At least two types of jellyfish have been visible from the docks in Menemsha in recent weeks — sea nettles and comb jellies. Sea nettles may be responsible for the stings several swimmers experienced this summer, according to Larry Madin, deputy director and vice president for research at the Woods Hole Oceanographic Institution. Madin is a former jellyfish researcher.
Sea nettle populations are kept balanced by predators like ocean sunfish, tuna, dogfish, and sea turtles, according to the National Aquarium in Baltimore.
“All seven species of sea turtles include them in their diets,” the aquarium site states. “The largest sea turtle species, the leatherback, depends on jellies for food. Because jellies are more than 90 percent water and an adult leatherback can weigh more than 2,000 pounds, one turtle can consume a lot of jellies.”
Comb jellies, a.k.a. ctenophores, are common in local waters in summer and fall, Madin said.
“They’re not closely related to true jellyfish, and don’t sting.”
“Ctenophores (Greek for ‘comb-bearers’) have eight ‘comb rows’ of fused cilia arranged along the sides of the animal,” according to the University of California Museum of Paleontology. “These cilia beat synchronously and propel ctenophores through the water. Some species move with a flapping motion of their lobes or undulations of the body. Many ctenophores have two long tentacles, but some lack tentacles completely.”
Ctenophores can’t sting prey. “Instead, in order to capture prey, ctenophores possess sticky cells called colloblasts,” the museum site states. “In a few species, special cilia in the mouth are used for biting gelatinous prey.”
The museum describes them as “voracious predators.”
“Jellyfish and ctenophores are carnivorous,” according to the Smithsonian Institution, “and will eat just about anything they run into! Most jellies primarily eat plankton, tiny organisms that drift along in the water, although larger ones may also eat crustaceans, fish, and even other jellyfish and comb jellies.”