During environmental anaerobiosis, when the whole organism is exposed to anoxic conditions caused by external, physical factors in the microhabitat, arginine phosphate, glycogen, and aspartate (only mollusks) are the substrates for the metabolism. Main end products formed are lactate (Crustacea, Xiphosura, some Gastropoda and Bivalvia), alanine, succinate, and the volatile fatty acids, propionate and acetate (most Gastropoda and most Bivalvia). Because of reduction of the overall metabolism in bivalves, utilization and production rates of substrates and end products, respectively, are low. They are generally faster in Crustacea, which do not reduce their metabolism.During functional anaerobiosis, when the muscle tissue becomes anoxic because of increased work done by the animal, energy is derived from arginine phosphate and glycogen. The main end products formed are lactate (Crustacea), octopine (active Gastropoda and Bivalviaj, and strombineialanopine (some gastropod species). Utilization rates of the phosphagen and production rate of lactate and opines are sometimes more than 500-fold higher than during environmental anaerobiosis. These strong variations in the glycolytic flux are probably regulated by the influence of the phosphagen and adenylates on regulatory enzymes and by the NADHNAD ratio. Key words anaerobiosis in mollusks, anaerobiosis in crustaceans, opine metabolism, succinate pathway, regulation during anaerobiosis Living organisms require continual energy expenditure for the maintenance of function and structure. This energy must be present in the form of ATP' molecules provided by the animal's energy metabolism. Generally, the production of chemical energy from the foodstuff molecules-carbohydrates, fats, and, to some extent, proteins-is achieved by their complete oxidation to COz and H20 with the major amount of ATP produced via oxidative phosphorylation. These processes take place inside the mitochondrion by use of the electron transport chain ending with the reduction of molecular oxygen. Thus, sufficient availability of oxygen is an absolute requisite for the functioning of anaerobic metabolism.Generally, air-breathing animals have no problems in terms of obtaining adequate amounts of oxygen. These organisms typically rely on aerobic energy metzbolism, although under circumstances some tissues (even in mammals) may rely partly or exclusively on anaerobic energy production. These tissues produce lactate anaerobically via glycolysis, which is transported via the blood to aerobic tissues for oxidation (heart) or for resynthesis of glucose (liver). These organisms become only partly anaerobic during exercise when muscle activity needs more energy than is produced aerobically.Animals living in water may continuously face problems of oxygen availability. Compared to air, the amount of oxygen dissolved in water is quite small. Furthermore, the amount of dissolved oxygen can vary dramatically depending on temperature, salinity, and the presence of decaying organic matter. Intertidal species may...
