Amino acids are the basic building blocks of living organisms and typically occur in a configuration known as the L-form. However, there are a few exceptional examples of the structural mirror image of the L-form (known as the D-form) present in animals. D-serine is a representative D-form amino acid and has crucial roles in mammals, but its role in non-mammals is unclear. Researchers from Japan recently uncovered a functional role of D-serine in a marine invertebrate, which may provide insight into the evolution of D-amino acid function in organisms.