SCOLIODON TYPE STUDY

Scoliodon
1. Distribution: The genus Scoliodon is widely distributed in the Indian, Pacific West Indies and eastern coasts of South America and Atlantic Oceans. Genus scoliodon is distinguished from other sharks, in having elongated snout, depressed head and compressed body.
2. The common Indian dog fish is Scoliodon sorrakowah which means ‘black shark’ in Tamil.
3. Habit and Habitat of Scoliodon: The shark is a marine, carnivorous and predaceous animal. It eats small pelagic schooling and bottom living bony fishes, including anchovies, codlet as well
as shrimps and cuttle fish.
4. External Structures of Scoliodon: Scoliodon is an elongated spindle shaped animal. It has a laterally compressed body. A fully developed specimen of the genus attains a length of about 60 cm.
5. The body is divisible into head, trunk and tail. The head is dorso-ventrally flattened and terminates anteriorly into a dorso-ventrally compressed snout. The dorsal side of Scoliodon is darkgrey while the underside is pale white.
6. The trunk is more or less oval in transverse section. It attains maximum thickness in the middle region and the body gradually tapers posteriorly into a long tail. The tail is also oval in crosssection and bears a heterocercal type of caudal fin, i.e., the posterior end of the vertebral col umn is bent upwards and lies in the dorsal or epichordal lobe.









7. Body surface is rough due to backwardly projecting spines of placoid scales embedded in the skin.
8. Eyes: Two prominent circular eyes are present. Each eye is provided with movable upper and lower eyelids. The third eyelid or nictitating membrane can cover the whole eye in emergency. The pupil is a vertical slit like aperture.
9. Body apertures: The following important apertures are present on the body surface
10. Mouth: The mouth is a very wide crescentic aperture lying on the ventral side of the head near its anterior end. It is bounded by upper and lower jaws; each is beset with one or two rows of sharply pointed and backwardly directed teeth to catch the slippery prey. The teeth are replaced if these are broken. The teeth of Scliodon are modified scales.
11. Nares: The nostrils are placed one at each angle of the mouth. These are exclusively olfactory in function and have no connection with the mouth cavity. Each nostril is partly covered by a small fold of skin.
12. External gill slits: Posterior to the eyes there are five vertical slits on each side. They are called gill or branchial slits. The branchial slits lead into the gill pouches which in turn open into the pharyngeal cavity.
13. Cloacal apertures. The cloaca opens to the exterior by a cloacal aperture which is located in between the two pelvic fins. The cloacal aperture is an elongated opening. The cloaca is a common chamber, into which anus, urinary and genital apertures open. On each side of the cloaca lies the abdominal pore.
14. Abdominal pores: The abdominal pores are paired structures and situated on elevated papillae to communicate the coelom to the outside. A faint lateral line is present. Beneath this line a canal is present. The canal opens to the exterior by minute pores at intervals. Many pores, called ampullary pores, are also present on the head.
15. Caudal pits. At the base of caudal fin, the tail bears two shallow depressions, one dorsal and one ventral, known as caudal pits, which are characteristic of genus Scoliodon.
16. Fins: As in other fishes, Scoliodon bears unpaired and paired fins which are actually flaplike integumentary extensions of the body. These are flexible and are stiffened by cartilaginous rods or horny fin rays. All the fins are directed backwards which is of positive advantage in swift forward movement in water.
17. Median unpaired fins: The fins under this category include two dorsal, one caudal and one ventral fin. The dorsal fins are triangular in outline. The anterior dorsal is larger and situated at about the middle of the body. The posterior dorsal is comparatively small and occupies a median position between the first dorsal and the tip of tail.
18. The caudal fin has one ill developed ventral lobe (hypochordal) which is divided into two parts. Two shallow depressions, called caudal pits, are regarded as the diagnostic features of the genus. These are pre sent at the root of the tail, one at the dorsal surface and another on the ventral. The median ventral fin is located in the midventral line and just anterior to the caudal fin.
19. Lateral paired fins: Two pectoral and two pelvic fins constitute the lateral paired fins. The pectoral fins are large and are situated posterior to the gill clefts. The pelvic fins are much smaller. In females, these are simple but in males each of them is connected with a copulatory organ called myxipterygium or clasper. Clasper is rod like in appearance having a dorsal groove leading to a siphon at its base.

