Jellyfish in a Lake
Quarter-sized freshwater jellies show up sporadically all over the state.
By Dyanne Fry Cortez
John Newman was consulting on an East Texas ranch when he saw something he never expected: a flock of round, whitish blobs bobbing in the clear water of a spring-fed lake.
Newman, who owns Newman Wildlife Management near Frankston, had a hunch what the blobs were. "I was in the Navy for four years, and I know what jellyfish look like," he says. The things in the lake were "just like a miniature saltwater jellyfish, but about the size of a quarter." Short tentacles rimmed the near-transparent bell. Internal organs formed a cross at the center.
That was August 1997. Newman called the Texas Parks and Wildlife Department. Jim Matthews, exhibits curator at the Texas Freshwater Fisheries Center, came out with fisheries biologist Rick Ott to collect specimens. Newman found out about the existence of a freshwater jellyfish - not a true jellyfish, but a member of a related family. Biologists call it Craspedacusta sowerbii, and it's found in lakes and ponds all over the world.
Terry Peard, who studies C. sowerbii at Indiana University of Pennsylvania, lists some 50 Texas sightings on his Web site. Freshwater jellies have appeared in Lakes Amistad, Cisco, Grapevine, Joe Pool, Limestone, Nacogdoches, Medina and Travis, as well as several private lakes and ponds.
The free-floating medusa is the most visible stage of the jellyfish's complex life cycle. In most seasons, C. sowerbii lives in colonies of tiny, stalked polyps attached to underwater surfaces. A polyp reproduces by budding. It may produce a branch that remains connected or a frustule larva that breaks off and crawls away. Every so often, the polyps will bud off a crop of medusae, which develop sex organs and go looking for mates.
Both the medusa and polyp forms of C. sowerbii feed on zooplankton. Like its saltwater relatives, the freshwater jellyfish uses stinging cells to capture its food. The stingers aren't tough enough to have much effect on a human, although some people have reported a tingle when one touches a sensitive spot.
Newman observed medusa "blooms" at his client's ranch each summer from 1997 through 2005. The sight reminds him of bubbles in a boiling pot. "They just appear from the depths and come up toward the surface," he says. "Sometimes you don't see more than 10. Sometimes, there are hundreds."
This dance may not accomplish much, in biological terms. Peard's research suggests that most United States populations are all male or all female. All the jellies in a given water body may be descended from a few dormant polyps (podocysts) that arrived on a bird's foot or in a hatchery tank with stocked fish.
Blooms last only a few weeks, and they don't happen on a predictable schedule. If you want to get lucky, Newman suggests gazing into water on a still day in late summer when the sun is high in the sky. "The hottest dog days of summer are when we see them," he says. "The hotter the water, the more active they are."
For more information on freshwater jellyfish, visit www.jellyfish.iup.edu.