ZOOLOGY OF CHORDATES: STUDY GUIDE - Zakharenko M.O. - 2015

CHAPTER 3. INFRAPHYLUM GNATHOSTOMATA

3.1. Superclass Fishes (Pisces)

Fishes are the most ancient primary aquatic jawed vertebrates capable of living exclusively in Water. They breathe through gills; in some species inhabiting water bodies with low oxygen concentration, additional Organs for breathing atmospheric air are formed. The modern fauna includes about 20-22 thousand species of fish.

The General characteristics of fishes are:

1. Fishes are the most ancient primary aquatic jawed vertebrates capable of living exclusively in water.

2. Most fishes are agile, strong swimmers with a diverse body shape.

3. The primary type of forward locomotion is lateral undulatory Movements of the entire body or solely of the powerful tail.

4. Paired fins—pectoral and pelvic—function as stabilizers, lifting surfaces, rudders, and less frequently as organs of locomotion; unpaired fins ensure body stability.

5. The high activity and maneuverability of fishes are associated not only with the refinement of the Locomotor System but also with The Development of the Forebrain and Sense Organs.

6. The digestive tract is differentiated: in most species, The Stomach is distinct, and the intestine is divided into small and large sections.

7. They breathe through gills; in species inhabiting oxygen-deficient waters, additional respiratory organs capable of absorbing atmospheric oxygen are formed.

8. A single circulatory loop. In lungfishes, The formation of a second, pulmonary, circulatory loop begins.

9. Protective bony formations—scales—arise in the Skin, sometimes having a complex Structure; in some species, scales are reduced.

10. The skin is rich in mucous glands.

11. Well-developed lateral line organs.

Fishes exhibit A wide variety of body shapes (Fig. 28).

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Fig. 28. Types of fish body shapes

A – arrow-shaped (garfish - "sea pike"); B – torpedo-shaped (tuna); C – bream-shaped (common bream);

D – flounder type (river flounder); E – ocean sunfish type (mola mola); F – eel-shaped (eel);

G – ribbon-shaped (oarfish); H – globose (boxfish); I – flat

Fishes are characterized by a considerable diversity of modes of locomotion, which can be grouped into six main types (Fig. 29):

Undulatory (sinusoidal) locomotion is performed by means of lateral undulating bends of the entire body. Most fishes move by means of frequent lateral oscillatory movements of the posterior part of the body (sometimes just the caudal peduncle). Propulsion via wave-like movements of the fins alone is characteristic of sluggish pelagic and especially benthic fishes (rays, flounders). Flapping fin movements, particularly of the pectoral fins, serve as an auxiliary propulsion mechanism in many teleosts during slow swimming. Some fishes (gobies, scorpionfishes, etc.) use their pectoral fins for crawling along the substrate. Flying fishes of the order Beloniformes, which glide on motionless, widely spread

elongated paired fins acting as lifting surfaces, are capable of gliding for 200-400 m. A special category is represented by the so-called parasitic modes of locomotion. "Pilot-fish behavior" is quite widespread: the movement of small companion fishes in water layers alongside a large fish or other animals, a boat, or a ship.

Fig. 29. Modes of locomotion in fish

A - undulating (sinusoidal) (eel); B - locomotion via oscillatory movements of the posterior body (cod);

C - wave-like movements of the fins only (rays, flounders); D - parasitic forms of locomotion, "pilotage"

The body of fish consists of the HEAD, trunk, and tail (Fig. 30).

Fig. 30. External anatomy of fish

1 - snout (rostrum); 2 - head; 3 - gill slits; 4 - gill cover (operculum); 5 - dorsal fin; 6 - adipose fin; 7 - caudal fin; 8 - lateral line; 9 - anal fin; 10 - pelvic fin; 11 - caudal peduncle; 12 - pelvic fin; 13 - pectoral fin.

The head is immovably joined to the trunk and varies in shape (Fig. 31).

