One of the oldest families of bony fish in existence, they 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 . 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 either vulnerable, endangered or critically endangered.
Sturgeon and related paddlefish first appear in the fossil record approximately 200 million years ago, making them among the most ancient of teleost fish. 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. Despite the existence of a fossil record, it has been difficult to fully classify the sturgeon species or unambiguously determine their phylogeny. This is in part due to the high individual and ontogenic variation, including geographical clines in certain features, such as rostrum shape, number of scutes and body length. A further confounding factor is the peculiar ability of sturgeons to produce reproductively viable hybrids, even between species assigned to different genera. The wide range of the Acipenserids and their endangered status have made collection of systematic materials difficult. These factors have led researchers in the past to identify over 40 additional species that were rejected by later workers. It is still unclear whether the species in the Asipenser and Huso genera are monophyletic (descended from one ancestor) or paraphyletic (descended from many ancestors)- though it is clear that the morphologically motivated division between these two genera is not supported by the genetic evidence. There is an ongoing effort to resolve the taxonomic confusion using a continuing synthesis of systematic data and molecular techniques.
Along with other members of the Chondrostei and the Acipenseriformes order, sturgeon are primarily cartiligenous, lack a vertebral centrum, and are covered with bony plates called scutes rather than scales. They also have four barbels - unique 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 and even baby seals.
Sturgeon have been referred to as both the Leviathans and Methuselahs of freshwater fish. They are among the largest fish: some beluga (Huso huso) in the Caspian Sea reportedly attain over 5.5 m and 2000 kg while for kaluga (H. dauricus) in the Amur River similar lengths and over 1000 kg weights have been reported. They are also probably the longest-lived of the fishes, some living well over 100 years and attaining sexual maturity at 20 years or more. The combination of slow growth and reproductive rates and the extremely high value placed on mature egg-bearing females make sturgeon particularly vulnerable to overfishing.
Sturgeons are polyploid; some species have 4, 8, or 16 sets of chromosomes.
Throughout this extensive range, almost all species are highly threatened or vulnerable to extinction due to a combination of habitat destruction, overfishing and pollution.
Most species are at least partially anadromous, spawning in fresh water and feeding in nutrient rich brackish waters of estuaries or undergoing significant migrations along coastlines. However, some species have evolved purely freshwater existences, such as the lake sturgeon (Acipenser fulvescens) and the Baikal sturgeon (A. baerii baicalensis), or have been forced into them by anthropogenic or natural impoundment of their native rivers, as in the case of some subpopulations of white sturgeon (A. transmontanus) in the Columbia River and Siberian sturgeon (A. baerii) in the Ob basin.
In Russia, sturgeon fisheries are of immense value. Early in summer the fish migrate into the rivers or towards the shores of freshwater lakes in large shoals for breeding purposes. The ova are very small, and so numerous that one female has been calculated to produce about three million in one season. The ova of some species have been observed to hatch within very few days after exclusion. In sturgeons that have attained maturity their growth appears to be much slower, although continuing for many years. Frederick the Great placed a number of them in the Garder See Lake in Pomerania about 1780; some of these were found to be still alive in 1866. Professor von Baer also states, as the result of direct observations made in Russia, that the Hausen (Acipenser huso) attains an age of 100 years, but can live over 210 years.
In countries like England, where few sturgeons are caught, sturgeon is included as a royal fish in an act of King Edward II, although it probably only rarely graces the royal table of the present period, or even that of the lord mayor of London, who can claim all sturgeons caught in the Thames above London Bridge. Where sturgeons are caught in large quantities, as on the rivers of southern Russia and on the great lakes of North America, their flesh is dried, smoked or salted. The ovaries, which are of large size, are prepared for caviar, for this purpose they are beaten with switches, and then pressed through sieves, leaving the membranous and fibrous tissues in the sieve, whilst the eggs are collected in a tub. The quantity of salt added to them before they are finally packed varies with the season, scarcely any being used at the beginning of winter. Finally, one of the best sorts of isinglass is manufactured from the airbladder. After it has been carefully removed from the body, it is washed in hot water, and cut open in its whole length, to separate the inner membrane, which has a soft consistency, and contains 70% of glutin.
Sturgeon (and, therefore also the caviar trade) are under severe threat from overfishing, poaching and water pollution.