Sturgeon

Sturgeon is the common name for the 27 species of fish belonging to the family Acipenseridae. The earliest sturgeon fossils date to the Late Cretaceous, and are descended from other, earlier acipenseriform fish, which date back to the Early Jurassic epoch, some 174 to 201 million years ago. The family is grouped into four genera: Acipenser, Huso, Scaphirhynchus, and Pseudoscaphirhynchus. Two species (A. naccarii and A. dabryanus) may be extinct in the wild, and one (P. fedtschenkoi) may be entirely extinct. Two closely related species, Polyodon spathula (American paddlefish) and Psephurus gladius (Chinese paddlefish, extinct) are of the same order, Acipenseriformes, but are in the family Polyodontidae and are not considered to be “true” sturgeons. Both sturgeons and paddlefish have been referred to as “primitive fishes” because their morphological characteristics have remained relatively unchanged since the earliest fossil record. Sturgeons are native to subtropical, temperate and sub-Arctic rivers, lakes and coastlines of Eurasia and North America.

Sturgeons are long-lived, late-maturing fishes with distinctive characteristics, such as a heterocercal caudal fin similar to those of sharks, and an elongated, spindle-like body that is smooth-skinned, scaleless, and armored with five lateral rows of bony plates called scutes. Several species can grow quite large, typically ranging 2–3.5 m (7–12 ft) in length. The largest sturgeon on record was a beluga female captured in the Volga Delta in 1827, measuring 7.2 m (23 ft 7 in) long and weighing 1,571 kg (3,463 lb). Most sturgeons are anadromous bottom-feeders, which migrate upstream to spawn, but spend most of their lives feeding in river deltas and estuaries. Some species inhabit freshwater environments exclusively, while others primarily inhabit marine environments near coastal areas, and are known to venture into open ocean.

Several species of sturgeon are harvested for their roe, which is processed into the luxury food caviar. This has led to serious overexploitation, which combined with other conservation threats, has brought most of the species to critically endangered status, at the edge of extinction.

Evolution

Fossil history

Acipenseriform fishes appeared in the fossil record some 174 to 201 million years ago, during the Early Jurassic, making them some of the earliest extant actinopterygian fishes. True sturgeons appear in the fossil record during the Upper Cretaceous. In that time, sturgeons have undergone remarkably little morphological change, indicating their evolution has been exceptionally slow and earning them informal status as living fossils. 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 bony plated armor, or scutes, and the abundance of prey items in the benthic environment. Their evolution seems to have been remarkably slow, leading to them being called living fossils, although they do not closely resemble their ancestral chondrosteans. They do, however, still share several primitive characteristics, such as heterocercal tail, reduced squamation, more fin rays than supporting bony elements, and unique jaw suspension.

Phylogeny and taxonomy

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. While ray-finned fishes (Actinopterygii) have a long evolutionary history culminating in the most familiar fishes, past adaptive evolutionary radiations have left only a few survivors, such as sturgeons and gars.

The wide range of the acipenserids and their endangered status have made collection of systematic materials difficult. The factors have led researchers in the past to identify over 40 additional species that were rejected by later scientists. Whether the species in the Acipenser and Huso genera are monophyletic (descended from one ancestor) or paraphyletic (descended from many ancestors) is still unclear, though the morphologically motivated division between these two genera clearly is not supported by the genetic evidence. An effort is ongoing to resolve the taxonomic confusion using a continuing synthesis of systematic data and molecular techniques.

The phylogeny of Acipenseridae, as in the cladogram, shows that they evolved from the bony fishes. Approximate dates are from Near et al., 2012.

• A young lake sturgeon (Acipenser fulvescens)
• Great sturgeon or beluga (Huso huso) feeding on another fish
• Pallid sturgeon (Scaphirhynchus albus) showing siphoning feeding behaviour
• Beluga sturgeon in an aquarium.

In currently accepted taxonomy, the class Actinopterygii and the order Acipenseriformes are both clades. The family Acipenseridae is subdivided into 2 subfamilies; Acipenserinae, including the genera Acipenser and Huso, and Scaphirhynchinae, including the genera Scaphirhynchus and Pseudoscaphirhynchus. However, multiple recent studies have recovered this arrangement as paraphyletic, instead finding A. oxyrhinchus and A. sturio to form the most basal clade among sturgeons, and all other species being in a separate clade, with the various other species of Acipenser, Scaphirhynchus, Pseudoscaphirhynchus, and Huso to have varying levels of relationship with one another.

