By Denise Richardson
ONEONTA _ After more than 20 years studying amphibians, Hartwick College Professor Stanley Sessions has revealed another discovery about how tadpoles turn into frogs with deformities.
His early studies were about deformities caused by parasites. He responded to arguments by researchers and environmentalists about the effects of chemical pollutants. His most recent collaboration has revealed that legless frogs are the result of dragonfly nymphs taking bites of tadpoles.
Sessions and Brandon Ballengee, of the University of Plymouth in the United Kingdom, submitted their findings last year in a paper to the Journal of Experimental Zoology. Their article, ``Explanation for Missing Limbs in Deformed Amphibians,'' appeared online June 8 pending publication in its next issue, according to the journal's website.
In 1990, the same journal published Sessions' research results about frogs with extra limbs.
The findings about parasites and predatory nymphs make ``bookends'' to his work studying frogs, Sessions said during an interview last week in his office at Hartwick College.
Shelves in his office are heavy with books, photographs on the wall feature amphibians and a horse skull sits among other natural artifacts on his desk. He illustrates with his hands how a dragonfly nymph finds a meal.
Around the world, frogs have been found with missing or deformed limbs, Sessions said. Researchers studied the possible causes, and the search for answers sparked controversy and produced books on the issues, he said.
For a decade, Ballengee and Sessions have collaborated on art and science projects that show details inside amphibians' bodies. The BBC's Earth News has reported the recent findings on the dragonfly nymph. In a news release from Hartwick College, he describes the results:
"What we've found is that these predators grab tadpoles and almost surgically remove their tender hind limbs with their mandibles. They then let the tadpole swim away, and it can metamorphose into a frog that's missing a hind limb or part of hind limb. We call this phenomenon selective predation' since the predators consume only selected parts of the prey. It seems pretty obvious in hind sight, but this solves the whole rest of the problem."
For years, researchers have debated the roles of parasites, predators, chemical pollution or UV-B radiation caused by the thinning of the ozone layer, was triggering the deformations, the BBC report said.
"Deformed frogs became one of the most contentious environmental issues of all time, with the parasite researchers on one side, and the chemical company' as I call them, on the other," Sessions said in the recent news report. "There was a veritable media firestorm, with millions of dollars of grant money at stake."
Sessions, 60, who joined Hartwick in 1989, teaches animal development, evolution, natural history of vertebrates, and developmental genetics plus an off-campus course on the natural history of Costa Rica.
Hartwick is a private, liberal arts college with about 1,480 undergraduates.
Hartwick students were co-authors on about half of the dozen papers Sessions has published about deformed amphibians, college officials said. He started his research on deformities during his postdoctoral work at the University of California's Berkeley and Irvine campuses.
Sessions published his research indicating that extra limbs on frogs were caused by parasitic flatworms known as trematodes. But what about frogs without legs?
"People said, Okay, you explained the frogs with extra limbs, but what about these?'" Sessions said. "We thought it couldn't be predation because predators eat frogs, usually killing them in the process, and these frogs were otherwise healthy and intact. It turned out the missing limbs were much more difficult to explain than extra limbs, but I think we've finally cracked it!"
Ballengee and Sessions ran tests to see if stickleback fish, diving beetles, water scorpions, dragonfly nymphs or other suspected predators attacked tadpoles, the BBC said. Only three species of dragonfly nymph did.
Adult amphibians with one hind limb appear able to live for quite a long time, explaining why so many deformed frogs and toads are discovered, he told the BBC.
Sessions said he doesn't completely rule out chemicals as the cause of some missing limbs, but selective predation' by dragonfly nymphs is the leading explanation.
"Are parasites sufficient to cause extra limbs?," he said in a prepared statement. "Yes. Is selective predation by dragonfly nymphs sufficient to cause loss or reduction of limbs. Yes. Are chemical pollutants necessary to understand either of these phenomena? No."