Much-maligned jellies offer plenty of good to their ocean ecosystems
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Robots that hunt down and exterminate jellyfish: Good or bad idea? Discuss.
A 2013 video from robotics designers at the Korea Advanced Institute of Science and Technology shows three jelly-killer prototypes gliding as a metallic fleet over gently rippling water. An underwater video demonstrates the cunning plan. Pale jellyfish bells drift into view, and a blenderlike slicer whirs into action. Jellies explode in beige puffs as they are sucked into the spinning blades.
“This invention is fantastic,” commented Veronica Bingham on the IEEE Spectrum news website. “Jellyfish are a plague,” added commenter Soulshock.
People don’t see that jellyfish are doing some good for the ocean, says marine biologist Thomas K. Doyle of National University of Ireland, Galway. He doesn’t deny that they sting. But snakes and spiders bite, and they get more regard, acknowledged as useful in controlling pests. When talk turns to jellyfish, however, people just want them gone, he says.
The widespread negative attitude toward jellyfish drove Doyle and his colleagues to contribute a cautionary chapter to the 2014 book Jellyfish Blooms. They compiled 22 pages summarizing research on what jellyfish do for the neighborhood.
Though jellies are not easy to study, researchers have learned enough to say they are important to the fates of plenty of ocean creatures. In spite of their stings, jellyfish are fuel for other animals, nourishing some charismatic species that people would never feed to a robot. They offer shelter, creating floating refuges for tinier sea animals. As predators, jellyfish help regulate food webs, hunting with surprising sophistication for flimsy beings with no centralized brain.
There’s a peculiar contrast between our views of the much-maligned jellyfish and much-loved corals. They are cousins in the same phylum, Cnidaria, named for specialized stinging cells. An individual coral is just a brainless tube with stingers and one opening that doubles as mouth and anus. But corals get public love and conservation dollars when they come together as architects of reefs that host a rainbow of darting, drifting, lurking, dazzling life. Anyone old enough to leave nose-smudges against aquarium tanks can witness how corals matter in a community. The unfolding story of what good jellyfish do, however, is not getting through.
What’s a jelly
The meaning of the word “jellyfish” can drift depending on the currents of the conversation.
“There’s sometimes quite a lot of …” — Cathy Lucas pauses to choose her words carefully — “discussion and disagreement about what exactly a jellyfish is.” As one of the editors of the new Jellyfish Blooms volume, Lucas points out that the sea has many filmy, gelatinous organisms, some not closely related at all.
In the more traditional sense, Lucas says, the term “jellyfish” applies just to the watery species in the stinging cnidarians, and she sometimes narrows even further to a subgroup called scyphozoans. A fine example is the moon jelly (Aurelia aurita) which stars in much of Lucas’ research at the University of Southampton in England. Bobbing in oceans like pale, wavering reflections of the moon, the broad, shallow bells of the adult moon jelly pulse gently along, with tentacles trailing and the circular, sometimes pinkish gonads hazily visible as four moonlets.
Just what jellyfish in the broadest sense do in their watery ecosystems has become a topic of urgent interest in recent decades. Biologists are debating whether jellyfish blooms, great sudden aggregations of sexually reproducing adults, are increasing, and whether gelatinous species are on their way to overrunning the oceans. More blooms could mean more beach closings, more clogged fishing nets or intake pipes (jellies have temporarily shut down at least one power plant) and, some argue, fewer fish. That long-running, ongoing discussion has prompted new insights into what jelly animals mean for the rest of sea life.
That’s a huge question, and researchers are going to creative lengths to track and measure jellies’ impact. They have learned that jellyfish may move so much that they help mix ocean water, spreading nutrients around. Jellies might be big players in transferring carbon from the upper ocean into deep storage way below, preventing it from wafting into the atmosphere as greenhouse gases. For example, the gelatinous salps, noncnidarians that look like plastic party garlands, do an efficient job of sending their carbon-rich fecal pellets down deep, fast: up to 2,700 meters a day versus the otherwise impressive krill deposit rate of 862 meters per day.