Fig. 31. Various head shapes of fish and fish-like vertebrates: 1 - Northern anchovy (Engraulis mordax); 2 - peacock flounder (Bothus lunatus); 3 - white sturgeon (Acipenser transmontanus); 4 - yellow seahorse (Hippocampus kuda); 5 - Chinese sucker (Myxocyprinus asiaticus);

6 - bobtail snipe eel (Cyema atrum); 7 - secretary blenny (Acanthemblemaria maria); 8 - tiger shark (Galeocerdo cuvier);

9 - pebble butterflyfish (Chaetodon multicinctus); 10 - wrasse (Macropharyngodon meleagris); 11 - clown triggerfish (Balistoides conspicillum); 12 - swordfish (Xiphias gladius); 13 - sockeye salmon (Oncorhynchus nerka); 14 - king mackerel (Scomberomorus cavalla); 15 - sea lamprey (Petromyzon marinus); 16 - American paddlefish (Polyodon spathula); 17 - red-bellied piranha (Pygocentrus nattereri); 18 - longnose gar (Lepisosteus osseus); 19 - sharpbelly (Culter alburnus); 20 - bristlenose pleco (Ancistrus triradiatus); 21 - pelican eel (Eurypharynx pelecanoides); 22 - deep-sea anglerfish (Ceratias holboelli); 23 - bicolour parrotfish (Cetoscarus bicolor); 24 - green moray eel (Gymnothorax funebris)

There are no clear boundaries between the body regions. They transition smoothly into one another, providing a streamlined body shape. The Mouth opening is surrounded by upper and lower jaws. The lower jaw is movable, enabling active prey capture (the Eurasian perch is a predatory fish). The head also bears eyes and organs of smell: paired openings known as nostrils. Gill covers are located on the sides of the head, protecting the respiratory organs—the gills. Fins provide propulsion or regulate body position. They are folds of skin that typically feature supportive outgrowths called rays. Fins are divided into paired and unpaired. Paired fins include the pectoral and pelvic fins, while unpaired fins include the caudal, anal, and dorsal fins (one or more). Fish exhibit several types of caudal fins (Figs. 32, 33).

Fig. 32. Fin Morphology based on lobe and vertebral Symmetry

A - protocercal; B - diphycercal; C - hypobatic heterocercal; D - epibatic heterocercal; E — homocercal

Fig. 33. Types of caudal fins classified by lobe configuration: 1 - lunate (tuna); 2 - fork-shaped (herring); 3 - truncate (cod); 4 - emarginate; 5 - rounded (burbot); 6 - double-emarginate (salmon); 7 - pointed (viviparous eelpout)

In most fish, scales appear as thin, translucent plates that overlap like roof tiles (Fig. 34).

Fig. 34. Types of scales and their modifications

1 - placoid, 2 - ganoid, 3 - cycloid, 4 - ctenoid, 5 - fulcra of sturgeons, 6 - enlarged caudal scales of sprats; 7 - modified scale of Schizothorax

Fish scales are dermal derivatives. The skin contains glands that secrete mucus, which protects against pathogen invasion and facilitates swimming by reducing water friction. In some species, this mucus is toxic.

In some fish, the Skeleton is composed entirely of Cartilage, whereas others possess both cartilage and Bone tissue. The first type of Skeletal structure is characteristic of cartilaginous fish (Chondrichthyes), while the second is found in bony fish (Osteichthyes). In most adult fish, the notochord is partially or completely replaced by vertebrae. The Skull may be cartilaginous or bony. Based on the method of upper jaw attachment to the skull, several types are distinguished (Fig. 35).

Fig. 35. Types of skulls based on the attachment of visceral arches to the braincase:

a - hyostylic skull; b - autostylic skull; c - amphistylic skull; 1 - hyomandibula, 2 - hyoid, 3 - Meckel's cartilage, 4 - lower jaw,

5 - posterior otic process, 6 - palatine process, 7 - palatoquadrate cartilage

The skull also incorporates gill arches, which bear the gills. Externally, the gills are covered by opercular bones. The trunk Muscles form broad longitudinal bands, alongside specialized muscles that move the jaws, opercula, and fins. Located within the Oral Cavity is a muscular outgrowth known as the Tongue. In most fish species, Teeth are present on the jaws and other cranial bones, aiding in capturing and holding prey. The posterior part of the pharyngeal cavity is pierced by gill slits. Food passes from the pharyngeal cavity into a long Esophagus, which in most species expands into a stomach. Posterior to the stomach lies the Small Intestine, into which open the ducts of specialized digestive glands—The Liver and the Pancreas. While Digestion of nutrients begins in the stomach, it is completed in the small intestine, where nutrients are absorbed into the bloodstream. Undigested food residues then pass into the Large Intestine and are subsequently expelled through the rectum (or cloaca). Most bony fish species possess a swim bladder. This thin-walled outgrowth of the esophagus is filled with gases and helps fish maintain their buoyancy in the water Column. The swim bladder is found exclusively in bony fish; cartilaginous fish lack it.

