Craspedote, a freshwater jelly

They frighten, fascinate: jellyfish inhabit the seas and oceans, and leave neither vacationers nor naturalists indifferent. And when we meet a small species in our lakes, ponds, canals, after a moment of surprise, we can only go to the discovery of Craspedacusta sowerby, present in the fresh waters of all continents.

Craspedacusta sowerbyi or Crasp
Craspedacusta sowerbyi or Freshwater Craspdote

A small jellyfish in our bodies of water

In 1880 London scientists discovered, surprised, a jellyfish in a basin of exotic plants. Translucent, it is clearly visible to the naked eye with its 2 cm diameter. Named Craspedacusta sowerbyi, Where Freshwater craspedote, it belongs to Cnidarians, cousin of sea and ocean jellyfish, but also anemones and corals. The only freshwater jellyfish, in its class of Hydrozoans, it will meet only Hydras, polyps that are always fixed.

Since its first description in London the Craspedote has been observed on all continents, in 1929 in the Garonne basin then in all the rivers of France.

Fragile and elegant

The encounter is surprising: leaning over the waters of the lake, a canal, an old gravel pit, even the pond, we see floating small organisms the size of a 2 euro coin, translucent, in parachute shape. The Freshwater Craspedote is a Typical hydrozoan : its parasol is lined with more or less long tentacles, up to 400, a canopy closes the parachute on its underside. Under the umbel, crossing the velum, springs a tube, the manubrium, with an external orifice and connected to 4 radiary canals internally. The entire anatomy of the jellyfish is visible by transparency.

She floats in the midst of her prey

The Freshwater Craspedote can move by vigorous contractions of its parasol, but floats with the movements of the water, among zooplankton. It is thus within reach of its prey. Copepods, daphnies, all the small animals of the plankton will be captured by its tentacles, carried to the orifice of the manubrium, the mouth, to be swallowed up. Once digestion is complete, the waste will be rejected through the same orifice, serving as an anus this time.

In Craspedotes the tentacles are provided with cnidocytes, stinging cells provided with harpoons with paralyzing effects. Only on small preys, the man does not seem to have to worry about his presence, his epidermis could not be crossed.

She suddenly appears and disappears

The Freshwater Craspedote can go unnoticed one year, or proliferate in abundant populations the following year. Then disappear again. Before being a floating jellyfish observable with the naked eye, it takes the shape of a tiny polyp a few millimeters fixed on the seabed, vegetation, rocks. Its small tentacles allow it to feed on animal micoplankton. This polyp will reproduce asexually, by budding, release of crawling “larvae” moving and forming a new polyp, and production of medusoid buds. Some will become jellyfish leading to sexual reproduction. The fertilized eggs will pass through the stade planula, small floating ciliated larva that will attach to become a polyp.

Present on all continents and therefore considered as species invasive the freshwater Craspedote would not represent a threat to ecosystems. Its supply of zooplankton remains limited, its presence remains discreet. The impact of climate change on the evolution of populations of this species may, however, modify current data in the years to come.

Source: Au Jardin, conseils en jardinage by

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