Exercise 11 Chondrichthyes and Osteichthyes: Observation and Classification of Specimens
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EXERCISE 11 CHONDRICHTHYES AND OSTEICHTHYES: OBSERVATION AND CLASSIFICATION OF SPECIMENS Structure 11.1 Introduction Objectives 11.2 Materials Required 11.3 General Features and Type Study of Class Chondrichthyes Sphyrna Pristis Torpedo 11.4 General Features and Type Study of Class Actinopterygii Labeo rohita Anguilla Exocoetus 11.5 Terminal Questions 11.1 INTRODUCTION This laboratory exercise involves the study and observation of jawed fishes which is based on Unit 13 of Block 3 of the Animal Diversity theory course. You will recall that the cartilaginous fishes are known as Chondrichthyes (Gr., chondros , cartilage and ichthyes , fish) Whereas Osteichthyes includes bony fishes. In this exercise you will study some representative members of cartilaginous fishes belonging to Subclass Elasmobranchii, (Gr. elasmos , plate and branchia , gills) and some representative members of ray-finned bony fishes included in Class Actinopterygii . All fishes are aquatic gnathostomes (with true jaws)having a streamlined body. The paired appendages are pectoral and pelvic fins supported by fin rays. The median fins are one or two dorsal fins, an anal fin and a caudal fin. The exoskeleton is in the form of scales, dermal denticles or horny scutes. The notochord is constricted into vertebrae. Respiration is by gills. Most fishes have a well developed lateral line system composed of sensitive receptors that can sense motion by nearby fishes and prey. The general position of fish in the animal kingdom is shown in the following schematic diagram. Chondrichthyes and KINGDOM ANIMALIA Osteichthyes: Observation and Classification of Specimens CHORDATA NON CHORDATA Groups Protochordata Vertebrata (Craniata) Superclasses Agnatha Gnathostomata (without jaws) (Jawed fishes and tetrapods) Chondrichthyes Osteichthyes Amphibia Reptilia Aves Mammalia Subclasses Elasmobranchii Holocephali Fig. 11.1: Schematic diagram depicting the general position of fish in the animal kingdom. Objectives After performing the exercise you will be able to: • identify and give the scientific and common names of the specimens of cartilaginous fishes like Sphyma , Pristis , Torpedo and bony fishes like Labeo, Exocoetus and Anguilla , • classify the identified cartilaginous and bony fishes up to the level of orders, • list characters justifying the classification of the identified genera of cartilaginous and bony fishes, draw their well labelled diagrams and mention special features, if any, • mention the habit and habitat and geographical location and economic importance, if any of each identified genus of the cartilaginous and bony fishes. 11.2 MATERIALS REQUIRED 1. Museum specimen of the following cartilaginous and bony fishes: i) Sphyrna ii) Pristis (sawfish) iii) Torpedo (electric ray) iv) Labeo v) Exocoetus 131 Animal Diversity: vi) Anguilla Laboratory 2. Laboratory Practical file. 3. Pencil, eraser and ruler. 11.3 GENERAL FEATURES AND TYPE STUDY OF CLASS CHONDRICHTHYES Members of class Chondrichthyes retain their cartilaginous embryonic endoskeleton throughout their life. It is not replaced by bone though calcium salts are sometimes deposited within it in order to strengthen it. The general position of cartilaginous fish is given in Fig. 11.1.Class Chondrichthyes is divided into two sub-classes: (i) Elasmobranchii and (ii) Holocephali. We will be studying some representatives of elasmobranchs. Features of Chondricthyes i) Cartilaginous fishes are mostly marine and predaceous. ii) They have fusiform or spindle-shaped body. iii) Their fins are both median and paired, all supported by fin-rays. Pelvic fins bear claspers in males. iv) Tail is heterocercal. v) Skin is tough. It contains minute placoid scales (Fig. 11.2) and mucous glands. vi) Endoskeleton is entirely cartilaginous without any bones. vii) Mouth is ventral and jaws have teeth. viii) Sexes are separate, gonads are paired and gonoducts open into cloaca. ix) These fishes are oviparous or ovoviviparous and fertilization is internal. x) No swim bladder present. Fig. 11.2: Small placoid scales are conical tooth-like structures are characteristic of cartilaginous fishes, the Chondrichthyes. 132 Sub-class Elasmobranchii Chondrichthyes and Osteichthyes: Observation and Elasmobranchs have five to seven separate gill openings on each side; Classification of operculum or gill-cover is absent, dorsal fin(s) and spines, if present, are rigid; Specimens males usually possess modified pelvic fins with claspers; dermal placoid scales often present; palatoquadrate (upper jaw) not fused to cranium (suspension amphistylic or hyostylic); branchial basket mostly behind the neurocranium; both upper and lower jaws have rows of sharp teeth which are replaced relatively rapidly; some ribs usually present; spiracle opening (remains of hyoidean gill slit) usually. Elasmobranchs are typically predaceous fishes that rely more on smell than on sight for obtaining their food (the olfactory capsules are relatively large and the eyes are relatively small). In the present laboratory exercise you will study the specimens of three elasmobranchs-Sphyrna, Pristis and Torpedo. 