Beach season has just started in the US, and in the Chesapeake Bay, that means swimmers are enjoying that brief period of time when the air is warm, the water is cool, and stinging jellies are nowhere in sight. Known as sea nettles, or Chrysaora quinquecirrha, stinging jellies are a ubiquitous summertime feature of the Chesapeake Bay. They’re reviled as menacing pests that, like mosquitoes, have little ecological value.
Except, they do. Sea nettles are voracious consumers of one the Bay’s other gelatinous creatures—the much more charismatic ctenophore known as the comb jelly. These walnut sized jellies don’t sting. Kids scoop them easily out of the water to roll them in their palms. At night they ripple with bioluminescence, and boaters enjoy watching them light up in their wakes.
But they’re not as benign as they seem. Comb jellies have an appetite for oyster larvae, chowing down on one of the Bay’s critically important species. (Oysters have been seriously depleted in the Chesapeake, with a current population lower than one percent of historic levels.) So as far as oysters go, comb jellies are considered far more menacing than sea nettles that scare swimmers out of the water.
Despite their unsavory stinging properties, sea nettles help keep the comb jelly population in check, which means, anyone concerned with the health of the Bay should be mildly pleased to see them.
Ironically, sea nettles also need oysters for their survival. In the fall, as the adult nettles are dying off, their larvae begin to drift down through the water to the sandy bottom of the Bay. Only when they find a hard surface to land on can they settle out and grow into the polyps that can survive a cold winter and develop into adults in the spring. Rocky bottom structures are rare in the Bay, which means oyster shells provide the most likely habitat for sea nettle polyps. It’s a relationship that seems to work for both of them.
So, come July, when the nettles are thick in the Bay, would-be swimmers can sit on the dock looking out over the unwelcoming water and say to themselves, “This is a good thing.”
Of course, they may also prefer to find a sea nettle free place to swim, so that means heading upstream to fresher waters (10 ppt salinity or less) or anywhere the water is cooler than around 70 degrees F.