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Clinging Jellyfish Hang Out on Eelgrass, Pack Powerful Sting

The clinging jellyfish, Gonionemus vertens.
Annette Govindarajan

The sting of a jellyfish can range from a mild annoyance to a life-threatening incident, depending on the species. Reports of severe stings in coastal ponds along the coast of Cape Cod in recent years have sparked concern that a new, more virulent jellyfish may have entered the area. It turns out the jellyfish responsible – known as a clinging jellyfish - may have been here for more than a century.

Clinging jellyfish (Gonionemus vertens) are small, somewhere between the size of a dime and a quarter, with a reddish-brown cross and sticky tentacles that allow the jellyfish to anchor itself to its preferred habitat - eelgrass.

The first records of the species in the area date back to the 1890s. They appear to have been common, but stings don't seem to have been an issue. Clinging jellyfish numbers declined in the 1930s, when a fungal disease killed off eelgrass beds. Then, a few years ago, a researcher and some summer visitors reported powerful stings causing severe pain, even temporary paralysis, in one case. 

Researchers initially thought that environmental factors may have caused clinging jellyfish to become more virulent. Now, however, their leading hypothesis is that a second wave of clinging jellyfish with stronger stings may have been introduced to the area around 1990. They're planning to perform genetic tests, comparing jellyfish from around the region with samples from different parts of the Pacific, to determine whether that's the case.

Annette Govindarajan, a research specialist at Woods Hole Oceanographic Institution, is leading investigations of the jellyfish, and she cautions against over-reacting. While the stings are painful, the effects are temporary; victims recover fully within a few days. Also, clinging jellyfish are found almost exclusively in calm ponds and back bays, stuck to either eelgrass or floating docks. Swimmers at ocean beaches are unlikely to encounter them. In fact, Govindarajan says eelgrass researchers have been the population at greatest risk of stings, so far.

If you think you have seen or been stung by a clinging jellyfish, you can contact Govindarajan.

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