Scoliodon laticaudus: Spadenose shark -Notes for T.Y.B.Sc. Sem VI

Notes: Zoology- Semester VI, University of Mumbai, India Presented by Prof. Sudesh D. Rathod

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SCOLIODON LATICAUDUS

Spadenose shark

Systematic position

Kingdom: Animalia Synonyms

Phylum: Chordata Carcharias muelleri J. P. Müller & Henle,

1839

Class: Chondrichthyes Carcharias palasoora Bleeker, 1853

Subclass: Elasmobranchii Carcharias sorrahkowah Bleeker, 1853

Order: Carcharhiniformes Carcharias sorrakowah Cuvier, 1817

Family: Carcharhinidae Reference Book: ‘On the spadenose shark,

‘SCOLODON LATICAUDUS’

-by R. V. Ranade

Genus: Scoliodon

Species: S. laticaudus

Habit & habitat

Scoliodon laticaudus is marine; brackish; demersal; amphidromous shark. It is typically found in coastal

waters at 10–13 m (33–43 ft) deep, often close to rocky bottoms and in lower reaches of tropical rivers. It is

uncertain, however, if this species can live in perfectly fresh water for extended periods. They form large

unisexual shoals for feeding purpose. It feeds on shrimps, small benthic fish, cephalopods and crabs.

Viviparous, with an unusual columnar placenta; litter size varies from 3 to 14.

Distribution

Indo-West Pacific, Scoliodon laticaudus is found in Eastern Africa near Somalia, Tanzania, Mozambique.

Scoliodon laticaudus is native of Bangladesh; Borneo; Cambodia; China; India; Indonesia; Japan; Macao;

Malaysia; Myanmar; Oman; Pakistan; Thailand; Philippines; Singapore; Bangladesh; Sri Lanka; Taiwan,

Province of China; Thailand; Viet Nam. Apparently absent from Australasia and Oceania. Kasim (1991)

reported that the annual recorded catch of Spadenose Shark in the Veraval coast, India from 1979-1981

averaged 823 tons. These were taken mostly by trawl and gillnet fishing as by-catch. Along Mumbai coast

spadenose are found to about five kilometers area.

External characters

Body moderately stout, fusiform in shape and can be distinguished into head, trunk and tail; color bronze

grey above, white below, fins sometimes darker than body; no conspicuous markings. Head broad, greatly

depressed, eyes and nostrils small. First dorsal fin extends up to or beyond middle of pelvic fin. Pupil vertical

and nictitating eyelid is in anterior and lower corner of eye. Nostrils are closer to mouth than the snout.

Mouth is ventral, crescentic, lined by indistinct upper jaw but a well-defined lower jaw. Gills are five pairs

and without operculum. Two dorsal, a pair of pectoral, a pair of pelvic, one ventral and one caudal fin are

present. Caudal is heterocercal and bears a crescent shaped depression called as precaudal pit. In males the

inner margin of each triangular pelvic fin is rolled forming a finger-like tapering appendage, the clasper


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or myxopterygium. Shark grows maximum about 74 cm, but mostly found smaller in size. Body size at

birth is 12 to 15 cm.

Skin

The integument provides a characteristic form to the body. It also protects the enclosed organs. Skin is very

tough, slimy and firmly attached to underlying muscle coat. Skin is comprised of outer epidermis and inner

dermis. Epidermis is derived from ectoderm and dermis from mesodermal layer of the embryo. The

epidermis has stratified epithelium and consists of basal germinativum layer of cuboidal cells followed by 4-

5 layers of progressively flattened cells and cornification is not complete. The outermost layer forms a

squamous epithelium. In the epidermis may have mucus secreting glands. Unicellular mucous glands are

present in the dermis. Dermis has no layers but is made of scattered cells embedded in connetive tissue, and

contains chromatophores. They are of two types 1) black, stellate and highly branched melanophores 2)

rounded or oval yellowish xanthopohres.

