If you hurry, you can still catch a free wildlife show at the Inner Harbor. The Northwest Branch of the Patapsco River is alive with big, milky jellyfish swimming lazily about.
It's not unusual to see the balloon-shaped Atlantic sea nettles in the harbor in the fall, said Jack Cover, the aquarium's general curator. The weather tends to be drier in late summer and autumn, which creates good conditions for those translucent, pulsing blobs with the long tentacles.
Chrysaora quinquecirrha, as they're known to scientists, are the Chesapeake Bay's most common jellyfish, Cover said. In summer, they're the bane of swimmers who get stung. But here in the harbor, which isn't really fit for swimming anyway, they're more a welcome sign of life.
"They're just moving up and down in the water column," Cover said. "They're trolling for plankton, and on certain moving tides ... you can see large numbers of them .. They go where the food is."
Cover should know - he makes regular forays into the bay to collect Atlantic sea nettles for the aquarium, which has a standing jellyfish exhibit. He said the ones in the harbor seem to be larger than what he's been gathering out in Eastern Bay.
Size aside, many Baltimoreans are surprised to see them in the harbor, and even query Cover about whether they could be escapees from the aquarium.
"It's kind of curious," he said. "Folks see them in the Inner Harbor and go, 'What? I didn't know we had living things in the Inner Harbor.'"
Visitors to the waterfront Tuesday pointed to the jellies as well, and wondered if they were a sign of good or bad water quality.
"Neither,'' said Raleigh Hood, a professor at the University of Maryland Center for Environmental Science. He and his students have been studying jellyfish at UM's Horn Point laboratory near Cambridge for years.
Jellyfish are better able to survive in water with little oxygen, Hood said. That's a problem that plagues the Chesapeake Bay every summer, as nutrient pollution triggers the growth of an oxygen-starved dead zone up and down the estuary.
But their presence in the harbor is more likely an indication that harbor water has been salty and warm enough to attract them, Hood said. Though rainy weather early in the summer turned the upper bay too fresh to sustain jellyfish and delayed their arrival, salinity levels rebounded with drier condtions in late summer.
While jellyfish populations globally appear to be on the increase, there's some debate about that. Conversely, limited data in the Chesapeake suggest they may be gradually declining in the bay, Hood said, but that's also subject to scrutiny. Denise Breitburg of the Smithsonian Environmental Research Center in Edgewater, who's parsed the bay data, has suggested the bay decline may stem from the loss of oyster reefs, which had served as nursery grounds for young sea nettles.
(In one of those ecological chain reactions, Breitburg and a colleague also suggested the drop in sea nettles may have hurt oysters in turn, because they feed on comb jellies, which in turn feed on oyster larvae.)
But much is still unknown about jellyfish, including how they move, the UM professor said. Scientists are hoping everyday citizens will help them track the mysterious creatures by reporting sightings. People can report sightings to www.jellywatch.org, a site maintained by the Monterey Bay Aquarium. Hood said he hopes Marylanders will help fill in the Chesapeake portion of the map.
Here in Baltimore, Cover said nettles should be a sign to people of the life teeming in the murky depths. The harbor gets visited by rockfish, blue crabs, gizzard shad, menhaden and other aquatic creatures, as well as ducks and migratory waterfowl. And the jellies' appearance is an indication that the water is chock full of zooplankton, microscopic animals, on which they feed.
"There is nature in the Inner Harbor," Cover said. "Yup. It's pretty cool."
Sampling done in the Patapsco River Wednesday by the Maryland Department of Natural Resources indicate that salinity at the surface midriver was 7.6 parts per thousand, a bit low for jellyfish, but better on the bottom, 18.4 parts per thousand, according to Mark Trice, who runs DNR's Eyes on the Bay monitoring site. Jellies tend to be more abundant when salinities are 10 to 20 parts per thousand.
Water temperatures Trice reported, were around 64 to 66 degrees Fahrenheit -- still within the range where nettles could survive, said Hood.
But the season won't last much longer. As water temperatures drop, by the end of this month or early next, the nettles' signature medusa phase will die off. The eggs jellyfish produce in abundance have dropped to the bottom. They'll overwinter there as polyps, ready to start next year's crop of nettles when the water reaches the right conditions again.
So while the wild jellies may still be around a bit longer, this very well could be, as Cover called it, their "last hurrah" for the year in the Inner Harbor.