ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 14. INFRAPHYLUM GNATHOSTOMATA. SUPERCLASS PISCES

3. Class Chondrichthyes (Cartilaginous Fishes)

Representatives: sharks, rays and skates. Most species are marine predators or omnivores, although some freshwater species exist (such as the largetooth sawfish Pristis microdon). The size of cartilaginous Fishes varies from small rays about 15 cm long to sharks reaching 15-20 m in length.

The giants among modern cartilaginous fishes include:

✵ the whale shark (body length 15-20 m, weight 10-20 t; inhabits oceans, feeds on fish, Arthropods, and squid);

✵ the basking shark (body length 15 m, weight 5 t; feeds on planktonic arthropods);

✵ the manta ray (body length 6 m, weight 4 t).

General characteristics of the Class Chondrichthyes

The Skeleton is entirely cartilaginous. Certain areas may be calcified, which provides them with significant strength. The skeleton is divided into the Axial Skeleton, cranial skeleton (neurocranium and splanchnocranium), skeleton of paired and unpaired fins, and the girdles of paired fins.

The axial skeleton is formed by vertebrae (vertebra). The vertebral centrum is biconcave. Such vertebrae are called amphicoelous. In the center of the centrum, There is a canal through which the notochord passes. It no longer performs a real supportive function. Above the vertebral centra are the neural arches (which house the Spinal Cord). Neural spines are located on these arches. In the trunk region of the axial skeleton, poorly developed hemal arches form short transverse processes to which very short cartilaginous Ribs attach. In the caudal region, well-developed hemal arches form the hemal canal, through which the caudal artery and vein pass (well protected from compression during sudden tail movements). The notochord develops only as an embryonic Structure, and in adult fish, it is replaced by vertebrae, with remnants still remaining between them.

The Skull of cartilaginous fishes is divided into two regions: the neurocranium and the visceral skeleton.

The visceral region of the skull originated from modified branchial arches.

The Pectoral Girdle (girdle of the anterior paired fins) has the form of a cartilaginous arch lying within the muscular wall behind the branchial region. In sharks and chimaeras, it is not connected to the axial skeleton (indicating the minor role of these fins in the animal's movement), whereas in rays, due to the increased size of the pectoral fins, the upper PARTS OF THE cartilaginous girdle arch connect to the spine. On the lateral surface of the girdle on each side, there is an articular process—the site of attachment for the fin skeleton. The part of the girdle lying above the articular process is called the scapular region, and the part located below is the coracoid region (from the Greek korakoeides — resembling a crow). At the Base of the pectoral fin skeleton lie three massive, flat basal cartilages, which connect by their narrowed tips to the articular process of the girdle. Radial cartilages, each consisting of 2-3 segments, attach to the opposite (wider) side of the basal cartilages. The fin lobe itself is supported by elastotrichia (from the Greek elastos — flexible, thrix — Hair) — thin rays of horn-like substance produced by Skin Cells.

The Pelvic Girdle (girdle of the pelvic fins) is formed by a cartilaginous pelvic plate lying in the musculature of the abdominal wall directly in front of the cloaca. The Skeleton of the pelvic fin attaches to its lateral surfaces. It consists of a single elongated basal element, to the outer surface of which a series of radial cartilages is attached, the first of which is the largest. In male cartilaginous fishes, the elongated basal element extends beyond the fin lobe and serves as the skeletal framework of the copulatory organ — the pterygopodium.

The skeleton of unpaired fins is formed by rod-shaped cartilages of various sizes — radials embedded in the musculature. The fin lobe is supported by elastotrichia. The caudal fin in most cartilaginous fishes is heterocercal: the upper lobe is significantly larger than the lower lobe, and the terminal part of THE Vertebral Column extends into it. The skeletal framework of the caudal fin is formed by elongated neural and hemal arches of the vertebrae and cartilages.