The excretory organs of fish are the trunk Kidneys, which filter out harmful Metabolic waste products that are then excreted via the Urinary Bladder. However, the Functions of the kidneys extend beyond excretion; they also regulate the salt balance within the fish's body. This capability enables certain fish species to inhabit both freshwater and saltwater environments. For instance, various species of sturgeons and salmon spend their lives in the seas but migrate to rivers to spawn. Some species of sharks may also venture into river mouths.

The respiratory organs of fish are the gills, which are situated on the gill arches. Two rows of red gill filaments line each outer edge of a gill arch. Blood Vessels branch within these filaments, facilitating gas exchange. Coordinated movements of the opercula draw water in through the mouth, Pharynx, and gill slits, washing over the gill filaments. When the opercula return to their resting position, water is forced out from beneath them. Cartilaginous fish lack opercula; their gill slits open directly to the exterior through independent apertures. In certain fish—specifically lungfish—one or two Lungs are present In addition to gills, enabling them to breathe atmospheric oxygen.

The Circulatory system OF fish consists of a two-chambered Heart and blood vessels. The structure of The Heart varies among cartilaginous, chondrostean, and teleost fish (Fig. 36).

The fish heart contains exclusively venous blood. Upon contraction of the atrium, blood flows into the ventricle, from which the contraction of the ventricle pumps it to the ventral aorta and the gills. There, the venous blood is oxygenated, transforming into arterial blood. From the gills, arterial blood travels through blood vessels to various organs, where it releases oxygen and absorbs carbon dioxide, thus reverting to venous blood. Venous blood then returns from various Tissues and organs to the heart via Veins. Consequently, fish exhibit a single circulatory loop. Fish blood is red due to the presence of Hemoglobin contained within Blood Cells known as erythrocytes. In cartilaginous fish, an conus arteriosus and a sinus venosus are adjacent to the heart (Fig. 36).

Fig. 36. A - cartilaginous fish; B - sturgeons; C - bony fish.

1 - ventral aorta; 2 - conus arteriosus; 3 - ventricle; 4 - atrium; 5 - sinus venosus

The Brain of fish, like that of all vertebrates, comprises 5 main regions: the Medulla Oblongata, Cerebellum, optic tectum (Midbrain), Diencephalon, and Telencephalon (forebrain). The forebrain governs complex behavioral patterns and houses the olfactory center. The cerebellum, part of the Hindbrain, is responsible for motor coordination. The eyes feature a spherical, transparent lens that is incapable of changing either its position or its shape. Consequently, fish perceive objects primarily at relatively short distances, though they are capable of distinguishing shapes and colors. The Inner ear, which functions as the Organ of Hearing, is housed within the cranial bones of fish. The Organ of Equilibrium is also associated with the inner ear. Additionally, fish possess olfactory organs consisting of a pair of capsules at the front of the head that open externally via nostrils. SENSE OF SMELL is exceptionally well developed in fish, particularly in predators. Taste buds are located primarily on the tongue. However, the most critical role in the life of fish is played by the lateral line system.

The modern fauna comprises 20,000–22,000 fish species inhabiting virtually all aquatic biotopes. Fish display a wide variety of body shapes, with lengths ranging from 1 cm to 20 m and masses from 0.7 g to 20 t. According to the Classification system of T. Rass and G. Lindberg (1971), the superclass Pisces is divided into the following taxonomic units:

FISH SYSTEMATICS

Superclass Pisces Class Chondrichthyes

Subclass Elasmobranchii

Subclass Holocephali

Class Osteichthyes

Subclass Actinopterygii

Subclass Sarcopterygii

ORIGIN AND EVOLUTION of Fish. Fossilized remains of ancient fish discovered in older sedimentary rock layers of the Earth's crust structurally resemble modern jawless fish. Until recently, the earliest known jawless, fish-like animals were believed to date from the Early Ordovician (approximately 450–470 million years ago). However, in 1999, fossils of the fish-like creature Haikouichthys, belonging to the jawless group and dating back approximately 530 million years (Early Cambrian), were discovered in Yunnan Province, China. Such forms may have served as the ancestors of all vertebrates. Fossil remains of the lobe-finned fish Osteolepis macrolepidotus are known from the Middle Devonian period (Great Britain).

