Rohu: Systematic Position, Distribution and Structure | Bony Fish

In this article we will discuss about Rohu:- 1. Systematic Position of Rohu 2. Habit and Habitat of Rohu 3. Geographical Distribution of Rohu 4. External Structures of Rohu 5. Skeletal Structures of Rohu 6. Coelom of Rohu 7. Digestive System of Rohu 8. Hydrostatic Organ of Rohu 9. Respiratory System of Rohu 10. Circulatory System of Rohu 11. Venous System of Rohu 12. Nervous System of Rohu 13. Urinogenital System of Rohu.

Contents:

1. Systematic Position of Rohu
2. Habit and Habitat of Rohu
3. Geographical Distribution of Rohu
4. External Structures of Rohu
5. Skeletal Structures of Rohu
6. Coelom of Rohu
7. Digestive System of Rohu
8. Hydrostatic Organ of Rohu
9. Respiratory System of Rohu
10. Circulatory System of Rohu
11. Venous System of Rohu
12. Nervous System of Rohu
13. Urinogenital System of Rohu

1. Systematic Position of Rohu:

Phylum Chordara:

Subphylum Vertebrata (= Craniata)

Series Pisces

Class – Teleostomi

Subclass – Actinopterygii

Order – Cypriniformes

Family – Cyprinidae

Scientific Name Labeo rohita

Common Name:

English – Rohu

Vernacular Names

Bengali – Rui

Oriya – Rohu

Assam – Rohiti, Rui

Hindi – Rohu

Punjub – Tapra, Rohu

Marathi – Tambadamassa

Gujarati – Rohu

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2. Habit and Habitat of Rohu:

It is abundantly found in fresh water in ponds, lakes, rivers and reservoirs. It is chiefly vegetarian and bottom feeders but young fry feed on zooplankton. It breeds in June and July in running water.

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3. Geographical Distribution of Rohu:

Labeo rohita is found in tropical and tem­perate regions. It is the commonest cap in the plains of India, except in the south. It is also common in Bangladesh and Myanmar (Burma). Labeo rohita, commonly called the Rohu fish, is one of the typical fresh-water bony fish­es of India. This particular fish is studied as the type specimen of bony fishes in many Indian Universities.

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4. External Structures of Rohu:

The Rohu fish has a spindle-shaped body measuring up to 1 m in length and weighing about 20-25. kg. The dorsal side of the body is blackish in colour and the ventro-lateral sides are silvery. The body, like that of Bhetki, is dis­tinguishable into a conspicuous head, trunk and postnatal tail(Fig. 6.19).

The head extends from the snout up to the posterior margin of the operculum. The snout is depressed and projects beyond the jaws. Labeo rohita is characterised by having no lateral lobes in the snout. Two nostrils are present on the dorsal side of the snout.

The mouth is a crescentic transverse opening bounded by thick fringed lips. Teeth are absent in the jaws. The eyes are prominent and are lidless. One or two pairs of barbels are present on the dorsolateral sides of the mouth. The maxillary barbels are relative­ly short and delicate that the rostal barbels.

The trunk is elongated and oval in cross-sec­tion. It is covered over by thin overlapping cycloid scales. The lateral line runs along the lateral sides of the body. The scales along the lateral line contain pores which are connected with a tubular canal. The vent is situated ventrally and just in front of the anal fin.

Fins:

Both paired and unpaired fins are well- developed. The pectoral and pelvic fins are borne by the respective girdles. The pectorals are located at the anterolateral side of the trunk behind the operculum. Each pectoral fin is sup­ported by 19 fin rays. The pelvic are situated on the ventral side behind the pectorals.

Each pelvic fin contains 9 fin-rays. There is only one dorsal fin in Rohu which arises from the mid- dorsal line of the trunk half-way between the snout and the base of the tail. The anal fin lies posterior to the anus. The dorsal fin consists of 13 fin-rays and anal fin has 4-6 fin-rays. The tail fin is homocercal with two symmetrical lobes. Several fin-rays support the tail fin.

Chromosome Number:

The diploid chromosome number is 52 in Labeo rohita.

Skin:

The integument is covered by an outer soft epidermis and an inner dermis. The epidermis is composed of epithelial cells with numerous unicellular mucous glands. The musculature of the body wall lies below the integument. The disposition of the body wall musculature is similar to that of other fishes.