The musculature of cartilaginous fishes consists of Muscle segments — myomeres, separated from each other by Connective Tissue partitions — myosepta. Differentiated Muscles of the jaws, branchial arches, and paired fins appear. A characteristic feature of the musculature of cartilaginous fishes is its relative autonomy — retaining The ability to contract even when communication with the Central Nervous System is disrupted.

The CHARACTERISTICS OF THE musculature and the entire Locomotor Apparatus provide cartilaginous fishes with enhanced power reserves. For instance, in experiments, sharks did not reduce their speed and maneuverability when carrying an additional load equal to 1/4 of their body weight measured in air, whereas the mobility of bony fishes decreased significantly even with a small additional load. Sharks are constantly in motion and have a body Temperature 7-8°С higher, and sharks of the genus Isurus up to 10°С higher, than the surrounding Water. This is due to their high swimming speeds.

The body of cartilaginous fishes is covered with tough skin composed of a multilayered epidermis and connective tissue — the corium. Numerous unicellular glands are located in the epidermis, and their mucous secretion covers the entire body in a thin layer. Cells of the lower epidermal layers contain pigments. Specialized pigment cells are also located in the corium. Together, they create the coloration pattern characteristic of each species. Some rays are able to change their color depending on the substrate they rest on. This is achieved by moving pigment from the cellular processes into The Cell body and vice versa, or by contracting and expanding the entire pigment cell.

Placoid scales develop in the skin, consisting of a rounded or rhomboid basal plate embedded in the corium and a tooth-like spine whose tip projects outward through the epidermis. Inside, the scale has a cavity filled with Blood Vessels. Externally, the scale is composed of a durable substance — dentin, formed by corium cells; the tip of the spine is covered by a cap of a very hard substance — enamel, produced by cells of the basal layer of the epidermis. In sharks, scales cover the body uniformly, while in rays, large scales are scattered in the skin with small scales randomly distributed between them. In stingrays and electric rays, the scales are reduced.

Large placoid scales are located on the jaws, transforming into Teeth. The spine and its enamel layer increase in size. If a tooth

becomes worn or broken, it sheds, and a new one grows to replace it. Tooth replacement can occur throughout the fish's entire life.

The body is divided into three regions: HEAD, trunk, and tail. The head features eyes, paired nostrils leading to olfactory capsules, and a Mouth opening bounded by jaws. The anterior part of the head is elongated, forming a rostrum. On the sides of the head, there are 5-7 pairs of external gill slits.

The trunk has paired (pectoral and pelvic) and unpaired (dorsal, caudal) fins, which do not directly propel the fish but act as rudders. The caudal fin is heterocercal (the upper lobe is significantly larger than the lower one).

The Digestive System (Fig. 55) of cartilaginous fishes begins with the mouth opening, which leads to the Oral Cavity. At its bottom lies a small fold of the mucous membrane — the Tongue; it is supported by the unpaired element of the hyoid arch (copula).

Fig. 55. Internal anatomy of a shark (after Naumov, Kartashev, 1979):

1 — nostrils; 2 — mouth; 3 — gills; 4 — external gill slits; 5 — sinus venosus;

6 — atrium; 7 — ventricle; 8 — conus arteriosus; 9 — ventral aorta; 10 — afferent branchial Arteries;

11 — cardiac Stomach; 12 — pyloric stomach; 13 — Small Intestine; 14 — section of the Large Intestine with a spiral valve;

15 — rectum; 16 — rectal gland; 17 — cloaca; 18 — Liver; 19 — Gallbladder; 20 — Bile duct;

21 — Pancreas; 22 — Spleen; 23 — Kidney; 24 — vas deferens; 25 — clasper of the pelvic fin;

26 — Thyroid Gland; 27 — Testis.

