Chordate Zoology - textbook - Y. V. Tsaryk - 2013

Chapter 4. DIVISION GNATHOSTOMATA, or ECTOBRANCHIATA. SUPERCLASS PISCES

4.2. CLASS BONY FISHES OSTEICHTHYES

4.2.3. Taxonomy of the Class Bony Fishes and Characteristics of Representatives of Individual Orders

4.2.3.2. Subclass Ray-finned Fishes Actinopterygii

4.2.3.2.2. Superorder Ganoidomorpha

Direct descendants of palaeoniscids. Fossil remains are known from the Permian; during the Mesozoic era, The Diversity of groups and the number of species increased sharply, but as early as the Cretaceous, the extinction and decline of many groups began. Simultaneously, the flourishing and Structure/89.html">Rapid Evolution of other ray-finned fish groups began, collectively known as teleosts (Teleostei). In addition to the four orders of ganoids, a small number of species of which still survive today, up to 5–7 orders represented by extinct forms are distinguished.

Scales are ganoid (rarely) or bony cycloid; sometimes fused scales form bony plates. The caudal fin is heterocercal. The Heart of most species has a conus arteriosus, and the intestine contains a spiral valve. The swim bladder is reduced in some forms.

Order Acipenseriformes. Representatives of this order were previously grouped into the cartilaginous ganoids (Chondrostei). Modern species exhibit many archaic structural features. The upper lobe of the heterocercal tail bears ganoid scales; highly characteristic bony plates, or scutes (formed from fused scales), are arranged on the body in five regular longitudinal rows. A well-developed notochord in a dense sheath is retained; vertebral centra are absent, but cartilaginous dorsal and ventral (hemal) arches are present. A shield of numerous dermal bones has formed over the cartilaginous neurocranium. Radials are well developed in the Skeleton of the paired fins. In the upper part of the gill

cover, a vestigial spiracle is sometimes retained. A conus arteriosus is developed in the heart, and a spiral valve is present in the intestine. They are known since the Triassic, although they have never been numerous. Currently, 25 species belonging to two families exist; they are found only in the Northern Hemisphere.

The sturgeon family, Acipenseridae, comprises three genera with 23 species, six of which are found in our waters. The largest is the beluga (Huso huso), which inhabits the Black, Azov, and Caspian seas, as well as the eastern Mediterranean; it spawns in the rivers flowing into these seas at the age of 9–23 years, when it reaches a length of 1.5–2 m. Some individuals live up to 100 years, reaching 5.5 m in length and a mass of over 1.5 t.

The sturgeon genus, Acipenser, includes 16 species (5 species live in our waters).

The most famous are the Atlantic, Siberian, Amur, North American, and other sturgeons, starry sturgeon, sterlet (Fig. 4.41), and ship sturgeon. Of the mentioned species, only the sterlet is a permanent river resident. The others are anadromous Fishes that spawn in rivers on gravelly-stony substrates hundreds or thousands of kilometers from the river mouths. After spawning (which occurs annually or biennially), the adult fish return to the seas. Some sturgeon individuals live up to 30–60 years, reaching a length of 2.5 m and a mass of 100–120 kg; the maximum length of the sterlet is 1–1.2 m, and its mass is 16 kg.

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Fig. 4.41. Sterlet Acipenser ruthenus

In many species of anadromous sturgeons, two races are distinguished: winter-run fish enter rivers in autumn, overwinter there, and migrate back to the sea after spawning in spring; spring-run fish enter rivers in spring, spawn in late spring or early summer, and immediately return to the sea. The existence of two races allows for the alternating use of spawning grounds and food resources; furthermore, winter-run populations can more easily reach distant spawning grounds in the upper reaches of rivers.

Three species of shovelnose sturgeons (genus Pseudoscaphirhynchus) inhabit the rivers draining into the Aral Sea (Amu Darya, Syr Darya).

The paddlefish family, Polyodontidae, is currently represented by two species: the American paddlefish (Polyodon spathula) reaches a length of 2 m and a mass of 75 kg, inhabiting lakes and rivers of the Mississippi River basin; the Chinese paddlefish (Psephurus gladius) inhabits the Yangtze River basin (China) and sometimes reaches up to 7 m in length.

Adult acipenseriform fishes feed on benthic invertebrates, while large individuals prey on fish. The paddlefish is a typical planktivore; it swims with an open Mouth, filtering Water through its gill slits and retaining planktonic organisms with a dense mesh of gill rakers.

Acipenseriforms are highly valued for their delicious meat. 'Black' caviar (including pressed and granular varieties) is also a well-known delicacy. Overfishing and river pollution have led to a sharp decline in acipenseriform populations worldwide; in some areas, sturgeons have disappeared entirely.

Order Polypteriformes. The body of these fishes is encased, like armor, in rhombic three-layered cosmoid-ganoid scales (Fig. 4.42).

Fig. 4.42. Nile bichir Polypterus bichir

Amphicoelous vertebrae ossify, and bony Ribs are well developed. The mouth is terminal; a vestigial

spiracle is retained. The dorsal fin is divided into a series of small finlets. Externally, the caudal fin is almost symmetrical. The 'lung'—a cellular bladder, as in lungfishes—buds off from the ventral side of the Esophagus. A spiral valve is well developed in the intestine, and a conus arteriosus is present in the heart.

About 10 species belonging to 2 genera of bichirs inhabit the rivers and lakes of tropical Africa. They reach up to 1.2 m in length. Spawning occurs during the rainy season. The eggs are small, up to 1.3 mm in diameter, and adhere to underwater vegetation. The larvae have well-developed pectoral fins and large, feathery external gills.

Order Amiiformes. This order, along with Lepisosteiformes (see below), is often grouped into the superorder Holostei (the holosteans). Only two genera have survived to the present day. Holosteans are characterized by the presence of ganoid scales (which are reduced in amiiforms) and a lesser degree of Ossification of the neurocranium. The caudal fin is not distinctly heterocercal. The swim bladder, with its cellular walls, serves as an accessory respiratory organ and opens on the DORSAL SIDE OF the anterior esophagus.

The only extant species, the bowfin Amia calva, inhabits shallow, swampy waters of North America. The body is covered with cycloid scales. The vertebrae are amphicoelous. The heart retains a shortened conus arteriosus, and the intestine has a somewhat reduced spiral valve. It is an active predator. Before spawning, the male bowfin builds a nest from plant debris. The female deposits 20,000 to 70,000 eggs there. The fry attach themselves to the walls of the nest using an adhesive secretion from a gland located on the snout. The male guards the nest for several months. It has little commercial value.

Order Lepisosteiformes. Known from the Late Cretaceous. Currently, 6–7 species of a single genus of gars, or garpikes, Lepisosteus, inhabit lakes and rivers in North and Central America and Cuba. The elongated body is covered with a dense armor of articulately joined ganoid scales (Fig. 4.43). A layer of ganoin also covers the dermal BONES OF THE HEAD. In the heart

Fig. 4.43. Spotted gar Lepisosteus oculatus

There is a conus arteriosus, and a spiral valve in the intestine. Some species reach 3–4 m in length and over 150 kg in weight. They are predators that feed mainly on large fish, typically lying in wait for their prey. They compensate for oxygen deficiency in the water by gulping air into the swim bladder. In some areas, they are of commercial importance.