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5. Skeletal Structures of Rohu:

The endoskeleton of Rohu described by D. S. Sarbahi in 1932. The scales and fin-rays constitute the exoskeleton of the Rohu fish.

The endoskele­ton is completely ossified and consists of:

(a) An axial skeletal portion and

(b) An appendicular skeletal part.

Axial skeleton:

The axial skeleton is com­posed of the skull, vertebral column with the ribs and the skeletal elements supporting the median fins.

Skull:

The skull of Rohu has a very com­plicated structural organisation. Many inves­ting and replacing bones participate in the for­mation of the skull (Fig. 6.20). It is composed of the cranium, the sense capsules and the vis­ceral arches.

The cranium and the sense cap­sules are immovably united together while the visceral arches are loosely attached with the skull. Basically the skull is composed of a pos­terior basai plate and anterior trabecular region. The auditory capsules are united with the basal plate and the nasal capsules are attached with the trabecular region.

The lateral walls are mostly incomplete and are derived from the orbital cartilage. This carti­lage joins posteriorly with the auditory capsule and the nasal capsule on the anterior end. The palate quadrate (visceral arch) articulates ante­riorly with the trabecular region of the skull by a basal process and posteriorly with the audi­tory capsule by an otic process.

The skull in adult fish assumes an elonga­ted shape.

It is broadly divided into:

(a) A dorsal roof,

(b) A posterior occipital region,

(c) The otic region consisting of the bones of the auditory capsules,

(d) An orbitotemporal region and

(e) an anterior nasal (ethmoidal) region.

The dorsal side is more or less convex. A shallow supra-temporal groove is present on its posterodorsal side. This groove extends posterola­teral towards the main occipital spine. The posterior wall or the occipital region of the skull bears three apertures, a median foramen mag­num and two large oval fenestrae.

Each fenestra pierces the exoccipital bone and forms a char­acteristic feature of the cyprinoid skull. The occipital region is composed of a supraoccipi­tal, a basioccipital and a pair of exoccipital bones. The main posterior part of the skull is formed by the supraoccipital bone which does not form the dorsal boundary of the foramen magnum in Rohu.

It is divided into dorsal and posteroinferior portions. The wider anterior end of the dorsal portion is overlapped by the pari­etal bones. A median vertical occipital spine is present on the dorsal portion of the supraoccip­ital. The posteroinferior portion of the supraoc­cipital is composed of the occipital spine (or keel).

The exoccipitals are large bones, each consisting of:

(a) A basal plate,

(b) A paroccipital process and

(c) A small dorsal process.

The basal plate forms part of the floor of the cranial cavity and the paroccipital process forms the posterior boundary of the auditory capsule and the side wall of the cranial cavity. The dorsal process encloses the foramen magnum.

The basioccipital is a large bony piece and is roofed over by the occipital condyle. A deep depression is present on the posterior surface of the occipital condyle. A deep depression is pre­sent on the posterior surface of the occipital condyle. A large oval masticatory process is borne by the ventral surface of the basioccipital.

The otic region is represented by the paired auditory capsules each situated on the posterior side of the skull and lies between the seventh and the ninth cranial nerves. Each auditory capsule is derived from an otic carti­lage growing round the internal ear.

The otic cartilage is transformed into the prootic, epiotic, sphenotic, pterotic and opisthotic bones in other teleosts. But in Rohu, the opisthotic bone is lacking and the other four bones form a compact inverted cup-like structure. The orbit temporal region of the skull is composed of the temporal (or sphenoidal) region and the orbit.

The temporal region is subdivided into:

(a) The parietal region and

(b) The frontal region.

The parietal region is made up of the parietals, alisphenoids and Para sphenoid. The frontal region includes the frontals, orbitosphenoids and Para sphenoid. Besides these bones, the supra-temporal is located at the posterolateral angle of the skull.

The nasal (or ethmoidal) region of the skull comprises of the bones which develop in relation to the nostrils and the snout. The par­ticipating bones of this region are the paired nasals, ectoethmoids and lacrymals, a median mesethmoid, a vomer and a rostral. Of these bones, the mesethmoid ectoethmoid and ros­tral are replacing bones, while the nasals, lacrymals and the vomer are all investing bones.