Roughly speaking, filmy jelly animals consist of more than 95 percent water, with very little carbon. Yet even that can be worth eating.
A tally of stomach studies indicates at least 69 species of fish eat jellies as more than a random snack. The best example of a jellyvore may be the most improbable: the leatherback sea turtle, the largest turtle on Earth. Leatherbacks breeding in the Caribbean average some 400 kilograms and swim several thousand kilometers from their tropical breeding waters to northern latitudes for summer feasts on abundant jellyfish. After several months of chewing on stinging, watery prey, turtles actually gain weight.
People often ask sea turtle biologist Mike James of Fisheries and Oceans Canada in Dartmouth, Nova Scotia, how any animal endures mouthful after mouthful of jellyfish. That’s thinking like a human, he tells them. “We see turtles with tentacles all over them, on their heads, over their eyes.” To turtles, it’s just lunch.
The messy but traditional way of understanding diet involves poking into half-digested glop in stomachs. Jellies turn into glop just about at first gulp and slip speedily through a digestive tract, making them hard to identify and easy to undercount. So James has been working since 2006 with engineers and leatherback-savvy fishermen to customize cameras to see how a diet of jellies sustains these colossal creatures. The team managed to sneak up on jelly-distracted turtles, lean off a boat platform and attach cameras with suction cups to the oily, leathery skin covering the shells of 19 turtles.
Hours of dinner-cam footage showed jelly after jelly bobbing in the distance, nearing and finally disappearing as turtle head motions indicated chewing. Watching video from more than 600 jelly grabs was actually “soothing,” says collaborator Susan Heaslip of Dalhousie University in Halifax, Nova Scotia.
Turtles gain weight because they eat 73 percent of their massive weight in jellyfish a day, mostly lion’s mane jellies (Cyanea capillata). That’s an average of 261 daily jellies, about 330 kilograms, Heaslip, James and their colleagues reported in 2012.
The researchers went further, coaxing turtles to swallow capsules containing tiny temperature monitors. They revealed stomach temperatures rising at night after a daytime dip. That temperature boost from digesting a day’s jellies helps the tropical turtles cope with chilly northern seas, James and his colleagues report in the July 1 Journal of Experimental Biology.
Jellies can shelter as well as feed, but biologists needed to change their collecting strategies to appreciate just how much of a refuge they can be.
A mix of animals caught in the same net does little to reveal relationships, and jellies too easily slip through. With so little to go on, says Larry Madin of the Woods Hole Oceanographic Institution in Massachusetts, “it was easy to relegate the gelatinous things more to the category of curiosities than to really central components in the oceanic community.”
Madin and his graduate adviser, William Hamner of the University of California, Los Angeles, started changing that view in the 1970s with glass jars and a frightening notion for ocean research at the time: blue-water diving.
After much persuasion of nervous captains on research vessels, Madin, Hamner and their colleagues took glass sample jars far from ship interference, tethered themselves to a small inflatable boat that would be swept along with them in the current and dove in the featureless water of the open ocean.
“Everything is just blue all around you,” Madin says. Of the people who tried it, “some of them loved it and some of them found it very, very disorienting.”
But what a difference it made to see the gelatinous animals alive and whole, and to bring back jars of unscathed samples. Among the many surprises: “Almost all the jellyfish and all the other gelatinous jelly animals were hosts to a collection of … hitchhikers,” Madin says.
Before this, biologists had speculated that certain crustaceans might specialize in clinging to jellyfish. But Madin and his colleagues found hangers-on everywhere: larval fish, a juvenile octopus, polychaete worms, sea anemone larvae. Jellies may not be teeming like a coral reef, but they are “floating habitats,” he says.