With the evolution of jaws from one of the gill arches, the first true fish emerged. In addition to jaws, fish developed paired fins, an inner ear with three semicircular canals, and gill arches. Despite the appearance of the first jawed fish as early as the Ordovician, they remained subordinate until the Devonian period. Thus, fish and Jawless vertebrates coexisted for over 100 million years under conditions where jawless forms predominated, in sharp contrast to the present day.

Ancient fish possessed broad bodies covered in bony plates and likely lived near the sea floor. Fish with complete skeletons and jaws appeared much later. As early as 400 million years ago, they diverged into two main classes: cartilaginous fish (sharks, rays, and chimaeras) and bony fish.

The earliest fossil records of cartilaginous fish date back 220–265 million years. The ancestors of modern sharks already exhibited a relatively high level of Organization at that time. The oldest fossil sharks were discovered in the rich marine limestone deposits of Ohio (USA), where several teeth resembling those of modern sharks were found. Another discovery in Cleveland—comprising well-preserved body remains along with certain organs—allowed researchers to reconstruct a primitive shark. This specimen measured 45–120 cm in length. The ancestors of modern sharks died out during the Permian period, which saw the further evolution of hybodonts—shark-like animals equipped with sharp teeth in the front of their jaws and broad teeth in the back, adapted for crushing mollusk shells. Hybodonts lived in constant competition with the ancestors of teleost fish and carnivorous reptiles. Gradually, hybodonts began to give way to new shark species, one of which—the horn shark—still inhabits the waters near Australia. Throughout the Jurassic period, during the age of dinosaurs, sharks evolved successfully. Many families established themselves during this epoch, including the ancestors of modern rays. By the late Miocene, sharks were among the most widespread marine animals. Another Lineage of ancient cartilaginous fish gave rise to the classes of placoderms and bony fish. Modern cartilaginous fish are represented by two subclasses: Elasmobranchii and Holocephali. Modern elasmobranchs originated approximately 150 million years ago.

Cartilaginous fishes first appeared at the turn of the Silurian and Devonian periods, about 420 million years ago, and flourished during the Carboniferous. Bony fishes have inhabited the world's oceans since at least the Devonian; it is also possible they existed as early as the Silurian. Great white shark off the coast of Guadalupe Island (Mexico)

More than half of all currently living vertebrate species—specifically, around 31,000 species according to the FishBase database—belong to fishes. The number of recognized species continues to change due to the discovery of new species and taxonomic revisions of individual fish groups. Extant fishes are represented by two classes: cartilaginous fishes (Chondrichthyes) and bony fishes (Osteichthyes). Modern cartilaginous fishes are divided into two major groups: Holocephali (chimaeras) and Elasmobranchii (elasmobranchs, which include sharks and rays). Today, there are between 900 and 1,000 species of cartilaginous fishes. Bony fishes are divided into two subclasses: Sarcopterygii (lobe-finned fishes) and Actinopterygii (ray-finned fishes).

Review Questions.

1. General CHARACTERISTICS OF THE superclass Pisces (fishes).

2. Integument of fishes.

3. Types of fish scales.

4. Types of paired fins in fishes.

5. Types of caudal fins.

6. Modifications of fish scales.

7. Skeleton of fishes.

7. Digestive System of fishes.

5. Respiratory system of fishes.

6. Excretory system of fishes.

7. Circulatory system of fishes.

8. Nervous system of fishes.

9. Reproductive System of fishes.

10. Structure of a fish larva.

11. Lifestyles of fishes.

12. Reproduction and development of fishes.

13. Significance of fishes in nature and human life.

14. Classification of fishes.

15. Structural Features of the fish heart.

16. Excretory system of fishes.

17. Features of fish reproduction.

18. Viviparity and ovoviviparity in fishes.

19. Parental care in fishes.

20. Ecological groups of fishes.

21. Comparative characteristics of the respiratory system in lampreys and fishes.

22. Comparative characteristics of The Nervous System in amphioxus, lampreys, and fishes.

23. Comparative characteristics of the excretory system in amphioxus and fishes.

24. Sense organs in fishes.

25. STRUCTURE OF THE eye and accommodation features in fishes.

26. STRUCTURE AND FUNCTIONS of the lateral line.

27. Comparative characteristics of the skeleton in amphioxus, lampreys, and fishes.