Genus

Genera in fish taxonomy are defined by shared derived morphological and molecular characters — a combination of synapomorphies. The stability of genus boundaries is often contested: molecular phylogenetics frequently shows that traditional genera are para- or polyphyletic, requiring splitting or lumping. The genus Carassius, for example, includes the common goldfish (C. auratus) and crucian carp (C. carassius) — species that hybridize freely, blurring species boundaries. Genus names serve as informative shorthand: Salmo implies membership in the salmon family; Thunnus signals membership in the group of large, warm-bodied, commercially important tuna species.