Most members of a category of crustaceans called hyperiid amphipods are evolutionarily specialized for jellyfish riding. A genetic analysis in 2013 recognized 23 families, some with huge eyes — useful when living on jellies that float up to water levels where light penetrates. “Females live on a jelly host and use it as a nursery for the babies,” Madin says.
Tropical jellyfish often take in tinier partners, strains of yellow-brown algae. As the algae photosynthesize, the carbohydrates and oxygen they produce give a nutritional boost to the jelly.
Some of the most entertaining of these species are upside-down jellyfish in the genus Cassiopea. Lucas’ students surveyed them on a field trip to Bermuda this summer, in a mangrove-rimmed saltwater pond. Algae colonize the ruffled surfaces of the jellyfish’s branched feeding structures called oral arms. The jellies flip on their backs, and the algae bask in sunlight and photosynthesize. The algae-tinged jellies look “like a creamy-brown version of a broccoli or a cauliflower,” Lucas says.
Carb-loading is a nice perk from the algae, but most jellies are active hunters. Like any predator, they influence the balance of prey in a food web. Humans lament some of the jellies’ prey, like larvae of herring or other commercial fish, but for the most part, jellies feed on copepods and other jellies.
To figure out how jellyfish, most of which have no eyes and not much muscle, can hunt takes some clever tinkering. To try to follow their motions when they forage, Doyle and his colleagues decided to tag them. Never mind that they are watery and slippery. Australian researchers Matthew Gordon and Jamie Seymour of James Cook University in Cairns had tagged some box jellies by taking them out of the water, partly drying them and affixing a tag with surgical glue. Instead, Doyle and his colleagues found they could fasten a cable tie collar with a tiny depth recorder around the closest thing a barrel jellyfish (Rhizostoma octopus) has to a neck. Called the peduncle, the tubelike structure descends underneath the bell and branches out into more elaborate tubes for feeding.
Data recovered from 25 barrel jellyfish showed they were doing more than mildly wafting at one depth. They rose and sank an average 619 meters per day; that’s more than 60 times the depth of water where they were tagged, Doyle and his colleagues reported in February 2012 in Proceedings of the Royal Society B. Regardless of how intentional or accidental parts of their journeys were, the movements at times approximate a pattern good for searching out scarce prey. They don’t see and chase after prey the way fish do, but with their up and down motion, Doyle points out, jellyfish cover territory and can compete with foraging fish for food in open water.
Story continues below infographic.
Rising green dye shows water flows created by jellyfish contractions.
Matthew Takyi-Micah/Oklahoma State Univ.
To see if such behavior is common in the jelly world, Doyle and his colleagues need to tag other species. They’ve already tried to monitor lion’s mane jelly, which is quite a challenge. Its peduncle lies amid folds upon folds of fragile tissue above more than a thousand tentacles 3 to 4 meters long that make it the most venomous jelly in Irish waters. Hauling the jelly out of the water to try to glue on a tag is out of the question because air bubbles get trapped in its complex structures and ruin its buoyancy. Applying surgical glue underwater was a bust. “We actually got a needle and thread and tried to stitch a tag on,” he says. “We just put holes in the jellyfish.”
In the end, someone wearing as much protective gear as possible has to swim over to grope for the peduncle in hopes of attaching a cable tie collar without ripping the jelly or getting stung. “The only thing is, your lips protrude with the snorkel,” Doyle says. “The solution: layers of Vaseline.”
What and how much food jellies catch while hunting depends on how and how much they move. For decades researchers assumed that jelly contractions sent them forward by jet propulsion. Not always so, it turns out.
Small bullet-shaped jellies do jet, says Sean Colin at Roger Williams University in Bristol, R.I. Their thin muscles have enough power to clench the bells and shoot quickly forward. This helps them flee danger but it’s not a tool for hunting. “They’re like spiders,” Colin says. They hang motionless to hunt, with their tentacles out like the strands of a web, waiting for what food blunders in, such as small, zipping fish.
