The annual killifish Austrofundulus limnaeus inhabits ephemeral ponds that dry out on a seasonal basis, thereby killing the adult and juvenile forms. Populations persist because diapausing embryos become embedded in the pond sediments. The rate of oxygen consumption of diapause II embryos is depressed by up to 90 % compared with that of developing embryos, and a parallel reduction is observed in heart rate. Developmental arrest was identified by cessation of somite proliferation and blockage of the ontogenetic increase in DNA content. Surprisingly, the arrest of metabolism and development is temporally offset as embryos reach diapause II; metabolic rate begins to decline 12 days prior to arrest of development. Release of embryos from diapause II is facilitated by increasing the light phase of the photoperiod. The rate of oxygen consumption of diapause III embryos is 84 % lower than the value preceding diapause III. The total energy flow of diapause II embryos apparently includes a contribution from anaerobic processes on the basis of calorimetric/respirometric ratios that are above the oxycaloric equivalent. Accumulations of lactate and ethanol at the expense of glycogen reserves are small or undetectable and do not account for the excess heat signal. Diapause II embryos maintain high [ATP]/[ADP] ratios and adenylate energy charge during diapause, consistent with a simultaneous depression of energy use and demand. Levels of AMP increase during early development and diapause II despite the highly charged adenylate pool. High values for [AMP]/[ATP] ratios in diapause II embryos are correlated with decreased rates of oxygen consumption and heat dissipation, which suggests a role for AMP in the depression of metabolism during early development and diapause II.

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