Boston University undergraduate Jessica Rogge and associate professor Karen Warkentin, working at the Smithsonian Tropical Research Institute's laboratories in Gamboa, Panama, discovered that frog embryos at a very early developmental stage actively respond to oxygen levels in the eggas reported in the Nov. 7, 2008 issue of the Journal of Experimental Biology. These initial responses to the environment may be critical to the frogs' long-term survival.
Red-eyed treefrogs, Agalychnis callidryas, lay jelly-covered egg clutches on leaves overhanging tropical ponds. Each clutch of several dozen transparent eggs is ready to hatch after only four days. By delaying hatching by a few more days, frog embryos significantly increase their chances of survival as tadpoles in the pond below where predators lurk. But as each egg matures, and embryos need more oxygen, less and less of the vital substance is available inside.
Warkentin has measured oxygen levels as low as 2% air saturated in the middle of red-eyed treefrog eggsyet the embryos refrain from hatching. "You'd think they would be dead. You'd think they would hatch. But they continue to develop at the same rate as embryos in eggs with much, much more oxygen," she explains. "Jessica found that these embryos maintain high metabolic rates and rapid, synchronous development by behaviorally positioning their external gills in a small high-oxygen area, a sweet spot near the exposed surface."
The team used a video camera to record embryo movement in the egg. By gently manipulating the eggs with a probe, Rogge turned the embryos within the eggs so that the gills were positioned away from the surface, in lower oxygen. "It's a lot like trying to pick something up out of a glass of water with your fingers," says Rogge, "the embryo 'wants' to be at the front of the egg, so unless I turned the embryo completely around in one motion it would come back to the front before I could fini
|Contact: Beth King|
Smithsonian Tropical Research Institute