The exact placement of Scaphirhynchus varies depending on the study and the methods used, with some placing it within the second-most basal clade comprising primarily Pacific species (shown above), whereas others place it in its own clade that is more derived than the secondmost basal clade but less derived than the most derived Atlantic and Central Asian clade. No studies have yet delineated a relationship between it and Pseudoscaphirhynchus. In addition, the exact relationships of the members of the most derived, primarily Atlantic clade vary, although most analyses at least find all the species in it to form a monophyletic clade. The placement of A. sinensis also varies by the study, with some placing it as the only Pacific member of the otherwise Atlantic-based most-derived clade, whereas others place it with the rest of the Pacific sturgeons as a sister to A. dabryanus.

Species

The family contains 8 extinct fossil species and 28 extant species/subspecies (include 1 species of Sterlet and 2 species of living fossils), in 4 genera. This list uses the original classification scheme:

Family Acipenseridae

• Subfamily Acipenserinae
  • Genus Acipenser Linnaeus, 1758
    • †Acipenser albertensis Lambe 1902 >
    • Acipenser baerii J. F. Brandt, 1869 (Siberian sturgeon)
      • Acipenser baerii baicalensis A. M. Nikolskii, 1896 (Baikal sturgeon)
      • Acipenser baerii stenorrhynchus A. M. Nikolskii, 1896
    • Acipenser brevirostrum Lesueur, 1818 (Shortnose sturgeon)
    • Acipenser dabryanus A. H. A. Duméril, 1869 (Yangtze sturgeon)
    • †Acipenser cruciferus (Cope 1876)
    • †Acipenser eruciferus Cope 1876
    • Acipenser fulvescens Rafinesque, 1817 (Lake sturgeon)
    • †Acipenser gigantissimus Nessov 1997
    • Acipenser gueldenstaedtii J. F. Brandt & Ratzeburg, 1833 (Russian sturgeon)
    • Acipenser medirostris Ayres, 1854 (Green sturgeon)
    • Acipenser mikadoi Hilgendorf, 1892 (Sakhalin sturgeon)
    • †Acipenser molassicus Probst 1882
    • Acipenser naccarii Bonaparte, 1836 (Adriatic sturgeon)
    • Acipenser nudiventris Lovetsky, 1828 (Fringebarbel sturgeon)
    • †Acipenser ornatus Leidy 1873
    • Acipenser oxyrinchus Mitchill, 1815
      • Acipenser oxyrinchus desotoi Vladykov, 1955 (Gulf sturgeon)
      • Acipenser oxyrinchus oxyrinchus Mitchill, 1815 (Atlantic sturgeon)
    • Acipenser persicus Borodin, 1897 (Persian sturgeon)
    • Acipenser ruthenus Linnaeus, 1758 (Sterlet)
    • Acipenser schrenckii J. F. Brandt, 1869 (Japanese sturgeon)
    • Acipenser sinensis J. E. Gray, 1835 (Chinese sturgeon)
    • Acipenser stellatus Pallas, 1771 (Starry sturgeon)
    • Acipenser sturio Linnaeus, 1758 (European sea sturgeon)
    • †Acipenser toliapicus Agassiz 1844 ex Woodward 1889
    • Acipenser transmontanus J. Richardson, 1836 (White sturgeon)
    • †Acipenser tuberculosus Probst 1882
  • Genus Huso J. F. Brandt & Ratzeburg, 1833
    • Huso dauricus (Georgi, 1775) (kaluga)
    • Huso huso (Linnaeus, 1758) (beluga)
• Subfamily Scaphirhynchinae
  • Genus Scaphirhynchus Heckel, 1835 (native to North America)
    • Scaphirhynchus albus (Forbes & R. E. Richardson, 1905) (Pallid sturgeon)
    • Scaphirhynchus platorynchus (Rafinesque, 1820) (Shovelnose sturgeon)
    • Scaphirhynchus suttkusi J. D. Williams & Clemmer, 1991 (Alabama sturgeon)
  • Genus Pseudoscaphirhynchus Nikolskii, 1900 (native to Central Asia)
    • Pseudoscaphirhynchus fedtschenkoi (Kessler, 1872) (Syr Darya sturgeon)
    • Pseudoscaphirhynchus hermanni (Kessler, 1877) (Dwarf sturgeon)
    • Pseudoscaphirhynchus kaufmanni (Kessler, 1877) (Amu Darya sturgeon)

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 and Idaho to British Columbia. They occur along the European Atlantic coast, including the Mediterranean basin, especially in the Adriatic Sea and the rivers of North Italy; in the rivers that flow into the Black, Azov, and Caspian Seas (Danube, Dnepr, Volga, Ural 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 some rivers in northeast China.

Throughout this extensive range, almost all species are highly threatened or vulnerable to extinction due to a combination of habitat destruction, overfishing, and pollution.

No species is known to naturally occur south of the equator, though attempts at sturgeon aquaculture are being made in Uruguay, South Africa, and other places.

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 human 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.