Exoskeleton

Exoskeleton is comprised of tiny, microscopic placoid scales also known as dermal denticles because their

structural organization is similar to that of the vertebrate teeth. Placoid scales cover entire body of shark

except inner margin of the claspers. It is suggested that close placement of placoid scales channel water

resulting in a laminar flow that acts to reduce friction and also make shark hydro-dynamically quiet while

stalking prey. Each placoid scale is made up of a basal plate and a crown. Basal plate is embedded in dermis

and has an opening of pulp cavity on its ventral side which leads to the spines of crown and supply nutrition

from dermis for growth. The tridentate crown projects outside the skin and is protective to skin. The

outermost layer of crown is made of enameloid or vitrodentine material. The inner layer is called as

dentine.


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Arrangement of placoid scales in shark

Structure of placoid scales side view

Endoskeleton

Endoskeleton of Scoliodon laticaudus is formed of axial skeleton and appendicular skeleton.

Axial skeleton includes vertebral column and skull.

Vertebral column of Scoliodon laticaudus is made of amphicoelous type of vertebrae i.e. they have

concavity on either side of the centrum. Vertebrae are divided into two groups, anterior group of trunk

vertebrae and posterior group of caudal vertebrae.

1. Trunk vertebra

It comprises a neural arch around spinal cord, a centrum and a pair of ventro-laterally placed

transverse processes. Transverse processes in trunk vertebrae articulate with cartilaginous ribs

about half an inch length. The top of neural arch is known as neural spine. The vertebral neural

plates are in the adult fused with their respective centra, and are notched behind for the exit of the

ventral (motor) roots of the spinal nerves. The intervertebral neural plates are

1. Neural spine,

2. Neural arch,

3&12. Neural canal,

4 &13. Concave centrum,

5. Focus,

6. Haemal arch,

7. Haemal canal,

8. Haemal spine,

9. vertebral Neural plate,

10&11. Inter-vertebral neural plate,

15&16. Transverse processes


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polygonal pieces alternating with the vertebral neural plates; they are notched behind, but at a more

dorsal level than are the vertebral neural plates, for the exit of the dorsal or sensory roots of the

spinal nerves.

2. Caudal vertebra

In caudal vertebra the transverse processes instead of projecting laterally, are bent inwards beneath

the centrum and meet and fuse in the mid-ventral line to form the haemal arch. Haemal arch

encloses a haemal canal and is projected ventrally as haemal spine, supporting the caudal fin. The

posterior part of vertebral column is bent upward to support asymmetrical tail, known as

heterocercal tail.

3. Skull

Skull consists of a cranium with a flattened floor and a more irregular and incomplete roof. Its sides

are expanded in front owing to the olfactory capsules, and behind owing to the auditory capsules,

while in the middle they are deeply hollowed to form the orbits or eye-cavity.

Dorsal view Ventral view

The dorsal and ventral boundaries of the orbits are respectively formed by the prominent supra-orbital and

suborbital ridges. Posteriorly it has a wide opening, the occipital region. Anteriorly it gives off three long

processes which fuse together terminally to form the rostrum (two dorso-lateral and one ventro-median

cartilages). Behind the olfactory capsules comes a large, nearly circular, hole, the anterior frontanelle above

cerebrum on cranium. Behind above cerebellum chanber is a small dorsal opening called parietal fossa.

Olfactory capsule lies behind rostrum. The olfactory capsules have no ventral walls, and are separated from

one another by the internasal septum. It encloses the olfactory chambers and is open ventrally.

Behind the olfactory capsules comes a large, nearly circular, hole, the anterior frontanelle, slightly behind

which are the two ophthalmic foramina. The dorsal and ventral boundaries of the orbits are respectively

formed by the prominent supra-orbital and suborbital ridges. Behind are the auditory capsules, each of

which is marked by a pair of prominent ridges, which lodge respectively the anterior and posterior vertical

semicircular canals of the ear.

A number of ventral structures disconnected or only loosely connected with the cranium. These together

constitute the visceral skeleton forming the jaws and supporting the gills.


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Appendicular Skelton of shark includes pectoral and pelvic girdles

1. Pectoral Girdle

Two pectoral girdles are attached to the coracoid bar

on either side on the ventral side of the shark just

behind the gills. The paired scapular cartilages, and

their extensions, the suprascapular cartilages, extend

dorsally from their base at the lateral ends of the

coracoid bar, near the fin, on either side of the body in

the muscular wall. The glenoid surface is the surface of

articulation between the fin base and the coracoid bar.