Visceral skeleton:

The visceral skeleton consists of a series of seven half-hoops encir­cling the pharyngeal wall. The half-hoops of two sides unite with each other along the mid- ventral line forming seven visceral arches. All the visceral arches are united with one anoth­er mid-ventrally to form a basket-like visceral skeleton. The first visceral arch is called mandibular arch, the second is the hyoid and the rest five are the branchial arches.

Of the five branchial arches, the four support the gills while the fifth one forms the inferior pharyn­geal bones. The inferior pharyngeal bones develop into the masticating plates armed with large teeth. The mandibular arch is divided into a dorsal palatopterygoquadrate bar and a ventral Meckel’s cartilage forming the primary lower jaw.

The palatopterygoquadrate beco­mes closely associated with the cranium and forms the primary upper jaw. The primary upper jaw becomes ossified by the following replacing bones, palatine, meta-pterygoid and quadrate.

Two investing bones premaxilla and maxilla support the anterior margin of the mouth and together form the secondary upper jaw. Each half of the lower jaw is composed of a small articular, a large dentary and a small angular.

Two dentaries unite in the middle line. The hyoid arch is also divided into two parts: the upper hyomandibular and lower hyoid cornu. The hyomandibular forms the suspensorium by which the jaws remain sus­pended to the cranium.

Many investing bones are connected with the hyoid arch and support the operculum (Fig. 6.21 A). The bones of the operculum are opercular, pre-opercular, sub-opercular and inter-opercular. Each branchial arch is ossified by four replacing bones, pharyngobranchial, epibranchial, ceralobranchial and hypobranchial.

Vertebral column:

The vertebral column is a completely ossified structure and com­posed of 37-38 vertebrae (Fig. 6.21 B). The vertebrae are of amphicoelous (i.e., both the ends of the centrum bear concavity) type.

The vertebral column is distinguishable into the following parts:

(a) An anterior trunk region consisting of 21 trunk vertebrae (Fig. 6.21 C) and bearing movable ribs and

(b) A posterior caudal region. The vertebrae of the caudal region lack ribs and possess haemal arches (Fig. 6.21 D).

The first four trunk verte­brae are greatly altered since these vertebrae connect the swim-bladder with the internal ear. The last 3 or 4 trunk vertebrae bear posteroventral processes. The last three caudal vertebrae are modified for the support of the caudal fin.

The posterior most caudal vertebra is transformed into an upturned rod-like urostyle. It is a solid structure with a ventral groove wherein the proximal ends of the hypourals fit.

Typical trunk vertebra:

A typical trunk vertebra is composed of a deeply biconcave centrum (Fig. 6.21 C). In the embryonic stage the concavities are communicated by a nar­row notochordal canal perforating the body of the centrum. The notochordal canal becomes closed in an adult. The edges of the centra are united by connective-tissue liga­ments and the spaces enclosed by the verte­brae are filled with the remains of noto­chordal elements.

A pair of backwardly directed processes arising from the antero­lateral borders of centrum, enclose the spinal cord and unite above to form the neural arch. The neural arch gives a long dorsal back­wardly directed neural spine. A pair of small blunt processes, the prezygapophyses, is present anteriorly at the base of the neural arch.

Another pair of postzygapophyses arise from the posterolateral edges of the vertebra. The postzygapophyses are pointed upwards and backwards. A pair of short parapophyses originating from the ventrolateral surfaces of the centrum is directed downwards. The ribs are attached with the parapophyses by ligaments.

Typical caudal vertebra:

The caudal part of the vertebral column is composed of 16 or 1 7 caudal vertebrae.

Like that of a trunk verte­bra (Fig. 6.21 D), a typical caudal vertebra has:

(a) An amphicoelous centrum with a median dorsal, a medium ventral and two lateral depressions,

(b) A neural arch with a long backwardly directed neural spine and

(c) Artic­ulating processes like pre- and post­zygapophyses, are present, in the same posi­tion.

From the anterolateral margins of the centrum, a pair of backwardly directed pro­cesses meets in the mid-ventral line to form a canal for placement of the caudal blood ves­sels. This process is known as the haemal arch. This arch produces a backwardly direc­ted haemal spine.

A pair of small blunt anteroventral processes is present at the bases of each haemal arch. Similar processes on the posterolateral side of the centrum are present. These posteroventral processes are directed downwards and backwards as seen in posterior trunk vertebrae.

