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 l...