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

CHAPTER 3. SUBPHYLUM GNATHOSTOMATA

3.3. Structural features of selected representatives of cartilaginous fish.

Cartilaginous fish (Chondrichthyes) are divided into two subclasses: Holocephali (chimaeras) and Elasmobranchii (sharks and rays). The subclass Holocephali differs significantly in its structural Organization from Elasmobranchii (Fig. 52).

The main distinguishing characteristics are as follows:

- autostylic Skull;

- Axial Skeleton represented by a notochord, with incomplete, ring-like vertebral centra;

- Teeth fused into dental plates;

- body mostly naked, less frequently covered with placoid scales;

- spiracles and cloaca are absent;

- gill slits are covered by Skin flaps (folds).

The subclass Elasmobranchii is divided into two superorders: sharks (Selachii) and rays (Batoidea). Together, they encompass 13 orders. Opercula (gill covers) are entirely absent in elasmobranchs. The gills open via five to seven slits on both sides of the body. The gill lamellae are plate-shaped and attached to the arches along their entire length (hence the name elasmobranchs). The body shape of elasmobranchs varies greatly; some possess a torpedo-like body, while others are flattened (Fig. 52).

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Fig. 52. European rabbitfish (1) and longnose chimaera (2).

SUPERORDER SHARKS - Selachomorpha

This superorder includes 20 families and 350 species. They are distributed across all seas and oceans, and some also inhabit fresh waters.

Order Heterodontiformes (bullhead sharks) - represented by a single family (Fig. 53), Heterodontidae.

Fig. 53. Port Jackson shark

Modern bullhead sharks possess two dorsal fins, each equipped with a spine, and an anal fin. Their body is short and noticeably thickened anteriorly. The HEAD is large and high, featuring prominent supraorbital ridges. The teeth in the front part of the jaws are small and pointed, whereas the posterior teeth are large and flat. This order includes the horn shark family. Bullhead sharks are relatively small (up to 1.5 m in length). They deposit conical eggs on the seabed enclosed in horny capsules, the surfaces of which bear spiral crests with pointed apices.

Order Hexanchiformes (frilled and cow sharks). Representatives of this order retain several archaic structural features: six to seven gill slits, the absence of vertebral centra, and the lack of a nictitating membrane in the eyes. A typical representative is the frilled shark (Chlamydoselachus anguineus), which inhabits great depths of up to 1,200 m (Fig. 54).

Fig. 54. Frilled shark

Order Orectolobiformes (carpet sharks). Sharks of this order are characterized by two dorsal fins lacking spines. Many species possess barbels on the snout. The Mouth is small. A nictitating membrane on the eyes is absent (Fig. 55).

Fig. 55. Nurse shark

They inhabit tropical waters. The Atlantic Ocean is home to the nurse shark (Ginglymostoma cirratum). This ovoviviparous shark reaches up to 4 m in body length and weighs 170 kg. This order also includes the whale shark (Rhincodon typus), which grows 15 (18–20) m long, weighs up to 14 tons, and feeds on plankton (Fig. 56).

Fig. 56. Whale shark

Order Lamniformes (Mackerel sharks). Species of this order are characterized by two dorsal fins, a large mouth, and powerful teeth. A typical representative is the common thresher shark (Alopias vulpinus) (Fig. 57).

Fig. 57. Thresher shark

This group also includes the great white shark, or Carcharodon (man-eater shark, or "white death") — Carcharodon carcharias (Fig. 58).

Fig. 58. Great white shark

Order Carcharhiniformes (Ground sharks). This order encompasses the majority of modern sharks. They are typically characterized by the absence of spines on their fins and the presence of a nictitating membrane (third eyelid). They inhabit warm and temperate waters, with some species even living in freshwaters. This order includes catsharks, houndsharks, sawsharks, and the scalloped hammerhead (Sphyrna zygaena) (Fig. 59).

Fig. 59. Hammerhead shark

They reproduce via ovoviviparity or viviparity. Large-bodied species are dangerous to humans.

Order Squaliformes (Dogfish sharks). This order unites sharks with or without spines on their dorsal fins and lacking an anal fin. These fish inhabit both cold and warm waters, including pelagic and benthic species. A notable representative is the spiny dogfish (Squalus acanthias), common in the Black Sea (Fig. 60).

Fig. 60. Spiny dogfish

In England, dogfish meat, which lacks an ammonia odor, is prized higher than herring meat. This order also includes the Greenland shark (Somniosus microcephalus) (Fig. 61). This shark can reach 6.5 m in length and weigh up to 1 ton, alongside certain deep-sea sharks equipped with specialized luminescent Organs.

