| Sturgeon Temporal range: Upper Cretaceous–Recent | |
|---|---|
 | |
| Atlantic
sturgeon (Acipenser oxyrinchus oxyrinchus) | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Acipenseriformes |
| Family: | Acipenseridae Bonaparte, 1831 |
| Subfamilies | |
| Acipenserinae Scaphirhynchinae See text for genera and species. | |
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
food 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 to be at risk of
extinction, making them more critically endangered than any other group of
species.[2]
Evolution
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 their evolution has been exceptionally slow and earning them
informal status as living fossils.[3][4] This is explained in part by the
long generation interval, 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, full
classification and phylogeny of the sturgeon
species has been difficult to determine, 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.[5] It is still unclear whether the
species in the Acipenser 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.[4][6]
Physical characteristics
Along with other members of the subclass Chondrostei, sturgeon are
primarily cartilaginous, lack vertebral centra, and
are partially 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, wedge-shaped snouts, 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.[7]
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 (18 ft) and 2000 kg[8] (4400 lbs.) while for kaluga (H.
dauricus) in the Amur River, similar lengths
and over 1000 kg (2200 lb) weights have been reported.[9] 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.[10] 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 four, eight, or 16 sets of chromosomes.[11]
Range and habitat
Sturgeon range from subtropical to subarctic
waters in North America and Eurasia. In North
America, they range along the Atlantic coast from the Gulf
of Mexico to Newfoundland,
including the Great Lakes and the St. Lawrence, Missouri and Mississippi Rivers, as well
as along the West Coast in major rivers from California to British Columbia. They
occur along the European Atlantic coast, including the
Mediterranean basin, in
the rivers that flow into the Black, Azov and Caspian
Seas (Danube, Dnepr, Volga and Don), the
north-flowing rivers of Russia that feed the Arctic Ocean (Ob, Yenisei, Lena, Kolyma), in the rivers of
Central Asia (Amu Darya
and Syr
Darya) and Lake Baikal. In the Pacific
Ocean, they are found in the Amur River along the
Russian-Chinese
border, on Sakhalin island, and in the Yangtze and other rivers in
northeast China.[10][12]
Throughout this extensive range,
almost all species are highly threatened or vulnerable to extinction due to a
combination of habitat destruction, overfishing and pollution.[12]
No species are known to naturally
occur south of the equator, though attempts at sturgeon aquaculture are being made
in Uruguay, South
Africa and other places.[13]
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[14] and Siberian sturgeon
(A. baerii) in the Ob basin.[15]
Conservation status
Because of their long reproductive
cycles, long migrations, and sensitivity to environmental conditions, many
species are under severe threat from overfishing,[16] poaching, water pollution and
damming of rivers.[17] According to the IUCN,
over 85% of sturgeon species are classified as at risk of extinction, making
them more critically endangered than any other group of species.[2][18]
Uses
Globally, sturgeon fisheries are of
great value, primarily as a source for caviar, but also for
flesh.
Before 1800, swim
bladders of sturgeon (primarily Beluga sturgeon from Russia) were used as a
source of isinglass, a form of collagen
used historically for the clarification of
beer, as a
predecessor for gelatin, and to preserve parchments.[19]
The Jewish law of kashrut, which only permits the
consumption of fish with scales, forbids sturgeon, as they have ganoid scales
instead of the permitted ctenoid
and cycloid scales. While all Orthodox groups forbid
the consumption of sturgeon, some conservative
groups do allow it.[20][21] The theological debate over its
kosher status can be traced back to such 19th-century reformers as Aron Chorin, though its
consumption was already common in European Jewish communities.[22] It remains a high-end staple of
many Jewish delis and specialty
shops.
In England, the sturgeon, along with
whales and porpoises, is a
royal
fish, and every sturgeon caught in England is the property of the
Crown.
Speci
A short-nosed sturgeon (Acipenser brevirostrum)
es
In currently accepted taxonomy, the
family Acipenseridae is subdivided into two subfamilies, Acipenserinae,
including the genera Acipenser and Huso, and
Scaphirhynchinae, including the genera Scaphirhynchus and
Pseudosaphirhynchus.[12]
- Article Source: http://en.wikipedia.org/
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