The embryos and larvae of the Australian moss frog Bryobatrachus nimbus develop for over one year in concealed cavities within bryophytes or lichens. Oviposition occurs in drier summer months, and consequently embryos experience a variety of hydrous conditions in concert with the drying and rehydration of the nest material. When the hydrous environment was manipulated in the laboratory, B. nimbus embryos incubated in the wettest conditions (0 kPa) had significantly higher survival to hatching than embryos reared at three drier water potentials −5 −10 and, −25 kPa). Moreover, embryos incubated in drier conditions developed with pronounced trunk and hind limb-bud asymmetry and had significantly shorter and shallower tails and lower metabolic rates, relative to embryos incubated at 0 kPa. Measurement of capsule morphology showed that the volume of perivitelline fluid in −10 and −25 kPa embryos was only 9% of that bathing 0 kPa embryos. The reduced developmental space, coupled with adhesions between the yolk sac wall and perivitelline membrane, limited rotation of drier embryos and, hence, promoted asymmetric development. The severity of abnormalities at −25 kPa indicated that B. nimbus was less tolerant of low water potentials than other anuran species studied. The unusually large egg capsules of B. nimbus buffer embryos against water loss during intermittent dry periods, but the vegetative nest material must also provide appreciable protection from egg desiccation during natural incubation.
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