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Class Chondrichthyes (Cartilaginous Fishes) Module 7

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Class Chondrichthyes (Cartilaginous Fishes) Module 7 CLASS CHONDRICHTHYES (CARTILAGINOUS FISHES) MODULE 7 MODULE 7 CLASS CHONDRICHTHYES (CARTILAGINOUS FISHES) Unit 1 Subclass Elasmobranchii Members of the Elasmobranchii (Gr. elasmos, metal plate + branchia, gills) are the sharks, skates and rays. Sharks belong to the superorder Selachii while rays and skates belong to the superorder Batoidea. Most sharks are streamlined pelagic predators while skates and rays are dorso-ventrally flattened for a bottom existence. They are carnivores that track their prey using their lateral line system and large olfactory organs. Their vision is not well developed. Elasmobranchs have fusiform body, with heterocercal caudal fin; paired Figure 2: A shark pectoral and pelvic fins; two dorsal median fins; pelvic fins in males are modified for claspers; fin rays are present. They possess a ventral mouth and two olfactory sacs that do not open into the mouth cavity. Their skin is covered with placoid scales and possess mucous glands. The endoskeleton is entirely cartilaginous and notochord is persistent. The digestive system consist of J-shaped stomach and intestine with spiral Figure 1: A skate Figure 3: A ray valve. The circulatory system consist of several pairs of aortic arches, the heart is two-chambered. Respiration is by means of five to seven pairs of gills with separate and exposed gill slits. Operculum is absent. Swim bladder is absent. The brain comprises two olfactory lobes, two cerebral hemispheres, two optic lobes, a cerebellum and a medulla oblongata. There are ten cranial nerves and three pairs of semi-circular canals. Sexes are separate, gonads are paired and reproductive ducts open into cloaca. Fertilization is internal and development is direct. Kidneys are of mesonephros (opisthonephros) type. AOE 1 CLASS CHONDRICHTHYES (CARTILAGINOUS FISHES) MODULE 7 Sub-unit 1 Biology of West African Tiger Shark The West African tiger or sharp nosed shark, Rhizoprionodon acutus (= Scoliodon terranovae) of the family Carcharhinidae is a common example in inshore to moderately deep waters of Nigeria. The body of Scoliodon is streamlined; the upper surface is darker. The anterior tapers gradually towards the end of the tail. The tail is equipped with powerful muscles, it provides the propelling power for the shark. The eyes are laterally placed and paired nostrils are on the ventral surface just Figure 4: Rhizoprionodon acutus anterior to the mouth. On each side of the head are five elongate gill slits. They have no spiracles behind the eyes. The surface of the head bears small pits (pit organs or ampullae of Lorenzini). They also possess lateral line canals. The lateral line can be located as a faint mark along each side of the body. Characteristics of the genus Scoliodon are caudal moon-shaped pits in front of the dorsal and ventral lobes of the caudal fin. The pelvic fins are smaller than the pectoral and their posterior angle in the male is modified into a grooved intromittent organ, called claspers, which are used in copulation. There are two unpaired dorsal and one unpaired ventral or anal fin. The caudal fin is around the terminal region of the tail and consists of two parts, the dorsal and ventral lobes with the ventral lobe being larger. It has an heterocercal (unevenly divided) tail with the vertebral column continuing along the upper lobe of the tail. The skin is tough, leathery and covered with placoid scales, which are modified anteriorly to form replaceable rows of teeth in both jaws (placoid scales consist of dentin enclosed by an enamel-like substance, and very much resemble teeth of other vertebrates). The mouth is ventral and leads into a spacious buccal cavity which is rich in mucous glands. The buccal cavity leads into the pharynx on either side of which are the internal apertures of the five gill pouches (the pharynx has both digestive and respiratory functions). The pharynx leads into a short and narrow oesophagus which has thick muscular walls and much folded lining. Posterior to the oesophagus is a muscular J-shaped stomach followed by the intestine, which is a wide tube that runs straight back in the abdominal cavity until the narrower rectum is reached. The spiral valve of the intestine increases its absorptive surface and also slows down the rapid passage of food. Near the beginning of the rectum is the tubular rectal gland which excretes excess salt taken in with the food. The rectum finally opens into the cloaca. AOE 2 CLASS CHONDRICHTHYES (CARTILAGINOUS FISHES) MODULE 7 There are five pairs of branchial clefts, which are richly supplied with blood vessels. Each branchial cleft consist of an internal pharyngeal opening, the gill pouch and an external opening, the gill slit. These structures function as respiratory organs). The excretory organs are the kidneys. The kidneys are two long