How Red Eyed Tree Frog Embryos Hatch Early to Escape Dangerous Predators

How do embryos escape from danger? We studied this with Red Eyed Tree Frogs. These frogs lay their eggs on plants over
tropical ponds where many of them are attacked by snakes. The embryos can hatch themselves rapidly and
prematurely to escape, but how do they do it? We used high speed video to take a close look
at the hatching process. Most embryos performed a distinctive shaking
just before the membrane ruptured. Some did not touch the membrane with their
snout while shaking, and some also gaped open their mouths. Moments later we saw a leak form just in front
of the embryo’s snout. We hypothesized that embryos made a hole in
the membrane by releasing enzymes from their snout. To test this, we stimulated embryos to hatch,
after which they began shaking. We allowed them to shake for several seconds
before rotating them so that their snout faced a new location on the membrane. We observed that the rupture still formed
at the original location of the embryo’s snout As evidenced by leaking of fluid. We then used scanning electron microscopy
to look for hatching glands and found them densely concentrated on the
snout. They’re large cells that appear recessed among
the epithelial cells and they have short microvili. In cross section they are full of secretory
vesicles just prior to hatching and mostly emptied of their contents just
seconds after hatching. This rapid highly localized enzyme release
allows Red Eyed Tree Frog embryos to hatch at a moment’s notice across a broad developmental
period. This is much faster than the hatching mechanism
that’s been described in other frogs. And it’s a critical part of a highly effective
embryo self defense mechanism.

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