Respiratory system
Since dogfish is an aquatic animal, it depends wholly upon oxygen dissolved in sea water for respiration. Thus, respiration is aquatic and carried on entirely by vascular gills.
I. Respiratory organs:
1. The respiratory organs of scoliodon consist of 5 pairs of gill pouches containing gills. Five gill pouches are present in a series on either side in the lateral wall of pharynx, behind the hyoid arch.
2. Each gill-pouch is compressed anterio-posteriorly. It opens into pharynx by a large internal brachial aperture and to outside by a narrow vertical external branchial aperture or gill-slit. Two adjacent gill-pouches are completely separated from each other by a vertical fibro-muscular partition, the interbranchial or gill septum.
3. The inner or pharyngeal border of each gill septum is supported by a cartilaginous visceral arch or gill arch with its slender branchial rays. The septum is covered by an epithelium and contains blood vessels, nerves, etc.
4. The mucous membranes of a septum is raised into numerous horizontal leaf-like folds, called gill lamellae or gill filaments.
5. These constitute the gill proper and are richly supplied with blood-capillaries. Each septum bears two sets of gill-lamellae, one on its anterior face and the other on its posterior face. Each set makes a half gill called demibranch or hemibranch, while both the sets attached to a gill arch and its gill septum constitute a complete gill called holobranch.
6. The posterior demibranch of a septum has longer lamellae than those of the anterior demibranch. A gill pouch thus contains two demibranchs belonging to two adjacent gills.
7. In Scoliodon, the hyoid arch bears only a demibranch on its posteror face, the first four branchial arches bear holobranchs, while the fifth branchial arch is a branch or without any gill.





























Circulatory System of Scoliodon:
The circulatory system consists of:
(a) The circulatory fluid, called blood,
(b) The heart,
(c) The arteries and
(d) The veins.
Blood:
The blood consists of colourless plasma and corpuscles are suspended in the plasma. Two kinds of corpuscles are encountered; the RBC (or erythrocytes) and the WBC (or leucocytes). The erythrocytes are oval bodies containing a nucleus. The haemoglobin is present in the erythrocytes. The leucocytes are amoeboid in structure.
Heart:
• The heart is a bent muscular tube and consists of the receiving parts, comprising of a sinus venosus and a dorsally placed auricle, and the forwarding parts, consisting of a ventricle and a conus arteriosus.
• The heart is situated on the ventral side of the body between two series of gill pouches. Receiving parts of the heart: The sinus venosus is a thin walled tubular chamber. The sinus venosus is highly contractile and the beating of the heart originates from this part of the heart.
• Two great veins, the ductus Cuveiri, open into the sinus venosus, one on each lateral side. Two hepatic sinuses enter the sinus venosus posteriorly. The sinus venosus opens into the auricle by sinuauricular aperture which is guarded by a pair of valves.
• The auricle is a large, triangular and thin walled chamber situated dorsal to the ventricle but in front of the sinus venosus. The auricle communicates with the ventricle through a slit like auriculo ventricular aperture guarded by two lipped valves. The receiving chambers, (sinus venosus and auricle) receive the venous blood from all parts of the body.

Forwarding parts of the heart:
• The ventricle has a very thick muscular wall, the inner surface gives many muscular strands, thus giving it a spongy texture. It is an oval chamber and constitutes the most prominent part of the heart. The conus arteriosus is a stout median muscular tube arising from the ventricle. The lumen of the conus arteriosus is provided with two transverse rows of semilunar valves.
• To keep the valves in position the free ends of the valves are attached to the ventricular wall by fine tendinous threads, called chordae tendinae. The conus arteriosus is continued forward as the ventral aorta.









