Sturgeon is the common name used for some 25 species of fish in the family Acipenseridae, including the genera Acipenser, Huso, Scaphirhynchus, and Pseudoscaphirhynchus. The term includes over 20 species commonly referred to as sturgeon and several closely related species that have distinct common names, notably sterlet, kaluga, and beluga. Collectively, the family is also known as the true sturgeons. Sturgeon is sometimes used more exclusively to refer to the species in the two best-known genera, Acipenser, and Huso. Sturgeons have been referred to as "primitive fishes" because their morphological characters have remained relatively unchanged since the earliest fossil record.