Proximally fin is comprised of propterygium,

mesopterygium and metapterygium from outer to inner

side which articulates distally to their corresponding

radials. outline of the distal part of is supported by

horny fin-rays called as ceratotrichia.

2. Pelvic Girdle

Pelvic girdle is present in the posterior side of the body to which the pelvic fins are attached. The pelvic

girdle is connected directly to the vertebral column in the sacral region. The pelvic girdle has two equal

halves which are known as 'ossa innominata'. Each as innominatum is formed by three bones. They

are the dorsal bone ilium, the ventral bone ischium and the antero-ventral bone pubis. The pelvic girdle

has a depression at the junction of the three bones. It is termed as acetabulum into which the head femur

of the hind limb articulates and forms a ball and socket joint. Form acetabulum it projects a long curved

shaft, basipterygium. Basipterygium gives out a series of radiales. Posteriorly basipterygium is attached

with claspers in male shark.

1. Ischio-pubic bar of pelvic girdle

2. Basipterygium

3. Clasper

4. Radiale

Fig. 8. Dorsal view of the pelvic girdle and fins of a male Scoliodon laticaudus


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Digestive system

Buccal cavity is the area enclosed by the jaws (mandibular arch) and the cartilage of the throat (hyoid arch).

The triangular sharp teeth are arranged in several rows beginning at the outer edges of the upper and lower

jaws. Behind the functional teeth are additional rows folded downward ready to replace any that are lost.

The pharynx is the portion of the alimentary canal posterior to the hyoid arch and has five pairs of gill and

also contains the tongue in dogfish. The shark can bring water into its pharynx to the gills by way of the

spiracle and mouth. The raised floor of the oral cavity is referred to as the tongue. The tongue of the shark is

practically immovable and without muscles. It is

supported anteriorly and posteriorly by cartilage and

hence referred as false tongue. Posteriorly it narrows

to form the esophagus.

The esophagus is the short hollow tube connecting the

pharynx to the stomach. The esophagus is the thick

muscular tube extending from the top of the cavity

connecting the oral cavity and pharynx with the

stomach. The esophagus is lined with papillae, which

form a tight seal to keep water out.

The esophagus leads into the "J"-shaped stomach. The

upper portion, the cardiac region, continues as the

main body, and ends at the duodenal end. The mucosa

is the inner lining of the stomach. The rugae are

longitudinal folds that help in the churning and mixing

the food with digestive juices. A circular muscular

valve, the pyloric sphincter, is located at the far end

or pyloric end of the stomach. It regulates the passage

of partially digested food into the intestines.

The pancreas is located on the duodenum and the

lower stomach. The secretions of the pancreas enter the duodenum by way of the pancreatic duct. The liver

is the largest organ lying within the body cavity. Its two main lobes, the right and left lobes extend from the

pectoral girdle posteriorly most of the length of the cavity. A third median lobe much shorter lobe and

contains the green gall bladder along its right edge. The dark, triangular-shaped spleen is located near the

posterior end of the stomach. Although a part the lymphatic system, the spleen is closely associated with the

digestive organs in all vertebrates.

The small intestine is composed of, anteriorly, a duodenum, and posteriorly, an ileum (valvular intestine).

The duodenum is a short "U"-shaped and connects the stomach to the intestine. The bile duct from the gall

bladder enters the duodenum. The valvular intestine is the second and much larger portion of ileum, which

contains a scroll valve. The scroll valve is the longitudinal fold hanging from roof of ileum which is coiled

within the valvular intestine. It adds surface area for digestion and absorption to an otherwise relatively short

intestine.

The large intestine, rectum or colon, a shorter section than the small intestine has a rectal gland entering it.

The rectum is the narrowed continuation of the valvular intestine. It is located at the posterior end of the


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body cavity. The rectum, the most posterior portion, ends in the anus which projects into the cloaca, a

common opening with the urogenital ducts.

The rectal gland is a slender, blind-ended, finger-like structure that leads into the rectum by means of a duct.

It has been shown to excrete salt (NaCI) in concentrations higher than that of the shark's body fluids or sea

water. It is thus an organ of osmoregulation, regulating the shark's salt balance.