• The function of the heart is to receive the deoxygenated blood from all parts of the body and to pump it for aeration to the gills. Such a type of the heart is designated as the venous or branchial heart, because only the deoxygenated blood circulates through its different parts.

Arterial System of Scoliodon:
The arterial system of Scoliodon is divided into two distinct categories of arteries. These are:
(a) The afferent branchial arteries arising from the ventral aorta which bring the deoxygenated blood to gills for oxygenation and
(b) The efferent branchial arteries which originate from gills and convey the oxygenated blood to the different parts of the body.
Afferent branchial arteries:
• The ventral aorta is situated on the ventral surface of the pharynx and extends up to the posterior border or the hyoid arch. The ventral aorta divides into two branches called innominate arteries, which again bifurcates into the first and second afferent branchial arteries.
• The third, fourth and fifth afferent arteries arise from the ventral aorta. Each afferent branchial artery arises from the ventral aorta by independent opening except the anterior most pairs which arise by a common opening.
Efferent branchial arteries:
• The afferent branchial arteries break up into capillaries in the gills. From the gills the blood is collected by efferent branchial arteries. There are nine pairs of efferent branchial arteries and these are equally distributed on each side. The first eight arteries form a series of four complete loops around the first four gill slits and the ninth efferent branchial artery collects blood from the demibranch of the fifth gill pouch and from where blood is poured into the fourth loop.
• In addition to short longitudinal connectives connecting the four loops, these are further connected with each other by a network of longitudinal commissural vessels called the lateral hypobranchial chain.














• From each efferent branchial loop arises an epibranchial artery. The four pairs of epibranchials join in the middorsal line to form the dorsal aorta. The ninth efferent branchial artery has no epibranchial branch but joins with the eighth efferent branchial artery.

Anterior arteries:
• The head region gets the blood supply from the first efferent branchial artery and partly from the proximal end of the dorsal aorta. Arteries from the first efferent branchial (hyoidean efferent) are:
(a) The external carotid,
(b) The afferent spiracular and
(c) The hyodean epibranchial which in turn receives a branch from dorsal aorta.
• The external carotid artery originates from the first collector loop and divides into a ventral mandibular artery giving branches to the muscles of the lower jaw and a superficial hyoid artery which supplies the second ventral constrictor muscle, the skin and the subcutaneous tissue beneath the hyoid arch.
• The afferent spiracular artery after originating from the middle of the hyoidean efferent, proceeds for ward as the spiracular epibranchial artery and enters the cranial cavity. Just before its entry into the cranial cavity it sends a great ophthalmic artery to the eye ball.
• Immediately after the entry to the cranium it joins with a branch from the internal carotid to form the cerebral artery. The cerebral artery immediately divides into an anterior and a posterior cerebral artery which supply the brain. The hyoidean epibranchial artery runs forwards and inwards to the posterior border of the orbit and gets an anterior branch from the dorsal aorta.
It divides immediately into:
(a) the stapedial artery which gives off the inferior orbital artery and runs forward as the superior orbital artery supplying the six eye muscles and the superficial tissue above the auditory capsule. The superior orbital artery gives a large buccal artery which runs as the maxillonasal artery. The maxillonasal gives several arteries to the muscles of the upper jaw, the olfactory sac and the rostrum,
(b) the internal carotid artery passes inward and enters the cranium where it bifurcates into two branches. One of the branches unites with its fellow from the opposite side and other branch unites with the stapedial.
Dorsal aorta and its branches:
The dorsal aorta is formed by the union of epibranchial arteries. It runs posteriorly and is situated ven- tral to the vertebral column. It is continued up to the tip of the tail as the caudal artery. Along the anteroposterior direction the following arteries have their origin from the dorsal aorta:
(a) Several buccal and vertebral arteries are given off anteriorly.
(b) A pair of small subclavian arteries arises from near the origin of the fourth epibranchial arteries.