In all fish, the tongue lacks its own musculature, and its movements are driven by the hyoid arch. The oral cavity leads into the Pharynx, the walls of which are perforated by gill slits. Cartilaginous gill rakers prevent food from escaping through them. Mucus secreted by the cells of the oral cavity contains no digestive Enzymes and merely facilitates swallowing. A short Esophagus leads to a spacious, V-shaped stomach. Chemical Digestion begins in the first, larger cardiac region of The Stomach under METABOLISM/18.html">The Influence of Pepsin (in a highly acidic environment—up to 1.6% due to HCl). Digestion is relatively slow, taking over 5 days for large prey. The chyme then passes into the narrow pyloric region of the stomach, where it is processed by Trypsin. Trypsin is introduced there from the proximal section of the intestine, where pancreatic secretions are released. The acidity of the pyloric stomach contents is significantly lower.

The stomach is followed by the intestine, which is divided into three sections:

1) the small intestine — very short, separated from the pyloric stomach by a sphincter (a circular thickening of the muscular coat); the ducts of the pancreas and the gallbladder of the large liver open into the small intestine; in the small intestine, the chyme is processed by BILE AND PANCREATIC digestive juices;

2) the large intestine — wide and very long, its wall features an internal fold, the spiral valve, which forms 12-13 spiral turns; this valve slows down the passage of food and increases the internal absorptive surface of the intestine; in the large intestine, DIGESTION AND ABSORPTION are completed in an alkaline environment;

3) the rectum — short, through which undigested food residues enter the cloaca and are expelled outside through the cloacal slit (located between the pelvic fins).

Arising from the dorsal surface of the rectum is the rectal gland—a hollow, finger-like projection that Functions as an organ of salt regulation. The cells of this gland secrete a fluid in which the concentration of sodium chloride (NaCl) is twice as high as in Blood Plasma.

Cartilaginous fish lack a swim bladder. They possess a very large liver, which accounts for 14-20% to 25% of their body mass. The Biological Significance of the shark liver is as follows:

1) it stores significant reserves of fat, which serve as energy reserves (up to 70% of liver mass in the basking shark, and up to 60% in some rays);

2) it acts as a hydrostatic organ, increasing the buoyancy of the body;

3) it stores vitamin A (1 g of shark or ray liver contains between 8,000 and 60,000 international units of vitamin A).

Cartilaginous fish, particularly sharks, are capable of consuming large amounts of food in a short period and then fasting for a long time, slowly utilizing their stored resources.

The primary excretory Organs of cartilaginous fish are the paired mesonephric Kidneys, which appear as elongated bodies. The Structural and functional unit of the kidneys is the nephron, consisting of Malpighian corpuscles (capillary glomeruli enclosed by Bowman's capsules) and renal tubules. In some cartilaginous fish, nephrostomes—ciliated funnels that open into the body cavity On the surface of the kidney—persist alongside nephrons. In the kidneys, blood is cleared of Metabolic waste products, and urine is formed. It is excreted through excretory ducts—Ureters and the urogenital papilla—into the cloaca, and then outside. Marine cartilaginous fish excrete small amounts of urine: only 2 to 50 ml per 1 kg of body mass per day. Freshwater cartilaginous fish excrete 250 ml per 1 kg of body mass per day.

The respiratory organs of cartilaginous fish—gills (5-7 pairs)—are closely linked to the digestive system. The gills are separated by an interbranchial septum. On the outer side of the interbranchial septa are folds of the mucous membrane—gill filaments, which are of ectodermal origin. The gill filaments are densely vascularized. Gas exchange occurs within these gill filaments. Unlike bony fish, the gill filaments cannot excrete nitrogenous waste products and salts. On the inner side of the interbranchial septa are gill rakers, which collectively form a filtering apparatus that strains water and prevents food from entering the Respiratory system. The smaller the food particles the fish feed on, the more closely spaced the gill rakers are. Fish breathe by passing portions of water through the mouth, the buccopharyngeal cavity, and the gill apparatus.

The Circulatory system (Fig. 56) in cartilaginous fish is closed and features a two-chambered Heart (one atrium and one ventricle).

