An unauthorized biography of the #!@*%$#* sea nettle, aka Chrysaora quinquecirrha, the cnidarian we love to hate.
by Marty LeGrand
As weathered and fading as the tobacco barns that dot the Southern Maryland farmscape, the pier at the Chesapeake Biological Laboratory (CBL) in Solomons hardly seems noteworthy. But in the battle against our mortal summer enemy, the sea nettle, this is in fact hallowed territory. Here, 50 years ago, a pragmatic CBL researcher launched a fiendishly simple counteroffensive against Chrysaora quinque-cirrha (Latin for "g*dd****d jellyfish!"). He began counting them.
From spring through fall, over three decades until his retirement, the late David Cargo walked the pier at lunchtime, clicker in hand, tallying the sea nettles floating at the surface. By comparing his admittedly casual surveys with climatic conditions, he hoped to forecast infestations, warning, perhaps, of "dry weather with a strong chance of midsummer nettles." Not only does sophisticated nettle forecasting exist today, but Cargo's data also continues to shed light on an understudied invertebrate that bewilders scientists almost as much as it bedevils Bay boaters and swimmers.
One morning early last September, I followed Cargo's legacy to ground zero. It was a bright breezy day, more early-fallish than dead-of-summerish, a meteorological cue for the nettles to consider retirement. Shafts of sunlight pierced a row of clouds and glinted off the water's surface as Rebecca Burrell opened the pier's padlocked gate. Burrell represents the second generation of researchers to carry Cargo's work forward. A biological technician studying estuarine ecology, she conducts nettle counts twice weekly from mid-May until the creatures disappear in the fall, recording them in a well thumbed notebook with "Jellyfish Counts at CBL Pier, 2005 to--" on its faded green cover. Added to the nearly unbroken trail of data collected by Cargo and a handful of successors since 1960, the numbers give scientists a rare and invaluable long-term perspective on the nettles' comings, goings and abundances.
Burrell follows Cargo's method, fundamental enough for a third-grade science project: Walk to the end of the 750-foot pier, counting only nettles on the south side; turn around, head shoreward and count the north side. Other data are added later--air and water temperatures, salinity level, wind speed, tides--to assess their effects on nettle prevalence. "It's time to start the walk, the jellyfish waddle," Burrell joked. Shielding her eyes from the glare, she peered into the murky water and pointed to one of the ghostly, gently bobbing blobs. "They're beginning to lose their oral arms," she said, referring to the shorter, wispier feeding appendages beneath the nettle's body. "It's a sign of senescence."
Soon this nettle, like its mates, would sink to the bottom and stay there, pulsing weakly, until it dies and gradually becomes a lowly link in the food web it's accustomed to dominating. (A show of hands: how many of you long-suffering souls would pay good money to watch them melt, a la the Wicked Witch of the West?)
Burrell noticed the season's first deteriorated specimen a few days earlier, when she tallied 8 nettles (well, 7.75 anyway). By the time she completed today's count (5 . . . ish), we saw more comb jellies--a favorite sea nettle meal--than nettles, a signal that the latter were dying off and that the seesaw battle of abundance had tilted once more to the comb jellies, the Bay's benign summer jellyfish (benign unless you're zooplankton).
I'd met Burrell and her boss, senior scientist Dr. Denise Breitburg, earlier at the Smithsonian Environmental Research Center in Edgewater, Md., where they work--Burrell in a wet lab where artsy ersatz jellyfish hang from the ceiling. (They are in fact open umbrellas, with trails of curly pastel gift ribbon where their shafts and handles used to be.) One of their projects, funded by Maryland Sea Grant, involves reanalyzing Cargo's earlier observational data in order to quantify them. "That could be really useful in understanding how the Bay has changed and how abundances of jellyfish have changed," Breitburg, a marine and estuarine ecologist, told me.
It's been the petri dish of Chesapeake jellyfish research, but the Patuxent is just one of the brackish middle and lower tributaries that harbor large numbers of sea nettles--which prefer the middle range of salinity. They can't tolerate water that's either too salty or too fresh. The creeks and coves of the middle Bay are where you get the highest reproduction and the highest densities, according to Breitburg, whose trawl-net samples in St. Leonard, Helens and other Patuxent creeks have yielded gallons of gelatinous cargo in only 90 seconds. "You can practically walk on them in some of these areas."
But you already knew that. What you may not know is that evidence shows the Bay's sea nettle population has plummeted since the mid-1980s (to only 10 percent of pre-1985 numbers, by some accounts). That's good news for those who'd like to swim or water-ski after the Fourth of July--but perhaps not such good news for the estuary itself, say scientists. And the giddy prospect of a less nettlesome Bay is tempered by one very inconvenient truth: jellyfish are notoriously adaptable. They've been around for hundreds of millions of years, and growing evidence indicates that today's man-made calamities, like oxygen-deprived "dead zones" and overfishing, actually favor gelatinous species--even though the plummeting nettle population in the Bay seems directly contrary to that (more on that later). Scientists call it "the jellyfish joyride;" the creatures pop up in alarming numbers as they exploit distressed ecosystems around the globe. They've destroyed fisheries in Ireland and Spain; driven away beach-goers in Australia and the Mediterranean; even capsized a 10-ton fishing trawler in Japan. (The vessel had netted oodles of an Asian species, each of which outweighs the average sumo wrestler.)