The subclavian artery gets the epicoracoid artery on its way and divides into:
(i) A branchial artery to the pectoral girdle and pectoral fin, (ii) An anterio lateral artery to the body musculature and (iii) A dorsolateral artery to the dorsal musculature.
(c) A large coeliacomesenteric artery arises just behind the origin of fourth epibranchial artery. It divides into a smaller coeliac artery and a larger anterior mesenteric artery. (d) A lienogastric artery originates posterior to the coeliaco mesenteric artery and gives off (i) An ovarian (in females) or spermatic artery (in males) to gonad, (ii) A posterior intestinal artery to the posterior part of the intestine, (iii) A posterior gastric to the posterior part of the cardiac stomach and (iv) A splenic artery to the spleen.
(e) Series of paired parietal arteries emerge out behind the subclavian artery. Each parietal gives a dorsal parietal artery and a ventral parietal artery. The dorsal parietal artery supplies the dorsolateral musculature, the vertebral column, the spinal cord and the dorsal fin. The ventral and the peritoneum. The ventral parietal parietal artery supplies the ventral muscles gives renal branches to the kidneys.
(f) A pair of iliac arteries extends to the pelvic fin as femoral arteries.


14. Venous System of Scoliodon:
The deoxygenated blood from the different parts of the body is returned to the heart by veins which form irregular blood sinuses throughout their courses. The existence of extensive blood sinuses is a charac teristic feature of the venous system of Scoliodon.




















Scales in fishes
• In many vertebrates, the exoskeletal covering of body is made of two types of scales: epidermal and dermal. Epidermal scales are cornified derivatives of the Malpigian layer of epidermis. They are well developed in terrestrial vertebrates such as reptiles, birds and mammals.
• Dermal scales are mesenchymal in origin and especially developed in the fishes. They are small, thin cornified, calcareous or borny plates which fit closely together or overlap.
• As regards the arrangement of scales on piscine body is concerned, they are most often imbricated and thus, overlap like shingles on the roof, with their free margins directed towa4rds the tail, so as to minimize the friction of water. But sometimes total reversal of the pattern of arrangement is seen in some fishes.
• Among barbot (Lota) and freshwater eel (Anguilla) the pattern is mosaic rather than overlapping one another, they are separated minutely or meet their neighbours only at their margins. Scales vary in size and shape in different species.
• The body of all fishes except members of family Siluridae and a few bottom dwellers is covered by scales.
• Five types of dermal scales have been identified in fishes: cosmoid, placoid, ganoid, cycloid and ctenoid.
• 1. Cosmoid scales: These do not occur in living fishes. These were characteristic of certain ostracoderms, placoderms, and extinct sacropterygians (lobe finned fishes and lung fishes). These consisted of 4 distinct layers: an outermost thin enamel like ganoine, thick dentine like cosmine, spongy bone and innermost compact bone.
• 2. Placoid scales: These are characteristic of elasmobranc fishes only. Each placoid scale consists of a backwardly directed spine arising from a rounded or rhomboidal basal plate embedded in dermis.
• Spine is made of enamel like and basal plate of dentine like bony material. A pulp cavity inside spine opens through basal plate. Placoid scales are closely set together in skin giving it a sandpaper like quality.










3. Gonoid scales: Ganoid or rhomboid scales are thick, usually rhomboid or diamond-shaped plates closely fitted side by side, like tiles, providing a bony armour to the fish. In some cases they may overlap.
• Ganoid scales are characteristic of chondrosteans (Polypterus, Acipencer) and holosteans (Leipidosteus) so that these are often called ganoid fishes. Polypterus has palaeoniscoid ganoid scales composed of 3 layers: outer enamel like ganoine, middle dentine like cosmine and inner bony isopedine.
• Lepidosteus has lepidosteoid ganoid scales with only two layers: outer ganoine and inner isopedine.






