Zoology of Chordates - textbook - Y. V. Tsaryk - 2013

Section 4. DIVISION GNATHOSTOMATA, or ECTOBRANCHIATA. SUPERCLASS PISCES

4.2. CLASS OSTEICHTHYES

4.2.3. Systematics of the Class Osteichthyes and Characteristics of Representatives of Individual Orders

4.2.3.1. Subclass Sarcopterygii

4.2.3.1.2. Superorder Dipneustomorpha

Paleozoic lungfishes had a solid bony Structure or several separate bones in the neurocranium. In modern species, cartilaginous ossifications are underdeveloped. Internal nares (choanae) are present. The Skull is autostylic. The Teeth form powerful tooth plates. The notochord persists throughout life. There are dorsal and ventral (hemal) arches, and in some fossil forms, vertebral centra. The Skeleton of paired fins is of the biserial type. Evolutionarily, There is a backward shift of the unpaired fins and their fusion with the caudal fin. Scales are cosmoid or bony. Modern species have one or two Lungs, which are outgrowths of the ventral side of the Esophagus. A division of the atrium and The formation of a Pulmonary Circulation loop are beginning to emerge.

Lungfishes are a lateral, specialized branch that diverged from primitive lobe-finned Fishes as early as the Early Devonian. They are divided into three orders; the six modern species have a relict distribution.

Order Dipteridiformes. This group includes extinct lungfishes from the Middle and Late Devonian, distributed in freshwater bodies worldwide. They are characterized by cosmoid scales, a great diversity of dermal bones, reduction of secondary jaws, the presence of conical teeth (not fused into tooth plates), and the presence of rudiments of vertebral centra. Paleozoic forms likely already possessed pulmonary Respiration and The ability to enter a state of aestivation when Water bodies dried up (fossilized "cocoons" have been found in Permian deposits).

Order Ceratodiformes. The neurocranium is almost entirely cartilaginous. Dermal bones are few. Secondary jaws are absent. Paired fins are well developed. They possessed only one lung. Scales are bony and large.

Currently, only one species exists - the Queensland lungfish - Neoceratodus forsteri (a few water bodies in Western Australia). It reaches a length of up to 1.5 m and a mass of over 10 kg. It inhabits overgrown, slow-flowing rivers. During dry periods, it survives in pools left in the riverbed. Every 40-50 minutes, it surfaces to loudly ventilate its lung. If the pool dries up completely, it dies. It feeds on invertebrates, although its gut usually contains many finely ground plant remains. It deposits eggs (up to 6-7 mm in diameter) on aquatic plants. The fry breathe mainly through gills. After the yolk sac is absorbed, they become more active, staying in open water and feeding on filamentous Algae. Pectoral fins appear on the 14th day after hatching. From this time, the lung begins to function. Pelvic fins form after 2.5 months. Queensland lungfish were intensively hunted for their tasty meat. They are now protected.

Order Lepidosireniformes. The neurocranium is almost entirely cartilaginous. Dermal bones are few. Secondary jaws are absent. Tooth plates have sharp ridges. Paired fins are tentacle-like; their skeleton is formed only by a segmented central axis. Small cycloid scales are deeply embedded in the Skin. Lungs are paired. Development occurs with metamorphosis: larvae develop cutaneous external gills that disappear once the lungs begin to function. Few fossil remains have been found in the Permian horizons of the USA and on the Russian Platform. Currently, five species from two genera are known.

Four species of the genus Protopterus live in Central Africa (Fig. 4.40). The largest is 2 m long, while the smallest is only 30 cm. They inhabit river pools and temporary lakes. They are crepuscular and nocturnal. They feed on a variety of animal food: Annelids, Mollusks, amphibians, and fish. They swim by bending their bodies in a snake-like fashion or crawl among aquatic vegetation. Numerous taste buds are located in the skin of the fins, especially the pectoral ones: as soon as a fin touches an edible object, the fish turns and grabs the prey. Occasionally, they rise to the surface and take atmospheric air into their lungs through their nostrils.

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Fig. 4.40. Marbled lungfish Protopterus aethiopicus

During a drought, when the water level drops to 5-10 cm, the lungfish digs a burrow. It scoops up the soil with its Mouth, grinds it, and expels it through the gill slits. Having dug a vertical shaft, the fish widens its end and freezes in a folded-in-half position, HEAD up. After the water level recedes, the soil plugs the entrance to the burrow. The chamber of large fish is located at a depth of up to half a meter. Due to the hardening of skin mucus

a dense cocoon forms around the fish (its wall thickness is only 0.05-0.06 mm); a thin tube appears in the upper part of the cocoon, through which air reaches the head. In this state, the fish remains until the next rainy season, about 6-9 months (in experiments, over four years).

During aestivation, not only fat but also Muscle tissue serves as an energy reserve. Nitrogenous waste products are converted into urea, which is less toxic than ammonia (the end product of METABOLISM during active life). It is not excreted but accumulates, accounting for 1-2% of the fish's body mass by the end of aestivation. The mechanisms that ensure the Organism's tolerance to such high concentrations of urea are not yet fully understood.

During the rainy season, the soil gradually softens, water fills the air chamber, and the fish, breaking through the cocoon, pokes its head out to inhale air. After a few hours, it leaves the burrow. Over time, urea is excreted through the gills and Kidneys.

After aestivation, the breeding season begins. In a burrow dug by the male, the female deposits up to 5,000 eggs with a diameter of 3-4 mm. The larvae have a large yolk sac and 4 pairs of feathery external gills. Throughout the incubation period and During the first weeks of the larvae's lives, the male remains near the nest and actively defends it. The larval period lasts 3-4 weeks.

The South American lungfish, or lepidosiren, Lepidosiren paradoxa, lives in the Amazon basin. It mainly inhabits temporary water bodies and escapes drought by burrowing in the same way as Protopterus. However, a cocoon does not form in the lepidosiren—the body of the dormant fish is surrounded only by mucus.

The male also guards the eggs and young. During this time, highly vascularized, branched, fleshy filaments 5-8 cm long develop on his pelvic fins. It is not fully understood whether these filaments help oxygenate the water in the nesting chamber or compensate for the inability to use pulmonary respiration inside the burrow. After the breeding season, these filaments are reabsorbed.

The local population highly values the tasty meat of the lepidosiren and intensively hunts these fish. The meat of Protopterus is also delicious, so in some areas, they are of commercial importance.