narrow structures which lie dorsal to the peritoneum and just beneath the vertebral column. In the male, the very small collecting tubules of the non- functional kidney drain into the Wolffian duct (mesonephric duct), while those of the posterior functional open into the white flat urinary duct (ureter) which finally open into the urinogenital sinus at the cloaca. The Wolffian ducts also carry the sperm from the testes of the male, which uses claspers to deposit the sperm in the female oviduct. The ovaries are paired. The müllerian duct, or oviduct (paired), carries the egg from the ovary and coelom and is modified into a uterus (in which a primitive placenta may attach the embryo shark until it is born). Development of the embryo occurs within the uteri as Scoliodon is viviparous. The brain is of five divisions: telencephalon (cerebrum), diencephalon, mesencephalon (mid-brain), metencephalon (cerebellum) and myelencephalon (medulla oblongata). There are ten pairs of cranial nerves distributed largely to the head region. Surrounding the spinal cord are neural arches of the vertebrae. Along the spinal cord, a pair of spinal nerves, with united ventral and dorsal roots, is distributed in each body segment. Facts about Sharks 1. Sharks have a skeleton made of cartilage like primitive fishes “but it is calcified”, strengthened by granules of calcium carbonate deposited in the outer layers of cartilage. The result is a light and strong skeleton. 2. A large and efficient tail fin propels the fish through water. 3. The dorsal fin acts as a stabilizer to prevent rolling as the fish swims through water. 4. The ventral fin acts as a keel to prevent side slip. 5. The pectoral and pelvic fins acts like the elevator flaps of an airplane to assist the fish in going up or down through the water, as rudder to help it turn sharply left or right, and as brakes to help it stop quickly. 6. The large upper lobe of a shark’s caudal fin provides upward thrust for the posterior end of the body. 7. The pectoral fins add lift to compensate for the downward thrust of the tail fin. 8. To keep from sinking, sharks must always keep moving forward in water. AOE 3 CLASS CHONDRICHTHYES (CARTILAGINOUS FISHES) MODULE 7 9. Sharks are also aided in buoyancy by having very large livers containing a special fatty hydrocarbon called squalene that has a density of only 0.86. The liver, therefore, acts as a large sac of buoyant oil that helps to compensate for the sharks heavy body. Economic importance of sharks Sharks play important role, as apex predators, in controlling species below them in the food chain; and also serves as indicator species for the maintenance ocean ecosystem health. Sharks indirectly maintain the seagrass and coral reef habitats; the loss of sharks has led to decline in coral reefs, seagrass beds and the loss of commercial fisheries. Shark flesh and fins serve as food in maritime countries. Shark liver oil is used for tanning leather, for wood preservation, as lubricant and has medicinal value especially for rheumatism, cough, as a laxative and as cosmetics ingredients. The scales of sharks are used as leather and for decoration. Sub-unit 2 Skates and Rays Skates and rays belong to thee superorder Batoidea. Theys are dorso-ventrally flattened (have depressiform body). They are specialized for bottom dwelling. Skates are oviparous while rays are viviparous. The pectoral fins are greatly enlarged, extends forward and fuses to the head. The pectoral fins are used like wings in swimming. They have five pairs of gill openings which are ventral but large spiracles are dorsal. Water for respiration is taken in through these spiracles to prevent clogging of gills because the mouth is often buried in sand. The teeth are flattened and adapted for crushing their prey: molluscs, crustaceans and occasionally small fish. Figure 5: Labelled diagram of a skate In the stingrays, the caudal and dorsal fins have disappeared and the tail is slender, whip-like and armed with strong stinging spines which can inflict dangerous wounds. The spines of sting rays are useful as needles and awls; and because of the poison they contain, are used as spear heads and daggers. In electric rays, the skin is smooth and naked and has certain AOE 4 CLASS CHONDRICHTHYES (CARTILAGINOUS FISHES) MODULE 7 dorsal muscles modified into powerful electric organs that can give powerful shocks and stun their prey. Sting ray also have electric organs in the tail. Subunit 3 Advances of Elasmobranchs over Agnathans Although agnathans and elasmobranchs both have cartilaginous skeleton, the elasmobranchs show advances over agnathans in having: 1. A well differentiated head 2. Scales covering the body, 3. Two pairs of lateral fins, 4. Movable jaws articulated to the cranium, 5. Enamel-covered teeth on the jaws, 6. Three semi-circular canals in each ‘ear’, and 7. Paired reproductive organs and ducts Unit 2 Subclass Holocephali The members of the subclass Holocephali (Gr.
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