Most frog embryos take hours to hatch, but when snakes attack, baby red-eyed treefrogs make a quick, wriggly escape from their eggs within a matter of seconds. In a new Journal of Experimental Biology study published last week, researchers reveal that enzymes released from their snouts create a hole in their protective capsules.
The eggs of red-eyed treefrogs, Agalychnis callidryas, are attached to vegetation with a layer of jelly. And they’re well known as escape artists: They can burst out of their eggs in as little as 6.5 seconds, then they drop safely into the pond below. “We had seen them thrashing around and we thought they were somehow breaking out of the egg,” Boston University’s Karen Warkentin said in a statement. “This escape hatching is a mechanism for running away from a really important predator.” Undisturbed eggs hatch after six or seven days, but some threats – such as fungus infection, dehydration, flooding, or egg-eating snakes and wasps – cause them to hatch early and land in water at a less developed stage. Exactly how these tiny escape artists accomplish this feat remains a mystery.
To investigate this rapid premature hatching, Warkentin and a team led by Kristina Cohen of Boston University used high-speed cameras to record the escape in detail. They collected egg masses off leaves from Ocelot Pond and Experimental Pond near and in Gamboa, Panama. The embryos continued to develop on plastic cards suspended over tap water for five days. The eggs were then prodded with blunt forceps to trigger embryo emergence. Hatching took between 6.5 and 49 seconds.
After trembling for a bit, the embryos would open and close their mouths repeatedly until the egg membrane ruptured. “Following this,” Cohen explained, “you see fluid start to leak from the location in the membrane that is just in front of the embryo's snout.” The embryos then plugged the small hole with their snout and wriggled their way out. Because the embryos created the hole in their gelatinous capsules without even touching it, the team figured they were using enzymes to degrade the egg membrane.
To figure out where the enzymes were being released from, the team rotated the embryos around in the egg after they began their pre-rupture shaking and gaping. The rupture appeared in the original spot their snouts were pointed; embryos that were spun around would either try to relocate the hole, or they would shake again to create a second hole. Electron microscope images helped confirm that the embryos were releasing membrane-dissolving enzymes from glands densely concentrated on the snout during shaking. After hatching, these gland vesicles were empty.
Image in the text: Red-eyed treefrog embryos hatch to escape from a cat-eyed snake. Karen M. Warkentin