Fig. 61. Greenland shark

Order Pristiophoriformes (Sawsharks) comprises sharks characterized by an elongated, flattened, sword-like snout equipped with large teeth along the sides and a pair of long barbels at the midpoint of its length that serve a tactile function. The presence of a saw-like rostrum makes these sharks superficially similar to sawfishes from the superorder Batoidea. Sawsharks yield meat of good culinary quality (Fig. 62).

Fig. 62. Sawshark

Order Squatiniformes (Angel sharks or monkfish). Externally, members of this order resemble rays; however, their gill slits are positioned laterally on the body, as in all sharks. A typical representative is the angel shark (Squatina squatina) (Fig. 63).

Fig. 63. Angelshark (Squatina squatina)

The distribution range of these sharks encompasses the Mediterranean Sea and the European Atlantic coast. All angelsharks are ovoviviparous.

Unlike sharks, rays are characterized by gill slits located entirely on the ventral side of the body rather than on the sides. Their torso is strongly flattened, and the margins of the pectoral fins fuse with the sides of the body and head. In rays, the Eyeball is fused dorsally to the Orbit, a feature never observed in sharks. A nictitating membrane is always absent, and anal fins are lacking. The teeth of rays are awl-shaped or strongly flattened and rounded. None of them possess the sharp, blade-like teeth characteristic of many sharks. As a rule, spiracles are much better developed in rays than in sharks, which is related to their respiratory function. It is precisely through the spiracles that bottom-dwelling rays draw Water into the gill cavity. Only pelagic manta rays, which inhabit the water Column, take in water through the mouth like sharks; consequently, they have noticeably reduced spiracles. The dimensions of various rays range from a few centimeters to 6–7 m in length, while the heaviest specimens can reach nearly 2.5 tons. The majority of rays lead a benthic lifestyle, and only a few species are pelagic. Rays reproduce either by depositing capsule-enclosed eggs on the bottom or by giving birth to live young. Their diet comprises A wide variety of organisms, ranging from plankton and benthic invertebrates to fish. Rays are widely distributed across all seas and oceans, inhabiting both the cold waters of the Arctic and Antarctic, and the warmed shallows of tropical seas across a very broad Temperature range.

Superorder Batomorpha – Rays

Order Torpediniformes (Electric rays).

Order Pristiformes (Sawfishes).

Order Rajiformes (Skates).

Order Myliobatiformes (Stingrays and relatives).

Order Torpediniformes (Electric rays) is an order of flat marine Fishes with enlarged pectoral fins (Fig. 64).

Fig. 64. Electric rays: torpedo ray (1) and eyed electric ray (2)

Most representatives of this order are known for their ability to generate an electric discharge ranging from 8 to 200 volts, depending on the species, which is used for hunting and defense. The order includes about 69 species grouped into four families. The best-known genus of the order is Torpedo, which gave its name to the weapon type—the torpedo. This name originates from the Latin word torpere meaning "to paralyze," which precisely describes what happens to prey or a human stepping on it. The current generated by rays varies by species, reaching up to 50 amperes. Voltage also varies; for instance, the Atlantic torpedo ray (Torpedo nobiliana) can deliver discharges of 220 volts, but for most species, this figure is lower, ranging from 5 to 40 volts. The dimensions of electric rays also vary; the maximum length belongs to the aforementioned torpedo ray, reaching 1.2 m with a weight of up to 100 kg, whereas other species are smaller, measuring up to 50 cm.

Order Pristiformes (Sawfishes). The order consists of a single family and genus with 7 species (Fig. 65).

Fig. 65. Sawfish

They reach 5–6 m in length; the body is slightly flattened, and the pectoral fins are small. The gill slits are located on the ventral surface of the head. The elongated, saw-like rostrum is edged with large teeth, but unlike sawsharks, it lacks sensory barbels. They inhabit the shallows of tropical and subtropical seas and occasionally ascend into the lower reaches of rivers (the Australian sawfish permanently inhabits rivers). They are strong swimmers, feeding on small schooling fish—diving into a school and striking fish with lateral swings of their "saw"—and various benthic invertebrates (presumably using the "saw" to disturb the substrate). They are ovoviviparous. The teeth on the "saw" emerge on the skin of the young only after birth.

