Embryos of Artemia franciscana survive four years of continuous anoxia: the case for complete metabolic rate depression

Encysted gastrula embryos of the crustacean Artemia franciscana have acquired an array of adaptations that enable them to survive a wide variety of environmental extremes.The present paper shows that at least 60 % survive 4 years of continuous anoxia at physiological temperatures (20­23 °C) when fully hydrated. Although these embryos appear to carry on a metabolism during the first day of anoxia, no evidence for a continuing metabolism throughout the subsequent 4 years was obtained. During this period, there were no measurable changes in the levels of their stored, mobilizable carbohydrates (trehalose, glycogen, glycerol). Calculations indicate that, if these carbohydrates are being utilized at all during anoxia, the rate is at the least 50 000 times lower than the aerobic rate (lower limit of detection). Indications of proteolysis during prolonged anoxia were sought but not found. Under starvation conditions, the life span of larvae produced from embryos that had undergone 4 years of anoxia was not significantly different from that of larvae produced by embryos that had not experienced anoxia. Thus, all substrates and other metabolites required to support embryonic development to the nauplius, as well as endogenous (unfed) larval growth and molting, are retained during 4 years of anoxia. It is not possible to prove experimentally the absence of a metabolic rate in anoxic embryos under physiological conditions of hydration and temperature. Nevertheless, on the basis of the results presented here, I will make the case that the anoxic embryo brings its metabolism to a reversible standstill. Such a conclusion requires that these embryos maintain their structural integrity in the absence of measurable biosynthesis and free energy flow and are thus an exception to a major biological generality. Potential mechanisms involved in their stability are discussed.