Median fin skeleton:

The skeleton suppor­ting the median fin consists of a series of:

(a) Somactidia or endo-skeletal radials and

(b) The dermotrichia or dermal fin-rays (Fig. 6.21 E).

The somactids are parallel bony rods lying embedded within the body muscles. Each somactid is divided into a proximal, a mesial and a distal segment. The dermotrichia support the fold of the fin. In Rohu, the dermo­trichia are branched and jointed and are usu­ally called lepidotrichia. Besides these fin- rays, delicate horny-rays (actinotrichia) are present at the free edges of the fins.

The dorsal fin is supported by lepi­dotrichia. There are fifteen or sixteen lepi­dotrichia seated on fourteen radials. The prox­imal segment of each radial is enlarged and dagger-shaped. These are called interspinous bone or axonost. The median segment is short and the distal sector is greatly reduced. The anal fin bears a series of eight fin-rays support­ed by seven radials.

The first six are well- formed. The caudal fin is supported by a num­ber of flattened bony rods. Two epiurals and a radial are present on the dorsal side of the urostyle while nine hypourals are present on the ventral side. The fin-rays are attached with the hypourals and epiurals in two symmetrical halves.

Ribs:

The pleural ribs are segmentally disposed paired slender bony rods. The ribs are attached with the distal ends of the para­pophyses. There are seventeen pairs of ribs and the first pair are attached with the para­pophyses of the fifth trunk vertebrae. The ribs are present between the muscles and the peri­toneum and encircle the abdominal cavity.

Besides the ribs, there are series of rib-like Y- shaped inter-muscular bones which support the connective-tissue septa or myocommas. The inter-muscular bones originate from the neural arch of all the vertebrae composing the verte­bral column.

Appendicular skeleton:

The appendicular skeleton includes the supporting structures of the paired fins and the corresponding girdles (Fig. 6.22).

Pectoral girdle:

The pectoral girdle is situ­ated immediately behind the last branchial arch. It consists of a reduced ‘primary’ endo-skeletal and well-formed secondary der­mal girdle. The primary girdle is composed of two lateral halves, each is transformed into three replacing bones, scapula, coracoid and mesocoracoid (Fig. 6.22A).

The two halves of the girdle do not meet in the mid-ventral line. The secondary dermal girdle consists of inves­ting bones. Each side of this girdle is made up of a cleithrum (or clavicle) a supracleithrum, a post-temporal and a post-cleithrum. The dermal girdle is attached with the pterotic process of the skull by the post-temporal.

The scapula is a ring-lime bony piece with a large scapular foramen for the impass of the branchial artery and the nerve. The coracoid is an irregular triangular bone lying internal to the scapula but ventral to the mesocoracoid. The mesocoracoid is an inverted Y-shaped bone.

The coracoid and the scapula parti­cipate in the formation of the glenoid articula­tion to which three of the four radials of the pectoral fins are movably attached. The cleithrum of the secondary pectoral girdle is the largest bone and it completely covers the primary pectoral girdle.

The posterior inner surface of the cleithrum is connected with a stout curved bone, the post-cleithrum. The supracleithrum is an elongated dagger-shaped bone which covers the dorsal end of the cleithrum. The supracleithrum is articulated at its dorsal end with a minute conical post- temporal bone which in turn remains attached with the supra-temporal bone.

Pectoral fin:

The pectoral fin is supported by nineteen lepidotrichia attached with four radials. The radials are articulated directly with the scapula.

Pelvic girdle:

The pelvic girdle is located anterior to the anal fin. It consists of two simi­larly constructed halves. Each half is mostly composed of a large osseous pelvic bone with a small cartilaginous rod attached to the pos­terior end of the pelvic bone (Fig. 6.22B). The pelvic bone is distinguishable into an anterior elongated part and a posterior stout rod-like part.

The anterior part bears a ventral deep groove and its frontal end is forked. The forked end is connected with the ribs of the twelfth trunk vertebra by ligaments. The posterior rod like parts of the two halves of the pelvic girdle is united with the middle line.

Pelvic fin:

Each pelvic fin is supported by nine fin-rays and three radials. The fin-rays are attached to the radials proximally and the radi­als are articulated with posterior border of the pelvic bone. The first two radials bear two fin-rays while the rest are borne by the third radial.