11.3.1 Sphyrna Sphyrna (Fig. 11.3) is commonly known as hammerhead shark . Examine the specimen and note the following features: i) Body is elongated measuring about .09 to 6 metres in length and divided into head, trunk and tail: Colour of the body is grayish dorsally whereas ventrally it is yellowish. ii) From the front head is flattened and expanded sideways into two prominent lateral lobes hence, it looks like a hammer. iii) Eyes are found at the tips of lateral lobes and provided with 3 eye-lids (the third eyelid is a nictitating membrane that protects the eye in the water). iv) Mouth is crescentric and ventral, at the base of lateral lobes, nostril is located ventrally. v) There are five pairs of lateral gill slits. Spiracles are absent. vi) Trunk part of the body has median as well as paired fins. There are two dorsal median fins, caudal and anal fins. vii) Pectoral and pelvic fins are paired. viii) Viviparous, produces about 40 young. ix) Sphyrna is caught for its oil and for its fins that are considered a delicacy in shark fin soup. Habit and Habitat It is common marine shark living in deep waters. Hammerhead shark is ferocious and voracious feeder and it feeds on small fishes , crabs, lobsters etc. It is a swift swimmer and because of its panoramic vision catches its prey swiftly 133 Animal Diversity: Geographical distribution: Found in topical and sub-topical seas and coastal Laboratory warm waters of Pacific Ocean. Fig. 11.3: Sphyrna, in its natural habitat. Classification and its Justification Kingdom Animalia Animals, multicellular organisms with cells that lack a cell wall, many capable of movement or movement of some of their body parts or capable of movement at some time of their life cycle; heterotrophic nutrition. Phylum Chordata Dorsal tubular nerve cord, notochord and paired gill-slits are present. Group Vertebrata Notochord is replaced by vertebral column; two pairs of appendages; circulatory system closed; hepatic portal system present; bloodcontaining R.B.Cs. Superclass Gnathostomata Jaws and paired appendages are present . Class Chondrichthyes Cold blooded vertebrates, gills for respiration, fins for locomotion; streamlined body covered with scales. Cartilaginous endoskeleton; skin has tough but minute placoid scales. 134 Subclass Elasmobrachii Plate-gilled fishes. 5-7 separate gill Chondrichthyes and Osteichthyes: openings on each side in sharks and Observation and ventral in rays; Dorsal fin and spines if Classification of present are rigid; anal fin present in Specimens most sharks; spiracles present; upper jaw not fused with cranium; both lower and upper jaws have sharp teeth; lateral line system well developed. Superorder Galeopmorphi Usually 2 dorsal fins with or without spines; 5 gill slits present;anal fin present; caudal fin heterocercal to diphycercal; spiracles may or may not be present. Order Carcharhiniformes 2 dorsal fins without spines; eyes dorsal or dorsolateral with nictitating membrane; spiracles absent; pectoral fins behind 5 gill slits; Genus Sphyrna Common Hammerheadshark Name 11.3.2 Pristis Pristis is commonly known as sawfish (Fig. 11.4). Examine the specimen and note the following features: i) These are also hypotrematic elasmobranchs; the body is shark like with flattenedunderside.. ii) The sawfish may attain a length of 3 to 6 metres. iii) Rostrum or snout is an elongated blade-like structure armed on either lateral side with a row of sharp tooth-like scales (denticles), which make it a formidable weapon for defense as well as food capture. iv) Spiracles are present behind the eyes. v) Dorsal fins are large with the first dorsal fin being opposite to the pelvic fin. vi) Tail is well developed and terminates in heterocercal caudal fin. Features of Special Interest As the sawfish swims through groups of small fishes, it swings the saw from side to side, thus disabling many of them and eating them at leisure. Economic Importance This fish is invaluable for liver oil of high vitamin value and for skin which is used for making scale boards. 135 Animal Diversity: Laboratory (a) (b) Fig. 11.4: Pristis (sawfish). a) Museum specimen, b) In its natural habitat. Habit and Habitat The fish is predaceous with food chiefly comprising small fishes, flesh of whales and other marine animals. Geographical Distribution: This fish is marine and is found in Mediterranean and Atlantic Oceans particularly in America, West Indies, China and Gulf