In contrast, big, flattish jellies with wide bell openings aren’t great at jet propulsion (SN: 2/23/08, p. 122). They don’t have the muscle. Instead they flex the margins of their bells inward and then relax, in a rowing motion. This propels them, though not particularly fast. But importantly, the motion swooshes water — along with any tiny floating edibles — toward their tentacles.
Even those bottom dwelling jellies known as Cassiopea pulse while sunning their live-in algae. That pulsing intrigues researchers studying fluid motions, because these jellies are contracting their bells but not swimming. They may be using their minimal movements as a couch-potato way of hunting.Upside-down jellies often sunbathe on sand or other sediments with lots of tiny cracks and channels filled with a feast of tiny edibles for any organism that can extract it. Bell pulsing can suck goodies out of the sand, reported Carin Jantzen of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, and colleagues in 2010. Her team buried red-dyed sediment under an undyed layer. An upside-down jelly was settled on top, pulsing away. When the researchers lifted the jellyfish, a red circle marked where it had been. The jelly’s motions had drawn up the colored sediment.
The upside-down jellies may be such good pulsers that they serve as local ecosystem engineers, according to Arvind Santhanakrishnan of Oklahoma State University in Stillwater. Bells don’t just draw up potential food treasures from directly underneath, according to recordings he made with colleagues. The researchers analyzed green dye plumes and clouds of brine shrimp eggs (a jelly-safe alternative to plastic beads) as they swirled and sloshed in the surrounding water while the jellies pulsed in place. The lip of a bell flaps during a contraction and creates a strong swirl that pulls water toward it from across the sandy bottom, the researchers reported in July 2012 in the Journal of Experimental Biology.
“We were expecting more of a back and forth,” says coauthor Christina Hamlet, now at Tulane University in New Orleans. But the current goes forth and forth. As the bell relaxes, the complicated, frilly broccoli-arms prevent backwash, and the water pulled inward streams upward. In the fairly stagnant water where these jellyfish settle, the flow might provide a valuable bit of mixing, Santhanakrishnan speculates.
Ecosystem engineers or not, Cassiopea illustrate one thing jellies have going for them: They’re gorgeous. Jellyfish displays in public aquariums mesmerize visitors. “It’s like watching a screen saver,” says Jennie Janssen, who cares for the jellies at the National Aquarium in Baltimore. Does she find visitors anxious for gelatinous eradication, wondering what in the world jellyfish are good for?
“No” she says. “I get that about mosquitoes.”
Watery, filmy bodies make up substantial parts of at least seven phylum branches of the tree of animal life. “Jellyfish” often refers to the gelatinous cousins of corals in Cnidaria. They vary from scyphozoan tentacle-trailing bells to hitched-together garland colonies of siphonophores.
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J.P. Casey, M.C. James, and A.S. Williard. Behavioral and metabolic contributions to thermoregulation in freely swimming leatherback turtles at high latitudes. J. Exp. Biol. Vol. 217, July 1, 2014, p. 2331-2337. doi: 10.1242/jeb.100347.
G. Hays et al. High activity and Lévy searches: jellyfish can search the water column like fish. Proc. R. Soc. B. Vol. 279. Feb. 7, 2012, p. 465. doi: 10.1098/rspb.2011.0978.
C. Jantzen et al. Enhanced pore-water nutrient fluxes by the upside-down jellyfish Cassiopea sp. in a Red Sea coral reef. Mar Ecol Prog Ser. Vol. 411, July 29, 2010, p. 117. doi: 10.3354/meps08623.
A. Santhanakrishnan et al. Flow structure and transport characteristics of feeding and exchange currents generated by upside-down Cassiopea jellyfish. J. Exp. Biol. Vol. 215, Published online July 15, 2012, p. 2369-2381. doi: 10.1242/jeb.053744.
R. Ehrenberg. Jelly propulsion. Science News. Vol. 173, February 23, 2008, p. 122.
Kylie A. Pitt and Cathy H. Lucas (editors). Jellyfish Blooms. Springer, 2014.