Sturgeon

Evolution

Acipenseriform fishes appeared in the fossil record some 245 to 208 million years ago presumably near the end of the Triassic, making them among the most ancient of 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. Although their evolution has been remarkably slow, they are a highly evolved living fossil, and 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.

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 have a long evolutionary history culminating in our most familiar fishes, past adaptive radiations have left only a few survivors, like sturgeons and garfish.

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 scientists. 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.

Physical characteristics

Sturgeons retain several primitive characters among the bony fishes. Along with other members of the subclass Chondrostei, they are unique among bony fishes because the skeleton is almost entirely cartilaginous. Notably, however, the cartilagineous skeleton is not a primitive character, but a derived one: sturgeon ancestors had bony skeletons. They also lack vertebral centra, and are partially covered with 5 lateral rows of bony plates called scutes rather than scales. They also have four barbels—sensory organs that precede their wide, toothless mouths. They navigate their riverine habitats traveling just off the bottom with their barbels dragging along gravel, or murky substrate. Sturgeon are recognizable for their elongated bodies, flattened rostra, distinctive scutes and barbels, and elongated upper tail lobes. The skeletal support for the paired fins of ray-finned fish is inside the body wall, although the ray-like structures in the webbing of the fins can be seen externally.

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 (4400 lb) while for kaluga (H. dauricus) in the Amur River, similar lengths and over 1,000 kg (2,200 lb) weights have been reported. They are also among 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 four, eight, or 16 sets of chromosomes.

Life cycle

Sturgeons are long-lived, late maturing fishes. Their average lifespan is 50 to 60 years, and their first spawn does not occur until they are around 15 to 20 years old. Sturgeons are broadcast spawners, and do not spawn every year because they require specific conditions. Those requirements may or may not be met every year due to varying environmental conditions, such as the proper photoperiod in Spring, clear water with shallow rock or gravel substrate where the eggs can adhere, and proper water temperature and flow for oxygenation of the eggs. A single female may release 100,000 to 3 million eggs but not all will be fertilized. The fertilized eggs become sticky and will adhere to the bottom substrate upon contact. It takes 8–15 days for the embryos to mature into larval fish. During that time, they are dependent on their yolk saks for nourishment. River currents carry the larvae downstream into backwater areas, such as oxbows and sloughs where the free-swimming fry will spend their first year feeding on insect larvae and crustacea. During their first year of growth, they will reach 18 to 20 cm (7.1 to 7.9 in) in length and migrate back into the swift-flowing currents in the main stem river.

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 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.

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 are 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 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.

Behavior

Sturgeon are primarily benthic feeders, with a diet of shells, crustaceans and small fish. They feed by extending their syphon-like mouths to suck food from the benthos. Having no teeth, they are unable to seize prey, though larger individuals can swallow very large prey items, including whole salmon. Sturgeons feed non-visually. They are believed to use a combination of sensors, including olfactory, tactile and chemosensory cues detected by the four barbels, and electroreception using their ampullae of Lorenzini.

The sturgeons' electroreceptors are located on the head and are sensitive to weak electric fields generated by other animals or geoelectric sources. The electroreceptors are thought to be used in various behaviors such as feeding, mating and migration.

Many sturgeon leap completely out the water, usually making a loud splash which can be heard half a mile away on the surface and probably further under water. It is not known why they do this, but suggested functions include group communication to maintain group cohesion, catching airborne prey, courtship display, or to help shed eggs during spawning. Other plausible explanations include escape from predators, shedding parasites, or to gulp or expel air. Another explanation is that it "simply feels good". Leaping sturgeon are known to occasionally cause injuries to humans in boats; in 2015, a 5-year-old girl died after a sturgeon leapt from the Suwannee River and struck her.

Conservation status

Because of their long reproductive cycles, long migrations, and sensitivity to environmental conditions, many species are under severe threat from overfishing, poaching, water pollution, and damming of rivers. There is also a noticeable decline in sturgeon populations as the demand for caviar increases. 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.

A 2013 study on the critically endangered sturgeon populations in the Danube River Basin determined that ongoing illegal fishing activity and caviar trade is threatening the future of Danube sturgeons. Jutta Jahrl, a conservation expert with the World Wildlife Federation stated that "Romania and Bulgaria are home to the only viable wild sturgeon populations left in the European Union, but unless this sophisticated illegal fishing is stopped, these fish are doomed”.

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 wine and beer, as a predecessor for gelatin, and to preserve parchments.

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. 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.

Classification

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.

Species