4. Cycloid scales: Cycloid scales are thin flexible translucent plates, rather circular in outline, thicker in the centre and marked with several concentric lines of growth which cal be used for determining the age of the fish. They are composed of a thin upper layer of bone and a lower layer of fibrous connective tissue. They overlap each other, each scale embedded in a small pocket of dermis. Cycloid scales are found in lung fishes, surviving dipnoans some holosteans and the lower teleosteans such as carps, cods, etc.





5. Ctenoid scales: These are characteristic of modern higher teleosteans such as perch, sunfish, etc. in form, structure and arrangement they are similar to cycloid scales. They are more firmly attached and their exposed free hind parts which are not overlapped, bear numerous small comblike teeth or spines.
Intermediate types between cycloid and ctenoid scales also occur. Certain fishes, such as flounders, may bear both types, ctenoid scales dorsally and cycloid ventrally.




MIGRATION IN FISHES
1. Many fishes like birds perform seasonal migrations. The barracudas and swordfish of the warm (Xiphius gladus) of the warm tropical seas perform latitudinal migration, moving north in spring and south in autumn.
2. Some deep water fishes perform daily vertical migration. Migration is sometimes limited to freshwaters only; it is called potamodromy or limnodromy and sometimes limited within the sea called oceanodromy.
3. However, migration between the two is called diadromy, which is classified into two categories. Movement from fresh water to salt-water (sea) for spawning is called catadromous migration.
4. The most famous example of catadromous fish is the freshwater eel, Anguilla. The reverse movement, that is from salt-water to fresh, is termed anadromous migration. Examples of anadromous fishes are salmon, shad, striped bass, sturgeon, Alosa, Hilsa and some trout.
5. Completely free movement between fresh and marine water without the purpose of breeding is called amphidromous migration, exhibited by fishes like Megalopa, Chanos etc.
1. Eels: (a) The best example of catadromous migration is furnished by two common species of eels, Anguilla rostrata of European fresh water rivers and Anguilla vulgaris of America.
(b) With the advent of autumn, their colour changes from yellow to metallic silver. Feeding stops with the shrinking of their digestive tract.
(c) Eyes become larger, snout becomes sharper with thinner lips and the gonads are fully mature. The silvery eels then enter sea and migrate about 4500 kilomete4rs westwards from Europe or eastwards from America.
(d) Reaching their breeding place in the Sargasso sea off Bermuda, the adults die immediately after spawning in deep waters. The eggs hatch into little transparent, leaf-like flattened pelagic larvae, called leptocephalia, less than 6 mm long.
(e) They have sharp needle-like teeth for feeding. During their long return journey towards homes of their parents, they grow into elvers or glass eels about 8cm long, with cylindrical bodies.
(f) On reaching land, the males remain behind in brackish waters near coasts, while the females ascend freshwater streams and rivers. The elvers feed and grow to become yellow eels in some years. How the elvers without parents are able to find their way across the sea towards homes of their parents remains an unsolved mystery.
2. Salmon: (a) There is a single species of Atlantic salmon (Salmo salar) and 5 species of Pacific salmon (Oncorhynchus). They furnish the best example of anadromous migration.
(b) In winter, both the sexes leave their feeding grounds at sea to ascend the freshwater mountain streams, reaching the identical spot where they originally grew some years before. They stop feeding, change to a dull reddish brown from silver, and excavate shallow saucer-like pits in bottom gravel.
(c) After spawning the adults die, but some of the Atlantic species may survive, return to sea and spawn for a second or third time in life. After hatching the larval fish feed and grow for some time in the streams before going out to sea.
(d) The young salmons grow faster in the ocean because of abundant food there. Experimental evidence shows that strong olfactory sense of salmon determines its homing into the original birth place, for different streams have different odours.