The cloaca is the last portion of the alimentary canal. It collects the products of the colon as well as the

urogenital ducts. It is a catch-all basin leading to the outside by means of the cloacal opening.

Respiratory system

In Scoliodon respiration takes place through 5 pairs of gill slits or gill pouches. They are present in a series

on the wall of pharynx on either lateral side, behind the hyoid arch. Each gill pouch opens into the pharynx

by a large branchial aperture and to outside through an external branchial aperture or gill slit. Two adjacent

gill pouches are completely separated by a vertical fibro-muscular partition, the interbranchial or gill

septum. 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 epithelium and contains blood vessels,

nerves etc.

Gill chamber of shark Respiratory system of shark

The mucus membrane of a septum is raised into numerous horizontal leaf like folds called gill lamellae or

gill filaments. 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 hemibranch. A gill pouch thus contains two hemibranchs belonging to two different adjacent

gills. These two hemibranchs with their interbranchial septum and the visceral arch constitute a complete gill

or holobranch. The posterior hemibranch of each holobranch is larger than the anterior one. In Scoliodon,

the hyoid arch bears only a hemibranch posteriorly.

The first four branchial arches bear a holobranch each. Thus scoliodon has total nine hemibranchs while the

fifth branchial arch is devoid of gills and is called abranch.

The inner part of each interbrachial septum has a supporting visceral arch from which cartilaginous gill rays

arise in a single row and project into the interbranchial septum for further support. Visceral arches also give

out rigid comb-like gill rackers, which project the internal branchial apertures from food.


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Blood vascular system

Sharks have a two-chambered heart, with an atrium (also called the auricle) and a ventricle. The heart is an

S-shaped tube that is located in the head region of the shark. It is located at ventrally in thoracic pericardial

cavity lined by a thin pericardium. The blood is pumped by the heart through the afferent branchial arteries

(ventral aorta) to capillaries in the gills (where the blood is oxygenated). The blood then flows through

efferent branchial arteries (paired dorsal aorta), then through the tissues of the body, and then back to heart in

veins.

1. Heart of shark

The shark heart is two chambered heart having single atrium and a single ventricle. The heart of a

shark contains four important parts, the atrium and ventricle, the sinus venosus, and the conus

arteriosus. The sinus venosus is a small sac with thin walls which collects deoxygenated blood from

the fish’s veins, after which the blood flows into the atrium, a large muscular chamber. The atrium is

a one-way compartment for blood to flow into the ventricle (which does the pumping for the heart).

The bulbus arteriosus is a large tube into which the blood is pumped from the ventricle. The Conus

arteriosus then leads the blood to the ventral aorta, through which blood flows to the shark’s gills

2. The Aortic arches

The anterior end of the conus arteriosus continues forward as the ventral aorta. It gives off five

pairs of afferent branchial arteries which carry deoxygenated blood from the heart to the gills.

Each arterial collector loop is formed from the union of a pre-trematic artery from the anterior

side of a gill slit with a post-trematic artery from the posterior side of the gill slit. The pre and post-

trematic arteries receive tiny branches from the adjacent gill lamellae and both join together to give

rise to an efferent branchial artery. The four pairs of efferent branchial arteries collect

oxygenated blood from the gills. The four pairs of efferent branchial arteries join at the mid-dorsal

line to form the large dorsal aorta.

3. Arterial system:

Internal carotid artery Supplies brain; Dorsal aorta supplies entire trunk; Hyoidian artery supplies

carotid arteries; Pharyngoesophageal artery supplies esophagus and pharynx; Subclavian artery

supplies pectoral fin; Coeliac artery supplies gonads and entire digestive tract; Gastric artery supplies

stomach; Hepatic artery supplies liver; Ovarian artery/testicular artery supplies gonads;

Pancreaticomesenteric artery supplies pancreas and intestine; Anterior intestinal artery supplies

intestine; Anterior mesenteric artery supplies intestine; Gastrosplenic artery supplies spleen,

stomach, and pancreas; Iliac artery supplies pelvic fins.


