species on River Tisza, these insects swarm on River Danube at night. Photo credit: Attila Volgyi

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Photo credit: Attila Volgyi

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Photo credit: Attila Volgyi

Sources: Wikipedia, NatGeo, Cabinet of Water Curiosities

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