Order Rajiformes (Skates). This order includes over a hundred species across three families, characterized by a strongly flattened, more or less rhomboid body, a pointed snout, and a slender tail stem clearly demarcated from the trunk and terminating in a small caudal fin. The thornback ray (Raja clavata), a representative of the family Rajidae, inhabits the Azov and Black Seas (Fig. 66).

Fig. 66. Skates: thornback ray (1) and guitarfish (2)

In most (though not all) species, large placoid scales resembling embedded rounded buttons with slightly recurved teeth are scattered across various areas of the skin. They are bottom-dwellers inhabiting cold and temperate waters, while in the tropics they are found only at great depths. Their length ranges from 30 cm to 2 m. By day, they typically rest on the bottom or partially bury themselves in the substrate, swimming near the bottom at night. Their primary mode of locomotion is undulating Movements of the pectoral fins. They feed on small fish, benthic crustaceans, Mollusks, and Echinoderms. When attacking fish, they glide over them, press them against the bottom with their bodies, and then consume them. Females deposit only 1–2 eggs at a time, but over an extended spawning period, they lay several dozen eggs enclosed in horny capsules with tendrils. Development takes 4 to 6 months. In meter-long skates, the egg length reaches 6–10 cm, and newly hatched young measure 10–15 cm.

Order Myliobatiformes (Stingrays and relatives).

They are distinguished by significantly enlarged pectoral fins that extend far forward along the sides of the head, paired with a flattened body (Fig. 67).

Fig. 67. Stingrays: common stingray (1), freshwater stingray (2)

The undulating movement of the pectoral fins during locomotion serves as a diagnostic feature for this taxon. The eyes and spiracles are located on the dorsal surface of the body, while the gill slits are on the ventral side. They possess flat, small teeth and are generally carnivorous, although mantas are filter feeders (Fig. 68).

Fig. 68. Giant oceanic manta ray, or devilfish

Water filtration in manta rays is facilitated by a specialized Structure OF THE gill slits, which form a unique filtering apparatus.

Most ray species are viviparous and give birth to live young, although some lay eggs encased in a leathery horn capsule known as a "mermaid's purse".

Unlike other bottom-dwelling fish, rays never press the entire anterior portion of their body flat against the seafloor; instead, they prop themselves up on their pectoral fins, leaving a clear space underneath. To supply water to their gills, they open their spiracles while pushing back the valve, and once the gill pouches are filled, they close the spiracles and force the water through the gill slits. These rays propel themselves through undulating, wave-like movements of both fins that travel from the front of the body to the rear. The long tail, which is rarely used for propulsion, serves partly as a rudder.

Review Questions

1. General characteristics of cartilaginous fish.

2. Integumentary System of cartilaginous fish.

3. Types of scales in cartilaginous fish.

4. Types of paired fins in cartilaginous fish.

5. Types of caudal fins in cartilaginous fish.

6. Modifications of scales in cartilaginous fish.

7. Skeleton of cartilaginous fish.

7. Digestive System of cartilaginous fish.

5. Respiratory system of cartilaginous fish.

6. Excretory system of cartilaginous fish.

7. Circulatory system OF cartilaginous fish.

8. Nervous system of cartilaginous fish.

9. Reproductive System of cartilaginous fish.

10. Reproduction and development of cartilaginous fish.

13. Significance of cartilaginous fish in nature and human life.

14. Classification of cartilaginous fish.

15. Comparative CHARACTERISTICS OF THE respiratory system in lampreys and cartilaginous fish.

16. Comparative characteristics of The Nervous System in lampreys and cartilaginous fish.

17. Comparative characteristics of the excretory system in lancelets and cartilaginous fish.

18. Sense Organs in cartilaginous fishes.

19. Lifestyle of cartilaginous fishes.

20. STRUCTURE AND Functions of the lateral line.

21. Comparative characteristics of the skeleton of lampreys and cartilaginous fishes.

22. Ampullae of Lorenzini in cartilaginous fishes.

23. Origin and Structure of teeth in cartilaginous fishes.

24. Comparative characteristics of placoid scales in cartilaginous fishes and mammalian teeth.

25. Viviparity and ovoviviparity in cartilaginous fishes.

26. Distribution of cartilaginous fishes across aquatic environments worldwide.

27. Lifestyle features of sharks and rays.

28. Freshwater and marine cartilaginous fishes.

29. Giants and dwarfs among cartilaginous fishes.

30. Feeding habits of cartilaginous fishes.

31. Interactions between humans and cartilaginous fishes.

32. The Role of cartilaginous fishes in nature and human life.

33. Global fisheries of cartilaginous fishes.