The Heart contains only deoxygenated (venous) blood. Sequential contraction of the atrium and ventricle pumps venous blood into the ventral aorta, which branches into five pairs of afferent branchial arteries. Through these, blood reaches the capillaries of the gill filaments, where it is oxygenated. Oxygenated (arterial) blood is collected into efferent branchial arteries. The common carotid artery branches off from the first efferent branchial artery, supplying blood to the head. The efferent branchial arteries from both sides merge to form the dorsal aorta, which runs beneath the vertebral column along the body and branches to various organs. Blood supplies organs and Tissues with oxygen, absorbs carbon dioxide, and becomes venous. Venous blood flows through the Veins back to the heart.

Fig. 56. Diagram of the CIRCULATORY SYSTEM OF a shark (arteries shown in white, veins in black) (after Naumov, Kartashev, 1979):

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

6 — left afferent branchial arteries; 7 — left efferent branchial arteries; 8 — left carotid artery; 9 — dorsal aorta;

10 — left Subclavian Artery; 11 — abdominal arteries; 12 — caudal artery; 13 — caudal vein;

14 — left renal portal vein; 15 — left kidney; 16 — left posterior cardinal vein; 17 — left anterior cardinal vein; 18 — left Cuvierian duct;

19 — left lateral vein; 20 — left subclavian vein; 21 — HEPATIC PORTAL VEIN; 22 — liver; 23 — hepatic vein;

24 — stomach; 25 — large intestine; 26 — spleen; 27 — gonad.

The blood of cartilaginous fishes is red due to the presence of Hemoglobin in erythrocytes—a pigment that binds molecular oxygen.

Cartilaginous fishes possess a spleen—a large, compact organ located near the stomach. The spleen functions as a blood reservoir (storing A large number of erythrocytes) and is a hematopoietic organ (where formed elements of the blood—erythrocytes, leukocytes, and platelets—are produced).

Cartilaginous fishes are cold-blooded or poikilothermic (from Greek poikilos — variable and therme — heat, warmth) animals, meaning they have a variable internal body temperature that changes depending on the ambient temperature.

The Nervous System of cartilaginous fishes is more advanced than that of cyclostomes. In modern cartilaginous fishes, the Brain accounts for 0.06-0.44% of body weight. The brain consists of five well-developed regions: the Medulla Oblongata, Cerebellum, Midbrain, Diencephalon, and Forebrain.

In cartilaginous fishes, the relative size of the forebrain is significantly increased. Externally, a longitudinal groove divides it into hemispheres, but internally, such division does not yet exist. The forebrain functions as an olfactory center (olfactory bulbs) and participates in The regulation of movement and behavior. The massive diencephalon has well-developed optic thalami. Its dorsal side bears the Pineal Gland (epiphysis), and its ventral side bears the Pituitary Gland (hypophysis), which provide endocrine activity. An optic chiasm (crossing) of the optic nerves is formed. The diencephalon is the primary visual center, participates in Processing information from other Sensory Organs, plays a role in coordinating movements, and regulates metabolism and its seasonal adjustments. The midbrain is dorsally divided into two optic lobes, where the PATHWAYS OF THE visual analyzer terminate. Connections are established between the midbrain and the cerebellum, medulla, and spinal cord. The cerebellum is quite well developed. It maintains muscle tone, balance, and ensures general coordination of movements; Reflexes associated with the lateral line organ receptors close within it. In sharks, a complex system of gyri forms on The surface of the cerebellum. The medulla oblongata is the center of reflex regulation for the spinal cord and the Autonomic nervous system, which coordinates the skeletal-muscular, circulatory, respiratory, digestive, and excretory systems. The nuclei of the vestibular apparatus (balance) and lateral line organs are located here.

Ten pairs of well-developed Cranial Nerves arise from the brain of cartilaginous fishes. The medulla oblongata gradually transitions into the spinal cord, which is located in the spinal canal formed by the neural arches of the cartilaginous vertebrae. The spinal cord of cartilaginous fishes retains significant autonomy: decapitated sharks can perform undirected movements under the influence of strong stimuli.

