Freshwater jellyfish in France.

This summer we experienced a very long period of drought and two scorching heat waves which are probably the reason for an unusual and little known phenomenon to occur in the Vienne departement along with other parts of west and southwest France! These were exactly the right conditions when the water warms sufficiently for the emergence of a tiny freshwater jellyfish, craspedacusta sowerbii.  

They are normally only seen when they take the form of a small bell-shaped jellyfish known as a hydromedusa and float near the surface of the water but this requires a water temperature of at least 25°C and forms only one part of their interesting lifecycle.

As a jellyfish they are 20–25 mm in diameter, somewhat flatter than a hemisphere, and very delicate. They have a whorl of up to 400 tentacles tightly packed around the bell margin. Hanging down from the center of the inside of the bell is a large stomach structure called a manubrium, with a mouth-opening and four frilly lips. Food is taken in and waste  expelled through the mouth opening.

Click photo to enlarge

Craspedacusta sowerbyi more often exist as microscopic podocysts (dormant "resting bodies"), frustules (larvae produced asexually by budding), planulae (larvae produced sexually by the hydromedusae), or as sessile polyps, which attach to stable surfaces and can form colonies consisting of two to four individuals and measuring 5 to 8 mm.

This species, originally from China (Yangtze River Basin), probably originally arrived in Europe with aquatic plants imported for botanical gardens. In the ponds of Kew Gardens, near London, it was discovered in 1880 by the naturalist William Sowerby. Since then, it has conquered every continent thanks to the trade in aquarium plants.

In France they can be found in slow moving rivers, lakes and ponds, maybe even your garden pond if they have been transported there with pond plants or stuck to birds’ feet.  Should you come across them you need not worry, they present no danger to humans or other mammals.

Chris