Sturgeons are an ancient group of bony fish characterized by their elongated, scaleless bodies, partially covered by rows of bony plates called scutes. They belong to the family Acipenseridae, within the order Acipenseriformes and the class Actinopterygii. While members of Acipenseriformes first appeared during the Early Jurassic Period (~201 to 174 million years ago), the first true sturgeons are believed to have emerged in the Upper Cretaceous (~100 to 94 million years ago).
These bottom-dwelling fish rely on specialized sensory organs called barbels, located near their mouths, to detect food. Their diet primarily consists of shellfish, other small aquatic organisms, and fish eggs.
Today, 25 extant species of sturgeons inhabit subtropical to subarctic waters across North America and Eurasia. However, their populations have been severely impacted by slow growth rates, late maturity, and overexploitation, primarily due to the high demand for their caviar. As a result, most species are now classified as Critically Endangered on the IUCN Red List.
These fish are known for their large size. The largest recorded specimen was a female beluga sturgeon, caught in the Volga Delta in 1827, measuring 23 ft 7 in (7.2 m) and weighing 3,463 lb (1,571 kg). Another beluga from the Caspian Sea reportedly reached 18 ft (5.5 m) and weighed around 4,400 lb (2,000 kg).
Among other large sturgeons, the kaluga sturgeon of the Amur River weighs over 2,200 lb (1,000 kg). In contrast, the shovelnose sturgeon, North America’s smallest freshwater sturgeon, typically reaches 20 to 33 in (50 to 85 cm) and weighs about 5.5 lb (2.5 kg).
Sturgeons have a long, spindle-shaped, smooth-skinned body with five lateral rows of bony plates or scutes instead of scales. Although they possess many primitive traits of bony fishes, they stand out for having an almost entirely cartilaginous endoskeleton. In fact, they are also one of the few animals that retain a post-embryonic notochord.
Their wide, toothless mouth is surrounded by slender, whisker-like barbels, which are specialized sensory organs that help them sense their surroundings.
Initially, all sturgeons were divided into 2 subfamilies: Acipenserinae (which included the genera Acipenser and Huso) and Scaphirhynchinae (which included the genera Scaphirhynchus and Pseudoscaphirhynchus). However, recent studies suggest that this classification is paraphyletic and hence may be unreliable. Instead, these studies have revealed that the two species, Acipenser oxyrhinchus and Acipenser sturio, form the most basal sturgeon clade, while all other species (belonging to Acipenser, Scaphirhynchus, Pseudoscaphirhynchus, and Huso) form a separate group. Thus, currently, the 25 extant species of sturgeons are directly grouped under 4 genera. Among these, the Acipenser genus also includes eight extinct species.
Members of the order Acipenseriformes first appeared in the Early Jurassic Period (around 174 to 201 million years ago) and are thus one of the earliest extant ray-finned fishes. The first true sturgeons are believed to have appeared in the Upper Cretaceous Period, with the oldest remains being an incomplete skull that dates back to around 100 to 94 million years ago.
Since then, sturgeons have undergone almost negligible morphological changes and are thus referred to as living fossils. They retain a spiral valve intestine and a heterocercal tail, in which the vertebra extends into the upper lobe.
A fossil discovered in Morocco, dating back to the Maastrichtian Age, provides evidence that sturgeons once inhabited Africa.
Sturgeons are widely distributed across subtropical to subarctic waters in North America and Eurasia. However, they are not native to the Southern Hemisphere. In recent years, efforts have been made to introduce sturgeon populations in South Africa, Uruguay, and other southern countries.
Most sturgeons spawn in freshwater habitats and feed in nutrient-rich, brackish waters of estuaries and deltas. Some species, like the lake sturgeon, live exclusively in freshwater.
They are primarily benthic feeders that consume shellfish, including shrimp, prawns, crabs, lobsters, clams, mussels, and oysters. Moreover, they also feed on fish eggs.
These fish are typically long-lived, with an average lifespan of 50 to 60 years. However, some species, like lake sturgeons, may survive for as many as 100 years.
Sturgeons reproduce through broadcast spawning, where males and females release sperm and eggs into the water simultaneously. Spawning typically occurs in spring in clear water with a rock or gravel substrate. Female sturgeons can release 100,000 to 3 million eggs, which are fertilized externally. This migration from saltwater to freshwater for spawning is a typical feature of this fish, known as anadromous behavior. However, spawning does not occur annually, as it depends on optimal environmental conditions, including suitable water temperature, adequate daylight (photoperiod), and clear water with a gravel substrate (for adhering to the substrate).
Within 8 to 15 days, the embryo develops into a free-swimming larva, sustained by nutrients from the yolk sac. These larvae are carried downstream into backwaters, where they feed on insect larvae and crustaceans for about a year. Once they reach 7 to 8 in (18 to 20 cm) in length, they migrate back to swift-flowing currents in the downstream section of the river.
They are typically preyed upon by larger fish, including lampreys, walleyes, catfish, and even sharks.
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