The sensory organs of cartilaginous fishes are adapted to perceive stimuli in the aquatic environment:

✵ the most important organ of reception—smell—is represented by paired olfactory capsules that open externally via nostrils; experiments have proven the high sensitivity of the olfactory organs. Sharks can smell prey (blood, mucus) from 2-3 km away;

✵ cutaneous seismosensory organs:

— the lateral line, located in the skin along the sides of the body (hence the name) and branching out on the head (in benthic species, on the belly); it consists of canals that connect along their entire length to the external aquatic environment through pores; sensory cells at the bottom of the canals perceive all water movements and currents; thanks to this, fish react to the movement of objects around them, as well as to various obstacles;

- ampullae of Lorenzini, located in the skin of the head; functions: they act as thermal sensors that perceive changes in ambient temperature down to 0.05°C; they detect electric fields with a voltage of up to 0.1-0.01 mВ/см; they allow the detection of motionless but living prey (by picking up bioelectric currents generated in the gill muscles during respiration);

✵ organs of Vision—eyes—have a structure typical of all fish:

а) three layers — the sclera, choroid, and retina;

б) the sclera is transparent in front, forming a flat cornea;

в) the anterior portion of the choroid forms the iris with an opening in the center—the pupil; behind the pupil is the lens (lens)—a transparent, avascular, spherical body (lens) that refracts light rays passing through it and focuses the image on the retina; the lens is spherical, so fish can only see at a close range (up to 10-15 m); the outer part of the choroid forms the argentea (silvery layer); on the inner surface of the choroid, a tapetum lucidum is formed—a layer of flat cells containing shiny crystals (they reflect light rays that have passed through the retina, providing the ability to see even in low light); a thin outgrowth of the choroid—the falciform process—penetrates the retina and vitreous body, attaching to the lens; during the contraction of the falciform process, the lens shifts slightly inward (thus accommodating the eye for focusing on an object);

г) the retina contains sensory cells—photoreceptors—that perceive light rays; in cartilaginous fishes, photoreceptors perceive only black-and-white shades, and they do not recognize colors (achromats); from the photoreceptors, the Nerve Impulse travels along the Optic nerve to the brain;

д) the inner cavity of the eye is filled with a transparent substance—the vitreous body;

е) the distinctive Features of the visual organ of cartilaginous fishes are that the skin around the eye forms a low ridge—an immovable ring-shaped eyelid; some sharks have a movable nictitating membrane; in sharks and chimaeras, the eyes are mobile, whereas in rays, the outer fibrous tunic (sclera) often fuses with the Orbit (orbita)—a paired, symmetrical cavity in the skull of vertebrates where the eye is located;

✵ the organs of Hearing and Balance are represented only by the Inner ear, which is located in the cartilaginous otic capsules that are part of the skull; the inner ear—the membranous labyrinth itself—includes the saccule and utricle (organs of hearing, which primarily perceive low-frequency sounds: within 100-2500 Hz) and three well-developed semicircular canals connected to them, located in three mutually perpendicular planes (functioning as organs of balance);

✵ taste organs—taste buds—are found in the mucous membrane of the Oral Cavity and pharynx;

✵ electric organs in electric rays are modified striated muscles; they are capable of producing an electric current with a voltage of 8 to 220 volts; the discharge allows them to stun prey and defend themselves against predators.

The Reproductive System of cartilaginous fishes is represented by Gonads and reproductive ducts.

