The general term for all cases in which two (or more) biological species are found that do not differ in morphological (and other) characters that are generally accepted in the taxonomy of a given group. There are many twin species among molluscs, crustaceans and protozoa. Doppelganger species are groups separated by clear breaks. Most likely, in the process of speciation, the restructuring of the genotype is not always accompanied by changes in the morphology of the phenotype, making it difficult to distinguish between the two species. Examples of doppelganger species in fish are Neolamprologus pulcher and Neolamprologus brichardi.