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

Scoliodon laticaudus a spadenose shark is for the TYBSc course Semester-VI – USZ0601of University of Mumbai

1. 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

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

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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.

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

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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

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

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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.

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

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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

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

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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

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

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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.

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

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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.

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

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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.

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

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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

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

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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.

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

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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

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

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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.

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

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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

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

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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