Vertebrate Zoology - V. M. Konstantinov 2011

Chordates
Gnathostomes
Class Bony Fishes - Subclass Actinopterygii - Superorder Ganoidei - Order Acipenseriformes

Ray-finned Fishes are the most successful group of fish inhabiting Earth's waters today, comprising over 20,000 species, or 95% of all fish species.

Their diversity and major practical importance place them at the very forefront of The Study of bony fishes.

One of the key organizational features is the absence of a fleshy lobe at the Base of the paired fins. The Classification of this subclass is not yet fully finalized; the system adopted here divides the subclass into two superorders. These are the superorder Ganoidomorpha, with the main orders Acipenseriformes, Polypteriformes, Amiiformes, and Lepisosteiformes, and the superorder Teleostei, which includes about 38 orders. Individual orders are discussed below.

Acipenseriformes are a small, ancient group of primitive fishes that share A number of organizational traits with cartilaginous fishes. Externally, they somewhat resemble sharks. They possess a rostrum, which shifts the Mouth opening to the underside of the HEAD in the shape of a crescent-shaped transverse slit. Their caudal fin is asymmetrical and heterocercal, much like that of sharks. The paired fins are positioned horizontally. The chondrocranium retains a large amount of Cartilage, covered by dermal bones.

The Axial Skeleton is based on a lifelong notochord enclosed in a thick Connective Tissue sheath. Vertebral bodies do not develop, but cartilaginous upper and lower vertebral arches are present. The intestine features a spiral valve, and The Heart contains a conus arteriosus.

Features shared with teleost fishes include the presence of dermal bones in the neurocranium. These form the roof (nasal, frontal, parietal), sides, and floor of the Skull (parasphenoid, Vomer). The body is covered with five rows of large bony plates (scutes). They also have a bony operculum and a swim bladder connected to the gut. Fertilization is external, and the roe is small and numerous.

Only a few species of this order are distributed in the Northern Hemisphere, predominantly in its temperate latitudes (Fig. 36). The order Acipenseriformes comprises two families.

Representatives of the family Acipenseridae typically have a pointed rostrum, a small mouth, and lack Teeth in adults. This family includes the majority of modern species. Among them, we can mention several sturgeons: the Russian sturgeon (Acipenser güldenstädtii), distributed in the basins of the Black and Caspian Seas; the Siberian sturgeon (A. baeri), inhabiting our northern rivers from the Ob in the west to the Kolyma in the east, as well as the estuary areas of the Arctic Ocean seas; and the Amur sturgeon (A. schrenckii), which inhabits the Amur River. The stellate sturgeon, or sevruga (A. stellatus), is found in the Azov, Black, and Caspian Seas. Closely related to sturgeons is the sterlet (A. ruthenus), found in the river basins of the Caspian and Black Seas and the Arctic Ocean (extending east to the Yenisei inclusive).

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Fig. 36. Sturgeons:

1 — sturgeon; 2 — beluga; 3 — stellate sturgeon; 4 — sterlet; 5 — dwarf sturgeon

A distinct Lineage of sturgeons comprises two species of beluga: the European beluga (Huso huso), which inhabits the basins of the Caspian, Black, and Adriatic Seas, and the Far Eastern beluga (H. dauricus), commonly known as the kaluga, which lives in the Amur River basin. These are the largest freshwater fishes, reaching weights of over 1,000 kg and ages exceeding 100 years.

There are also some sturgeon species (though fewer in number) found in North America.

In fishes of the family Polyodontidae, the snout is elongated and expanded into a paddle-like blade, the mouth is large, the jaws bear small teeth, and the Skin is naked. Two species are distributed in North America and Southeast Asia, within the Mississippi, Yellow, and Yangtze River basins.

Sturgeons are partly freshwater and partly anadromous fishes. The sterlet and the American lake sturgeon spend their entire lives in fresh Water and do not undertake major Migrations. The beluga, Russian sturgeon, and sevruga feed in coastal marine waters and migrate into rivers to spawn, often traveling significant distances upstream.

The diet of sturgeons consists primarily of animal matter. The beluga is a predator that feeds on fish and occasionally preys on young seals. Sturgeons consume large quantities of Mollusks, while the sterlet feeds mainly on insect larvae.

Sturgeons are of great commercial importance, highly valued for their delicious meat and black caviar. The stocks of these fishes in Western Europe and North America, and now also in Russia, have been severely depleted. In Russia, sturgeon fishing is of greatest importance in the Caspian and Black Sea basins.