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1. Venous system:

Hepatic portal vein drains entire nutrient rich blood from digestive tract to liver which break into

capillaries; Gastric vein drains stomach; Pancreaticomesenteric vein drains intestine, spleen, and

pancreas; Anterior intestinal vein drains intestine; Anterior lienogastric vein drains spleen and

stomach; Leinomesenteric vein drains pancreas; Posterior intestinal vein drains intestine;

Posterior cardinal vein drains kidneys; Posterior cardinal sinus drains entire trunk.

Nervous system

1. Brain

Prosencephalon (forebrain) - subsequently divides into the telencephalon (cerebrum) &

Diencephalon (epithalamus, thalamus, & hypothalamus)

Mesencephalon (midbrain) - develops without further subdivision & forms the tectum

Rhombencephalon (hindbrain) - subdivides into the metencephalon (pons & cerebellum) and

myelencephalon (medulla oblongata)

a. Forebrain: Sharks have very well developed olfactory lobes, a mid-brain with a thalamus,

hypothalamus, pituitary gland, amygdala and a large pallium (forerunner of the cortex)

with well-defined basal ganglia, although the hippocampus has not much differentiated.

b. Midbrain: There are two large optic lobes situated on top of the midbrain.

c. Hindbrain: The hind-brain has a well developed brainstem with a large, elongated

cerebellum. Posteriorly it gives out a pair of auricles of cerebellum and medulla

oblongata which encloses fourth ventricle; and finally gives out a dorsal tubular spinal

cord.


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2. Cranial nerves

Terms Origin Definitions

Olfactory nerve Olfactory lobe Cranial nerve I, terminates in the olfactory bulb

Optic nerve Optic lobe Cranial nerve II, arises in the retina and extends to the diencephalon and then to the optic lobes

Oculomotor nerve Optic lobe Cranial nerve III, innervates 4 eye muscles (inferior oblique, superior, inferior and medial recti)

Trochlear nerve Betn. Optic & cerebellum

Cranial nerve IV, innervates the superior oblique muscle

Trigeminal nerve Internal surface of medulla

Cranial nerve V, (3 branches): opthamalic, maxillary, mandibular

Abducens nerve Ventral surface of medulla

Cranial nerve VI, innervates the lateral rectus

Facial nerve Common with trigeminal

Cranial nerve VII, (3 branches) arises from the medulla and base: superficial ophthalmic, buccal nerve, hyomandibular

Statoacoustic (Auditory)

Medulla

Cranial nerve VIII innervates the inner ear by means of a vestibular branch to the anterior part of the ear and a saccular branch to the posterior part

Glossopharyngeal nerve

Medulla Cranial nerve IX, has multiple branches, innervates the the third visceral arch

Vagus nerve Medulla Cranial nerve X, gives off lateral line trunk and four branchial rani to each of the remaining gills

Accessory

Medulla

Cranial nerve XI, accessory to vagus….Motor

Hypoglossal

Medulla

Cranial nerve XII, tongue muscles ………….Motor


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Receptor organs of shark

1. Skin (general cutaneous sense organs):

Skin has numerous free nerve endings enabling shark for the reception of sense of touch, pain and

temperature in the exterior. There are some touch corpuscles present at the bases of the fins.

2. Lateral line system (LL) and neuromasts:

LL system includes a network of canals and sense organ called as neuromast. Two longitudinal

canals run laterally along the flanks of the body from the tip of the caudal fin towards the anterior of

the shark. These lateral longitudinal canals divide into cephalic and orbital canals in the head

region. Orbital canals re-divide into supra-orbital and infra-orbital canals. Both supra orbital and

infra-orbital canals divide further and from a network on the head. Longitudinal canals also unite by

a transverse commissural occipital canal at the junction of head with trunk.

Neuromasts are special sense organs present in aquatic organisms. They are arranged in a series

along the LL canals. Each neuromast is a spherical or flask like hollow body located in a fibro-

cartilage. Internally it has a layer of connective tissue except its base. At the base it contains a lower

layer of sensory cells (hair cells) embedded in supporting cells and an upper layer made of

vacuolated gelatinous cells called as cupula. Cupula remains in close contact with the sensory cells.

Each sensory cell has sensory projections, a kinocilia and few stereocilia. When water enters in

canal system cupula displaces and bends kinocilia sending the impulses to axons attached to sensory

cells.