The paired male gonads are called Testes. The ducts of the testes open into the renal tubules of the anterior narrow part of the kidney. This part of the kidney does not function as an excretory organ and is transformed into the Epididymis; its tubules open into the Wolffian duct, which functions as a vas deferens. In sexually mature males, an expansion called the Seminal Vesicle forms in the posterior part of the vas deferens. The right and left vasa deferentia open into the cavity of the urogenital papilla. Alongside them, the openings of thin-walled, hollow sperm sacs also lead there. The ureters also empty into the cavity of the urogenital papilla. The urogenital papilla opens via an aperture at its tip into the cloacal cavity. The formation of male Gametes occurs in the tubules of the testis. Immature spermatozoa travel through the testicular tubules to the epididymis—the anterior part of the kidney—where they mature in its tubules. Mature spermatozoa pass through the vas deferens and accumulate in the Seminal Vesicles and sperm sacs. During Fertilization, contractions of the walls of the seminal vesicles and sperm sacs expel the sperm into the male's cloaca, which are then introduced into the female's cloaca using copulatory organs (claspers, or pterygopodia, developed from the pelvic fins).

Female gonads are called Ovaries. They have separate reproductive ducts—oviducts. Each oviduct forms an expansion in its anterior part called the shell gland, while the expansion in the posterior part is called the Uterus. The right and left oviducts open into the cloaca through separate apertures on the sides of the urogenital papilla. There is no direct connection between the Ovary and the oviduct. A mature egg cell is released through a rupture in the follicle wall into the body cavity and enters the oviducal funnel (infundibulum), which is enlarged during this period. Due to the peristaltic Movements of the oviduct walls, the egg cell moves along the oviduct, is fertilized in its upper section, and then, in the region of the shell gland, is first covered with an albumen layer and then with a tough, horny shell, which may have various outgrowths (Fig. 57). This shell protects the egg and embryo from dehydration in seawater, from many predators, and from mechanical damage. Using these outgrowths, the eggs are suspended from seaweed or cling to the uneven surfaces of underwater rocks. The eggs are large and contain a lot of yolk.

Fig. 57. Eggs of cartilaginous fishes (after Naumov, Kartashev, 1979)

A — daggernose shark; B — catshark; C — sawshark; D — ray; E — chimaera.

In cartilaginous fishes, the formed egg may be laid externally (oviparity) or may be retained in the lower part of the oviduct

— in the uterus, resulting in the embryo developing within the maternal body (ovoviviparity). During ovoviviparity, no special connection is established between the embryo and the maternal Organism: development occurs at the expense of the egg's energy reserves, though the embryo likely obtains oxygen via osmosis and diffusion from the maternal body. In some cartilaginous fishes, true viviparity is observed, where a close connection—a kind of Placenta—develops between the embryo and the maternal organism: the Blood vessels of the embryo and the mother lie close to each other; through osmosis and diffusion, oxygen and nutrients from the mother's bloodstream enter the embryo's bloodstream, while metabolic wastes pass into the mother's blood.

Internal fertilization is a characteristic feature of all cartilaginous fishes.

The fecundity of cartilaginous fishes is low: 1–2, rarely 10–12 eggs ranging from 6 to 20 cm in length. Only the Greenland shark lays up to 500 eggs about 8 cm long. The Development of a tough, horny eggshell restricts oxygen access. This has led to the appearance of external gills in the embryos of many cartilaginous fish species; these are resorbed before hatching. Embryonic development in cartilaginous fishes is slow, taking from 4 to 12 months. However, the young possess a high capacity for independent life.

Significance of cartilaginous fishes in nature

Cartilaginous fishes play a huge role in ecosystems by regulating the populations of many animal species or by feeding on organic remains.

Significance of cartilaginous fishes in human life

The meat of sharks and rays is quite tasty, rich in Vitamins (A, D), and is consumed by humans. In some countries, it is considered to have healing properties.

Substances obtained from shark liver are used in medicine in attempts to treat malignant tumors.

Shark skin is used to make various products (industrial raw Materials, drums, shoes), and some cultures fashion weapons from their teeth.

Shark and ray meat is used as animal feed or as bait for fishing.

The stingray (common stingray) is dangerous to humans: if stepped on, it can strike with the sharp spine on its tail; at the base of the spine is a gland that secretes venomous mucus; the sting is not fatal but causes severe pain, temporary paralysis, and general malaise.