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Old 11-09-2008, 04:50 AM
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Default Oxygen-seeking frog embryos boost survival

Red-eyed treefrog (Agalychnis callidryas) embryos can rotate inside their eggs to get more oxygen if levels start to drop, their prolonging chances for survival. The embryos are ready to hatch just four days after they are laid, but delaying hatching a few days can boost survival rates because tadpoles that are more mature can more easily swim away from predators. But as an embryo continues to develop, more oxygen is recycled from a limited supply inside the egg. Oxygen levels have been found to be as low as 2% in the middle of the egg. In response to the risk of suffocation, the embryos position their external gills in a small high-oxygen area near the egg’s surface. This rotation allows the embryos to maintain high metabolic rates and viability.

External gills and adaptive embryo behavior facilitate synchronous development and hatching plasticity under respiratory constraint. J Exp Biol. 2008 211: 3627-35
Plasticity in hatching timing allows embryos to balance egg- and larval-stage risks, and depends on the ability of hatching-competent embryos to continue developing in the egg. Hypoxia can slow development, kill embryos and induce premature hatching. For terrestrial eggs of red-eyed treefrogs, the embryonic period can extend approximately 50% longer than development to hatching competence, and development is synchronous across perivitelline oxygen levels (P(O(2))) ranging from 0.5-16.5 kPa. Embryos maintain large external gills until hatching, then gills regress rapidly. We assessed the respiratory value of external gills using gill manipulations and closed-system respirometry. Embryos without external gills were oxygen limited in air and hatched at an external P(O(2)) of 17 kPa, whereas embryos with gills regulated their metabolism and remained in the egg at substantially lower P(O(2)). By contrast, tadpoles gained no respiratory benefit from external gills. We videotaped behavior and manipulated embryos to test if they position gills near the air-exposed portion of the egg surface, where P(O(2)) is highest. Active embryos remained stationary for minutes in gills-at-surface positions. After manipulations and spontaneous movements that positioned gills in the O(2)-poor region of the egg, however, they returned their gills to the air-exposed surface within seconds. Even neural tube stage embryos, capable only of ciliary rotation, positioned their developing head in the region of highest P(O(2)). Such behavior may be critical both to delay hatching after hatching competence and to obtain sufficient oxygen for normal, synchronous development at earlier stages.

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