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3. Electroreceptors (ampullae of Lorenzini)

The ampullae of Lorenzini are small vesicles that form part of an extensive subcutaneous sensory

network system. These vesicles are found around the head of the shark. These are modified LL

organs and respond to detect weak magnetic fields produced by other fish at short ranges. This

enables a shark to locate prey buried in the sand or to orient to nearby movement. Each ampulla is a

bundle of sensory cells innervated by several nerve fibers. These fibers are enclosed in a jelly-filled

tubule that has a direct opening to the surface through a pore. These pores on the head of the shark

are visible to the naked eye, and appear as dark spots. These are two type ampullary and tuberous

receptors.

Ampulla of Lorenzini has a flask like ampullary sac with a long tube and is filled with gelatinous

substance. There are few chambers in the ampullary sac containing sensory cells which are

connected to neurons ventrally. A series of ampullae of Lorenzini are embedded in connective tissue

are mostly found in the head region. Tuberous organs lack the ampullary sac. In the marine water a

weak electric field is generated naturally in number of ways are received by shark to differentiate

between the objects event in the dark.

4. Internal ear

Only internal ear is present in shark. It is also called as

membranous labyrinth. Labyrinth is enclosed in auditory

capsule of the cranium floating in perilymph. It is made of

a vertical flattened utriculus and ventral sacculus.

Utriculus has outgrowths forming three semicircular

canals lying perpendicular to each other namely anterior,

posterior and horizontal canal. Sacculus gives out a

posterior outgrowth called lagena. The cavity of entire


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labyrinth is filled with fluid called as endolymph. Membranous labyrinth has four functions i.e.

controls musculature; change in direction during swimming; to regulate depth and sound reception.

5. Olfactory sac

There are two olfactory organs/sacs/chambers. Each olfactory organ is located in olfactory capsule of

skull which is guarded by a nasal fold. There is an incurrent opening placed anteriorly and an

excurrent canal posteriorly. Olfactory chamber has about 40 parallel, vertically arranged olfactory

folds or ridges internally. Each olfactory ridge gives out numerous lateral alternating leaflets like

outgrowths lined by olfactory epithelium. These outgrowths increase the surface area. The

epithelium is lined by olfactory chemoreceptor cells. Sense of smell is acute in shark at a capacity

to detect upto 1 ppm dilution (1 part in million parts of sea water). Taste buds are also similar to the

olfactory receptors but they are not distant field chemoreceptores.

6. Eye: In Scoliodon laticaudus have laterally

positioned eyes. Scoliodon laticaudus is

considered to have generally small eyes

compared to body size. The outer layer of the

shark eye comprises a thick cartilaginous

sclera. Shark eyes need to handle wide water

pressure variations, as they range from shallow

to deep water looking for food. Shark eyes

have a large, spherical lens, a cornea, a retina

(with both rods and cones), an iris, and a pupil.

The retina has a greater proportion of rods

(light intensity sensors) than cones (color

sensors), so Scoliodon laticaudus very sensitive

to small differences in light intensity (dark

versus light). They even have good vision in

dim light. Scoliodon laticaudus has a mirror-

like layer in the back of the eye, the tapetum

lucidum. This layer doubles the intensity of incoming light, enhancing light sensitivity. Unlike other

fish, shark's pupils can dilate and contract to control the amount of incoming light. Scoliodon has a

nictitating membrane, a type of eyelid that protects the eye during hunting. External eyelids are

immovable.


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Urogenital system

1. Male urogenital system:

The kidneys are flattened; ribbonlike, dark colored structures lying dorsally on either side of the midline,

along the entire length of pleuroperitoneal cavity and deeply placed dorsally. The upper portion of kidney is

rounded known as head of kidney is an active part of reproductive system. The male reproductive system

begins in the testes. These sex organs are located in the dorsal side of the shark next to the liver; however, the

testes are noticeably larger than the ovaries. The size of the testes, however, varies in size depending on the

month of the year (they are larger during the breeding months of January and February). The function of the

testes is to produce sperm that will later be used to fertilize the female ova. The sperm is taken from the

testes to the Archinephric Duct by few small tubes known as Vasa efferentia. The vasa efferentia dilate to

form a vide vas defferens transports sperm to the Seminal Vesicle. The Seminal Vesicle is directly attached

to the spiralled Archinephric Duct and is noticeably larger and straight. The Seminal Vesicle leads to the

