Monday, September 6, 2010
September 6, 2010 : Sturgeon
Sturgeon is the common name used for some 26 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.
One of the oldest families of bony fish in existence, sturgeon are native to subtropical, temperate and sub-Arctic rivers, lakes and coastlines of Eurasia and North America. They are distinctive for their elongated bodies, lack of scales, and occasional great size: Sturgeons ranging from 7–12 feet (2-3½ m) in length are common, and some species grow up to 18 feet (5.5 m). Most sturgeons are anadromous bottom-feeders, spawning upstream and feeding in river deltas and estuaries. While some are entirely freshwater, very few venture into the open ocean beyond near coastal areas.
Several species of sturgeons are harvested for their roe, which is made into caviar—a luxury good which makes some sturgeons pound for pound the most valuable of all harvested fish. Because they are slow-growing and mature very late in life, they are particularly vulnerable to exploitation and to other threats, including pollution and habitat fragmentation. Most species of sturgeons are currently considered vulnerable, endangered or critically endangered.
Sturgeon and related paddlefish appeared in the fossil record approximately 200 million years ago, making them among the most ancient of actinopterygian fishes. In that time they have undergone remarkably little morphological change, indicating that their evolution has been exceptionally slow and earning them informal status as living fossils. This is explained in part by the long inter-generation time, tolerance for wide ranges of temperature and salinity, lack of predators due to size, and the abundance of prey items in the benthic environment.
Along with other members of the Chondrostei and the Acipenseriformes order, sturgeon are primarily cartilaginous, lack a vertebral centrum, and are covered with bony plates called scutes rather than scales. They also have four barbels—tactile organs that precede their toothless mouth and are dragged along often murky river bottoms. Sturgeon are distinctly and immediately recognizable for their elongated bodies, flattened rostra, distinctive scutes and barbels, and elongated upper tail lobes.
They are primarily benthic feeders. With their projecting wedgeshaped snout they stir up the soft bottom, and use the barbels to detect shells, crustaceans and small fish, on which they feed. Having no teeth, they are unable to seize prey, though larger specimens can swallow very large prey items, including whole salmon.