Description
Gonionemus vertens is an small disk shaped jellyfish, probably imported from Japan in ballast water tanks of ships or with the import of oysters. It has long tentacles with nerve toxin all along the edge of the bell. The toxin can inflict a strong pain and even mild temporary paralysis. It prefers brackish waters with sea grass. The first recordings from the Grevelingen date from 1976. Because the lake was cut of from the North sea at that time and the salt level started to drop, scientists feared for an explosively increasing numbers of the animal. In 1979 an opening in the Brouwersdam was made, in order to increase the salt level and this jellyfish almost completely disappeared from the lake.

Size
The bell has a diameter of 20 to 25 mm. The tentacles are 1.5 time as long as the bell diameter.

Colour
The animals has a transparent bell with brown lobed gonads, shaped like a cross. The tentacles are light brown and at the edge of the bell you can see a row of green dots, at the base of each tentacle.

Habitat
It prefers sea grass fields or sea weeds, but can also be found in open water.

Distribution
Atlantic ocean and Pacific ocean.