Cannonball jellyfish: A favorite food for sea turtles, a minor danger to us
Sometimes they wash up by the hundreds on our beaches—those mushroom-shaped, gelatinous masses called cannonball jellyfish, or jellies (Stomolophus meleagris).
They’re a common species along the eastern shore of the U.S., as well as the Gulf and parts of the Pacific coast.
Like corals and sea anemones, cannonball jellyfish belong to the large invertebrate phylum Cnidaria, which includes over 10,000 species of mostly marine animals.
In jellyfish terminology, the familiar bell-shaped form is called the medusa. Its gelatinous consistency comes from a jellylike substance — the mesoglea — sandwiched between the body’s inner and outer cell layers.
The medusa is the mobile, sexually reproducing phase of the jellyfish life-cycle, which includes several inconspicuous developmental stages. Among these is a minuscule, tentacle-bearing form called a polyp.
The polyp lives attached to rocks or other substrates, feeding and eventually morphing into a segmented stage that releases multiple tiny, disc-like creatures. These grow and mature into a free-swimming medusae.
Although medusae can be widely transported by ocean currents, they also move on their own by alternately contracting and expanding their bells. These movements forcibly displace jets of water, propelling the animals along.
Cannonball medusae can be as big as 10 inches in diameter. Typically they’re pale-colored with a brown-pigmented border and a fringe of 16 short, fused “arms.” The arms transfer tiny crustaceans, larval molluscs, and other prey to a central mouth area on the undersurface. Mucus produced by multiple mouth folds at the bell’s base helps to ensnare food, as well.
Some kinds of jellyfish are commercially harvested for human food in China, Thailand, Indonesia, Korea, and elsewhere in the world, and cannonballs jellies may have some potential in that regard.
But they’re also a main food source for the endangered leatherback sea turtle, so careful management and conservation of cannonball jellyfish may become a growing ecological concern.
Do cannonballs sting?
Compared to some other species, they’re relatively harmless, though their toxin may cause minor skin or eye irritation in some people.
Still, it’s wise to avoid close contact with any kind of jellyfish, since the tentacles of some species can deliver painful and sometimes dangerous stings.
Vicky McMillan, a retired biologist formerly at Colgate University,lives on Hilton Head Island. She can be reached at vicky.mcmillan@gmail.com.
This story was originally published April 11, 2017 at 11:12 AM with the headline "Cannonball jellyfish: A favorite food for sea turtles, a minor danger to us."