Sperm Sac. The Sperm Sac is the last stop before excretion in the male reproductive system. The Sperm Sac

contains seminal fluid that will allow the sperm to pass safely to the female and fertilize the ova. The entire

embryonic developmental stage occurs in the female. For this reason, males must have developed certain

structures that allow them to fertilize the female egg (equivalent to the male penis). This structure is known

as the Clasper. The Clasper allows the male shark to hold on to the female shark during sexual reproduction.

Each Clasper posses a Clasper Groove, allowing the Clasper to bend at possible awkward angles. Also part

of the Clasper structure is the Siphon Sac. All of these structures are essential during sexual reproduction.

To start off sexual reproduction, the Siphon Sac absorbs some seawater. This seawater will help to propel

the seamen from the male into the female.

2. Female urogenital system:

The kidneys are flattened; ribbonlike, dark colored structures lying dorsally on either side of the midline,

along the entire length of pleuroperitoneal cavity. In female the upper portion of kidney is non-functional.

The female reproductive system begins in the ovaries. The ovaries are where the egg or ova is produced and

are located on the dorsal side of the shark, just next to the liver. Every egg yellow colored and is enveloped

in a follicle. The color is a result of the large amount of yolk in each ovum. From here, the ovum leaves the


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ovary, heading for the oviduct. The oviduct is a tube-like structure where the ovum is fertilized by the male

sperm. It is clearly visible in mature females due to its large size. The oviduct is connected directly to the

cloaca, allowing for sperm to pass directly to the ovum. Different areas of the oviduct have been shown to

perform different functions. One area of the oviduct that has been enlarged (distal to the ovaries) is knows as

the nidamental or shell gland. This gland has been shown to store sperm for great lengths of time to allow

for fertilization later. The oviduct thins out again, only to enlarge once more in a more caudal location to

form what is known as the uterus. The uterus is the location of embryonic development. The opening is

located towards the dorsal portion of the cloaca.

Copulation, fertilization and development

This shark is placentally viviparous, with arguably the most advanced reproductive mode of the

elasmobranchs. Male dogfish reach maturity at 11 years while female dogfish reach maturity at 19-20 years.

Eggs are ovulated at only 1 mm in diameter and the stalked placenta forms when the embryos are only a few

millimeters in length. The young are born at a length of 12-15 cm. Males mature at 24-36 cm and females at

33-35 cm. Breeding occurs throughout the year and females probably mate at least once each year.

Viviparous, with an unusual columnar placenta; litter size varies from 1 to 14. [Litter sizes range from 6-

18, with a mean of 13.] The young are born throughout the year, after a gestation period of five or six

months.

Economic importance

Although dogfish liver oil was once valued for its vitamin A content and dogfish supported a thriving fishery

off Washington state and British Columbia, they are now rarely landed by North American fisheries.

However, substantial quantities are marketed for human food in Europe and Asia.

When abundant, dogfish may become a serious pest and inflict severe economic losses on fisheries by

damaging fishing gear and the catches of the more desirable fish. Furthermore, huge schools of dogfish

sweeping over the sea bottom devour and all but eliminate the smaller fishes and invertebrates in their paths.

Despite its commercial importance, overall fishery statistics for the spadenose shark are lacking. A 1996

report found it to be the most common coastal shark on Chinese markets, and it is also one of the most

common sharks caught off northern Australia. Substantial numbers are caught by Indian and Pakistani

fisheries; from 1979 to 1981, an average of 823 tons were caught annually off Verval, India. The spadenose

shark is also caught as bycatch, particularly in gillnet fisheries off Kalimantan.

Made by

Mr. S. D. Rathod

Associate Professor

Department of Zoology

B. N. Bandodkar College of Science

Thanr-400605

Scoliodon laticaudus: Spadenose shark -Notes for T.Y.B.Sc. Sem VI

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Transcript of Scoliodon laticaudus: Spadenose shark -Notes for T.Y.B.Sc. Sem VI