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First published online October 31, 2008
Journal of Experimental Biology 211, 3627-3635 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.020958
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External gills and adaptive embryo behavior facilitate synchronous development and hatching plasticity under respiratory constraint

Jessica R. Rogge and Karen M. Warkentin*

Department of Biology, Boston University, Boston, MA 02215, USA


Figure 1
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  		Fig. 1. Examples of oxygen consumption rates measured at different oxygen levels for individual 5-day-old embryos and newly hatched tadpoles of red-eyed treefrogs with and without external gills. Embryo measurements stopped at hatching. Lines show regression fits used to estimate Pcrit (arrows); data points above and below Pcrit are indicated by filled and open circles, respectively. Note that the x-axis location for the gilless embryo is shifted relative to that for the other animals.

 

Figure 2
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  		Fig. 2. Effect of external gills on oxygen levels that (A) limited metabolic rate in red-eyed treefrog embryos and tadpoles and (B) induced hatching, measured using closed-system respirometry. External gills improved oxygen uptake for embryos in terrestrial eggs, lowering both Pcrit and the PO2 that induced hatching. Embryos without gills were oxygen limited even in air (~20.5 kPa). External gills had no effect on the Pcrit of hatched tadpoles in water. Data are means ± s.e.m.; N=11 animals per treatment.

 

Figure 3
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  		Fig. 3. Red-eyed treefrog embryo rapidly moves to reposition itself with external gills in the well-oxygenated region near the air-exposed egg surface, after experimental manipulation to position gills in the hypoxic rear of the egg. Embryos are 3 days old and egg diameter is ~4.9 mm; the arrow marks the experimental individual. Photos by K.M.W.

 

Figure 4
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  		Fig. 4. Time required for red-eyed treefrog embryos to reposition gills or head near air-exposed egg surface after experimental manipulation turning animals to face hypoxic rear of egg, and time until the next movement for controls manipulated similarly but positioned facing air-exposed egg surface. Embryos were watched until they moved or for 120 s (2 days old) or 60 s (3 days old). Many control embryos did not move; time watched was used as time to move for these animals. Data are means ± s.e.m.; N=25 embryos per treatment per age.

 

Figure 5
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  		Fig. 5. Red-eyed treefrog embryo uses ciliary rotation to return its developing head to the well-oxygenated region under the air-exposed egg surface, after experimental manipulation to position it facing the hypoxic rear of egg. Embryos are 1 day old and ~3.2 mm long; the arrow marks the experimental individual. Photos by K.M.W.

 

Figure 6
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  		Fig. 6. Angular rotation of 1-day-old (neural tube stage) embryos of red-eyed treefrogs in the 5 min after moving them to different initial positions within the egg. Grey background indicates egg surface covered by other eggs and leaf substrate; white background represents air exposure and zone of higher perivitelline oxygen levels. Experimental embryos positioned with their developing head at the hypoxic rear of the egg moved it toward the air-exposed surface (A,B), whereas controls positioned with their developing head at the front of the egg remained there (C,D). Data are means ± s.e.m.; N=25 embryos per treatment.

 

Figure 7
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  		Fig. 7. Effects of embryo development (age) and position of external gills in the oxygen gradient within eggs on the time red-eyed treefrog embryos remained in a position, after spontaneous movements. Deep positions within the egg are hypoxic, and positions near the air-exposed surface are better oxygenated. No embryos remained long with their gills in hypoxic regions of the egg, and younger embryos were more active. Data were taken from videotapes (means ± s.e.m., N=40 embryos per age).

 

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© The Company of Biologists Ltd 2008