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EVIDENCE FOR ACCELERATED METAMORPHOSIS IN BULLFROG (RANA CATESBIEANA) TADPOLES IN AN EPHEMERAL POND
Wegner, K. E., C. R. Hackmann, AND S. Heppell. EVIDENCE FOR ACCELERATED METAMORPHOSIS IN BULLFROG (RANA CATESBIEANA) TADPOLES IN AN EPHEMERAL POND. Presented at American Society of Ichthyologists and Herpetologists, Kansas City, MO, July 3-8, 2002.
It has been widely accepted that time to metamorphosis for non-native bullfrog tadpoles in the Pacific Northwest is greater than one year. We surveyed 22 ponds within the EE Wilson Reserve (Benton County, Oregon) for bullfrog tadpoles and metamorphs from April through September, 2000-2001. In August we observed small bullfrog tadpoles in late stages of metamorphosis at SnipePond, a uniquely warm pond on the Reserve. Snipe Pond is considered intermittently ephemeral and dried down completely in 1999 and 2000. Aerial photos revealed no connectivity between Snipe Pond and adjacent waterways through the wet season. When we first observed metamorphosing tadpoles in summer 2000, the water level was ~20cm, down from a high of ~3m, and at the end of September the water lever was ~5cm. We speculate that these tadpoles were going through metamorphosis in their first season, at an age of approximately 3-3.5 months. In the eastern United States, where the bullfrog is a native species, time to metamorphosis can be as little as 3 months or as long as 3 years, following a latitudinal gradient. The increase in time to metamorphosis is often attributed to cooler water temperatures along the gradient. Snipe Pond is shallow and sustained temperatures of 28 degrees C or more through August and early September. Each year of our study revealed high numbers of very small newly metamorphosed bullfrogs. We believe our observation shows plasticity in the bullfrogs tadpoles in response to high temperatures and declining water levels, which may have important management implications for bullfrog control in the western U.S.