Jellyfish In the Chesapeake
Until the mid-196Os, hardly more was known about the sea nettle (Chrysaora quinquecirrha) than its basic biology, namely that it has two life stages, one where small, swimming larvae fasten to a surface and develop into fixed polyps, and a second where the mature polyps begin to bud off (strobilate) young nettles (ephyra) which mature into the large bell-shaped medusae that produce eggs and sperm.
The medusa - with its semitransparent bell and streaming tentacles - is easy enough to see. But the polyp proved more elusive. "People knew what the polyp stage looked like back then" says Dave Cargo, "But nobody could actually find them in the Bay."
The problem, he says, is that they weren't looking in the right places. Like others, Cargo began looking for polyps on hard surfaces, primarily oyster shell, but he found those shells crowded with young oysters and other organisms, which left little room for nettle polyps. Only when he looked at the undersides of those shells did he discover them lurking there.
Now retired from the UMCES Chesapeake Biological Lab, he began the first extensive field studies of nettles, studies that described the nettles' general habitat needs, their salinity tolerance, and their temperature requirements. Unlike many other jellyfish which flourish at ocean salinities, greater than 30 parts per thousand salt, nettles, he found, do best between 7 and 25 parts per thousand.
Beginning in the 1960's, Cargo began what has become a thirty-year monitoring of sea nettle abundance in the Patuxent River. That monitoring was not, he says, very sophisticated: each day at lunch during July and August, he would walk out on the 200-foot pier at CBL and simply count the number of nettles. His aim was to try and correlate sea nettle prevalence with climatic factors-temperature, waterflow, salinity - to see if he could predict the intensity of sea nettle infestations each summer. Though not rigorous by scientific standards, he points out that "after 25 years you have numbers you can hang your hat on." Houde is more emphatic - "Dave Cargo's is the kind of long-term data we rarely have. That's what makes it so important."
Jellyfish and the Food Web
Some ecologists have hypothesized that, because of changes that have occurred at the bottom of the food chain, the sea nettle and other gelatinous species may be more plentiful in the Bay than when settlers first arrived in the Chesapeake region. According to one argument, the clearing of land and continued development of shorelines have made the Bay more susceptible to runoff of the soil's natural nutrients and, increasingly, more vulnerable to today's massive nutrient overloading from human, animal and agricultural wastes. The consequence, most researchers agree, has been explosive growth of phytoplankton, the single-celled plants that thrive on nutrients.
But how has massive phytoplankton growth affected the Bay's food web? For one thing, continuing overenrichment of nutrients, known as eutrophication, has been hypothesized to favor production of microorganisms like bacteria and microzooplankton (protozoans and rotifers) that feed on them. Some ecologists have proposed that these microscopic zooplankton support gelatinous species more than the "higher" forms of zooplankton, such as copepods, and fish larvae. Has the Bay's eutrophication meant more nettles?
Denise Breitburg is skeptical. She questions arguments that claim the Chesapeake Bay has a greater prevalence of sea nettles now than several hundred years ago simply because of changes at the bottom of the food chain. Estuarine ecosystems are not that simple, she says. "You have to look at details of trophic . interactions higher up in the food chain," she argues, "and the behaviors of the various species involved." There is some evidence in her recent studies, for example, that nettles can tolerate low dissolved oxygen concentrations better than the larval fish or zooplankton they feed on. Though nettles were impaired, says Breitburg, they continued to feed at significantly high rates.
The prevalence of sea nettles themselves appears to be conditioned by the timing and intensity of spring rains, which in turn affect the budding of ephyrae by polyps. This year, for instance, cold spring temperatures and heavy rains have kept nettle production down - there aren't very many to count at the CBL pier. But these same environmental conditions also affect zooplankton concentrations, larval fish populations and temperature, all of which have ramifications for sea nettle and ctenophore behavior that cascade up and down the rest of the food chain.
Teasing out these ramifications has led Breitburg, Jennifer Purcell and Ed Houde to look closely at how these gelatinous species compete for food, what they consume, what they don't consume, and how they behave under different environmental conditions. In effect, they have been working to quantify predation of sea nettles and other jellies on other Bay inhabitants. Purcell, for example, has done painstaking laboratory experiments to determine how sea nettles feed on oyster and other bivalve larvae. Because peak sea nettle abundance occurs in summer during the oyster spawning season, many scientists had assumed that nettles were consuming large numbers of the swimming larval oysters.
Research in Purcell's laboratory, however, has shown that sea nettles may help oysters. Oysters have a larval stage that spends about two weeks swimming in the Bay before they settle to grow into mature oysters. During this swimming stage, they are vulnerable to predators such as sea nettles and comb jellies. Although sea nettles can catch the larval oysters, Purcell discovered much to her surprise, that they spit them out undigested and unharmed. These findings may be the first reported evidence of oyster larvae passing alive through a carnivorous predator.
In contrast, comb jellies catch and digest the larvae readily. However, comb jellies are also a favorite food of sea nettles, and they reduce comb jelly populations to zero in the tributaries during the summer when oyster larvae are most abundant. "Therefore," says Purcell, "sea nettles appear to protect oyster larvae from a major predator."
Gelatinous species also feed on small zooplankton, fish eggs and larval fish. How much these species consume is especially important for bay anchovy, says Houde, which are hunting the same prey. As competitors with the anchovy, which are an important food for striped bass, bluefish and other Bay species, jellyfish could eventually have indirect impacts on their production. But explaining just what those impacts are will depend on experiments that detail predation rates among jellyfish and anchovy. In one related experiment, Houde - working with Jim Cowan, now at the University of South Alabama - found that the gelatinous predators "have the potential to consume 20 to 40 percent of the daily eggs and larvae of bay anchovy in mid-Chesapeake Bay." He also found, however, that when ctenophores and nettles occur in the Bay at the same time, there is a decrease in predation on larvae. This decrease may be due to diminished consumption of fish larvae by nettles because of their heavy consumption of ctenophores.
With sea nettle populations in the Chesapeake down this summer, you would guess, says Houde, that daily mortality of fish eggs and larvae would be down. But, he adds, "things are so variable - there could be compensating factors." In other words, the ever-changing food web may account for the decrease of one predator with the increase of another.
What Does the Future Hold?
Whether the Chesapeake Bay has more sea nettles now than in the past remains a provocative question, but one that seems too premature to answer. "I could not have predicted some of our research results from theory," says Denise Breitburg. "Our studies have confirmed my feeling - that you have to look at the details of these trophic interactions, the behaviors of the different species involved." In short, there is simply a great deal we do not yet know about jellyfish, says Jennifer Purcell.
As much as we might like to blame the presence of jellyfish in the Bay on increased nutrients or other changes, it will take a good deal of study before we fully understand the role of jellyfish in the Bay ecosystem.