Scary headlines and eerie exhibits like the popular "Jellies Invasion" at Baltimore's National Aquarium aside, the jellyfish's precise impact on and reaction to an unhealthy Chesapeake remains murkier than its waters. One thing's for sure, though: for an animal without a brain they do pretty well for themselves.
"They're very simple and yet they definitely make a good living," says nettles researcher Margaret Sexton of the Horn Point Laboratory in Cambridge, Md. (Both Horn Point and CBL are operated by the University of Maryland Center for Environmental Science.) Since 2005, the doctoral student has counted nettles at Horn Point's Choptank River pier, trying, among other things, to make sense of their wild fluctuations. "Why one day there are a handful and others there are hundreds," she says.
Here's how much we don't know about Chrysaora quinquecirrha: taxonomists aren't even sure if they've named it correctly. "It turns out that the sea nettles that occur in estuaries like the Chesapeake Bay and the ones that occur along the coast are two different species [although they share a name]," Breitburg said. "Some time, probably fairly soon, we're going to wind up with a new name for the one that's here."
Both the estuarine and ocean sea nettles, however, are considered "true" jellyfish, members of an anatomically simple family that also includes sea anemones, corals and hydroids. The sea nettle's body (the bell) contains a primitive neural network for sensing (perhaps chemically) the presence of food, muscles for limited propulsion and efficient food gathering, a gastric cavity for digesting meals and gonads for making more nettles. That's about it. No heart, no gills, no bones, no brain. What we might refer to as the business end of a nettle, its lower dangly bits, work together to stun and capture prey. The long stringy tentacles and the frilly oral arms wield nematocysts, the infamous stinging cells that inject toxins into any living entity they encounter, i.e. hapless prey (fish) and defenseless flesh (your cousin Steve). We humans endure skin pain and irritation, but for the smaller critters the outcome is way worse; they are paralyzed and eaten. While the sea nettle isn't the largest of the Bay's jellyfish (that distinction belongs to the less prevalent moon jelly), it does get fairly big--up to 10 inches in bell diameter, with retractable tentacles that can extend over six feet.
Estuarine sea nettles have two distinct life phases--one visible and the other hidden. Those we can see are adults, capable of stinging swimmers, entangling propellers, clogging commercial fishermen's nets and occasionally jamming seawater intakes at the Bay's power plants. They spend their brief lives in annoying synchronicity with the boating season. They appear by June and depending on a variety of factors--many not well understood--reach their peak sometime in late summer. (As we all know, nettle-wise, August is the cruelest month). After spawning, they senesce during the fall as water temperatures decline, then are generally gone by October or early November.
Sexton believes the raggedy, armless ones Burrell spotted were an anomaly that occurs only when nettle season ends early, as it did in 2009. She's tried duplicating the phenomenon in deep tanks at Horn Point without success; another sea nettle mystery yet to be unraveled.The nettles that boaters don't generally see or encounter live on the Bay's bottom in the form of polyps. To understand their function, you need to understand the peculiarities of nettle reproduction. Adult males and females exist adrift, moving, eating and procreating to the water's rhythm. On the other hand, their spawn (planulae) reproduce asexually by attaching themselves to hard bottom surfaces--mainly the undersides of oyster shells--and developing into sedentary polyps. Initially no wider than a match, the little polyps sprout more polyps, forming fertile nurseries that survive the winter by closing up (encysting) until spring. When water temperatures warm again, the polyps grow. From spring through early summer each polyp buds off dozens of floating baby nettles resembling upside-down saucers. They mature into sperm- and egg-producing adults and the cycle begins anew.
You'd think a race of gifted propagators joyriding around on the excesses of human habitation would be faring pretty well, population-wise. In regions where they're invasive, they clearly do. Breitburg theorizes why the opposite may be true for Chesapeake's resident nettles: "When oysters crashed in Maryland in the late eighties, sea nettle abundances just seemed to really decline. Sea nettles have an overwintering stage that is attached to hard substrates, primarily to oyster shells. So when oysters crashed there probably wasn't enough hard substrate around for [the polyps] to overwinter." (Virginia's oysters crashed before Maryland's, but there are no Cargo-length data sets available to compare oyster-to-nettle trajectories in the lower Bay.)
Not everyone agrees that oyster decline predetermines nettle futures. Research in the Bay by scientists Jennifer Purcell and Mary Beth Decker indicates that ocean climate cycles may regulate jellyfish abundance too, particularly the equilibrium between top plankton feeders, sea nettles and comb jellies. Known as ctenophores [pronounced TEEN-a-forz] to scientists, comb jellies are the creatures whose soft green glow can be seen at night in your boat's prop wash. Lacking stinging capabilities, they're not true jelly-fish, but they eat much the same things as sea nettles: small crustaceans called copepods and other zooplankton, as well as the eggs and larvae of an important forage fish, bay anchovies. What's more, while both sea nettles and comb jellies can tolerate a low-oxygen environment (nutrient-laden "bad water"), ctenophores are better at it. In Breitburg's controlled experiments they survived for several days at saturation levels so low they'd kill any Bay fish in less than 24 hours. Nettles tend to stay in bad water only as long as their dinner does.
