The question of the gap between ancestral unicellular and pluricellular eukaryotes remains open despite the gathering and analyses of data obtained from cell biology and genome composition studies. Sponges, which are among the oldest pluricellular forms of animal life, represent an attractive intermediate between simple and more complex forms of animal life. These animals are characterized by relatively sophisticated metabolic and physiological traits. Sponges can live in symbiosis with bacteria and archaea, and both microorganisms participate in the sponge amino acid metabolism in terms of amino acid uptake, synthesis, degradation, and synthesis of bioactive metabolites and peptides. Such amino acid metabolism appears central for the physiological functions in sponges.