Breitburg too admires sea nettles. "They're really just about top predators in the system. They're not feeding on adult fish, but they feed on the same species that the top [predatory fish] feed on." And she offers a good reason to root for them: "The other thing they feed on is ctenophores, which may protect oyster larvae."
Here's an important distinction between sea nettles and the smaller but more abundant and voracious comb jelly: When a nettle ingests an oyster larva it spits out the baby oyster uneaten; not so with the comb jelly, which gobbles the oyster larvae like so much aquatic popcorn. So in that context alone, it's a bit of a vicious cycle for the nettles. With fewer nettles there to eat the comb jellies, there are more comb jellies to eat the oysters, which means fewer oyster shells for the nettle nurseries, which means fewer nettles to eat the comb jellies [repeat verse].
Breitburg's study of the effects of sea nettle abundance on other species eventually led her to some old file cabinets at CBL, where she found a trove of Cargo's unpublished data. She and others are now reanalyzing this and other work saved by his colleagues. Sexton has discovered, for example, that Cargo's pier counts likely underestimated nettle abundance below the surface--scary, considering his average daily surface count one week in August 1964 was 1,840.
Given such proliferation, it's no wonder that back in those days even Uncle Sam wanted the critters gone, or at least subdued, offering big bucks to anyone who figured out how. Mother Nature doesn't provide satisfactory controls--not here, anyway. Composed almost entirely of water, sea nettles offer little nutritional value. No Chesapeake predators make a steady diet of them. Sea turtles and a few resident fish species that consume nettles are too sparse to dent the Bay's robust population.
At the urging of Baltimore Congressman Edward Garmatz, Congress passed the Jellyfish Control Act of 1966. Declaring coastal waters unsafe, gelatinously speaking, for commercial fishing, boating and other water-based recreation, the feds authorized as much as $1 million annually through 1977 for research toward "controlling and eliminating jellyfish, commonly referred to as 'sea nettles', and other such pests. . . ."
Many anti-nettle methods were proposed or tested; most failed. Chemicals that killed polyps also killed oysters and crabs. Scientists tried to raise a known polyp predator, the striped sea slug (a benthic creature resembling a tiny shag rug with tentacles). That didn't work either. The U.S. Navy accidentally stumbled on a remedy so promising it was patented. While testing transducers in the Severn River's Carr Creek for the Navy in the late 1960s, civilian engineer Edward Thomas and physicist Bruce Douglas made a curious discovery one day. "When we hit this one band of frequencies [2 to 3.2 kilohertz], I noticed all these jellyfish on the surface trying to get out of the water," recalls Thomas, a retiree from Severna Park. Every time the frequency sweep was repeated, the result was the same: Thomas would spot nearly a thousand nettles, alive but bobbing around the submarine doing a jellyfish impersonation of the painting "The Scream." In their 1970 patent, Thomas and Douglas posited that the stunned nettles "could be gathered by net, centrifugal separators, or other means, and disposed of." Was the method used? "Not that I know of," Thomas says. After preemptive solutions bombed, researchers put taxpayer money to better use studying nettle biology and behavior, figuring if you can't kill 'em, you learn to live with 'em.
David Cargo kept counting, but also conducted polyp research. Eventually he was able to combine better understanding of their life cycle with salinity and water temperature readings to predict annual abundance, advising bathers and boaters via the media how likely nettles were to ruin their summer. (Writer Tom Horton labeled him "the Chesapeake version of Punxsutawney Phil.") Colleagues describe Cargo as a no-nonsense Army Air Corps veteran whose biology career veered by happenstance into nettle research. He never earned a doctorate, but became a respected research associate at CBL and associate editor of the journal Chesapeake Science. His published works are still cited in present-day research, which tends to focus on sea nettles' function in the Bay's complex ecosystem. With the rediscovery of unpublished data, his is research that keeps on enlightening.
"People often question why scientists repeat the same measurements over and over for many years," Breitburg says, "but it's only through long-term data sets that we can understand how ecosystems, and the world as a whole, are changing."
The more I read about his pioneering work, the more I wondered what Cargo looked like. After Burrell finished her count last September and headed off to SERC, I walked up the hill to CBL's administration building. Upstairs in the library I located several of Cargo's scholarly papers and a photo of him. It showed a lean, square-jawed, balding man with horn-rimmed glasses hauling in a fine-meshed net.
Would he marvel that his counts--once stored on the lab's old mainframe Univac--helped scientists achieve high-tech sea nettle prediction? Maybe. But I suspect he'd never guess his former lunchtime promenade, the pier, still stands on the cutting edge of sea nettle science.