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6. Coelom of Rohu:

The coelom is lined with peritoneum and divisible into an anterior pericardial cavity containing the heart and a posterior perivis­ceral cavity accommodating the main viscera.

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7. Digestive System of Rohu:

The digestive system is composed of an extremely long alimentary canal and associa­ted digestive glands (Fig. 6.23). The descrip­tion of this system was presented by D. S. Sarbahi in 1939. This particular fish is herbi­vorous in habit. The alimentary canal is divid­ed into mouth, buccal cavity, pharynx, oesophagus, intestinal bulb, intestine and rec­tum with its external opening, the anus.

The mouth is bounded by soft upper and lower lips. The free edges of the lips are broad and are beset with four or five rows of black­ish conical papillae. The buccal cavity is a short dorsoventrally compressed cavity with a flat floor and an arched roof. The mucous membrane lining the buccal cavity contains minute papillae.

There is no distinct tongue in Rohu, but the mucous membrane lining the floor of the buccal cavity is provided with highly developed thick muscles. The buccal cavity leads into a dorsoventrally flattened pharynx. The pharynx is bounded by gill- arches and is well-demarcated into an anteri­or respiratory part and a posterior narrow masticatory part.

The anterior portion is nar­rower and is perforated laterally by gill-slits. The posterior portion of the pharynx bears closely set pharyngeal teeth on its ventrolat­eral walls and the ventral wall is highly fold­ed transversely. The pharyngeal teeth help to crush solid foods.

These teeth are all alike (homodont) and are arranged in three rows, one alternating the other. Each tooth has a narrow basal root and a cylindrical projecting crown. The root remains embedded in the mucous membrane and the crown is laterally compressed.

The posterior portion of pharynx leads into a very short tube, the oesophagus. The mucous membrane of the oesophagus is thrown into a number of longitudinal folds. The ductus pneumaticus of the swim-bladder opens into the oesophagus.

True stomach is absent in Rohu. The anterior part of the intes­tine becomes swollen into a sac just behind the oesophagus. This sac is designated as intestinal swelling or intestinal bulb which stores food.

The gastric glands are absent in the intestinal bulb and it resembles histologi­cally the intestine. The mucous lining of the intestinal bulb contains absorptive and mucous cells. Absence of stomach in Rohu and related forms like Labeo gonius, Cirrhina mrigala, Catla catla and many other cyprinids is difficult to explain.

Lack of stomach is not actually related to feeding habit but may be a case of neoteny as suggested by Barrington (1957). The intestine is extremely elongated (measuring about 7.5 metres) and thin-walled. The intestine is more or less of uniform diam­eter and forms a number of coils.

Elongation of intestine and its extensive coiling are related to its herbivorous food habit. The intestine is lined by absorptive and mucus-secreting cells. The absorptive cells bear a free striated mar­gin. The muscular layer of the intestine is thin.

The mucous membrane of the intestine pre­sents different types of foldings. The anterior portion of intestine shows oblique transverse folds, while the posterior part of the intestine is characterised by having distinct longitudinal folds. The terminal part of the intestine is slightly dilated and forms a thin-walled sac called rectum.

Like that of the anterior part of the intestine the mucous lining of the rectum exhibits indistinct oblique transverse folds. The rectum opens to the exterior through the anus located just in front of the urinogenital opening. The pyloric caeca are lacking in Rohu.

Digestive glands:

The digestive glands comprise of liver and pancreas. The liver is a massive gland of dark- brown colour. The liver is divided into a narrow right lobe and a broader left lobe. The two lobes of the liver are connected at three regions, ante­riorly by a median lobe, medially by a median connective lobe and posteriorly by a median mass.

The gall-bladder is an elongated sac of about 8 cm in length and 2.5 cm in diameter. The gall-bladder is located between the right and left lobes of the liver. A cystic duct, after originating from the anteroventral end of the gall-bladder, receives three hepatic ducts. The pancreas remains diffused along the coils of the intestine. It extends into the liver.

The exocrine part of the pancreas may form acini. The cells of the acini are large in size. Each cell is colum­nar in nature with a prominent nucleus and divided into two portions—basal and apical. The basal part contains homogeneous cyto­plasm while the apical end possesses large zymogen granules. Presence of exocrine pan­creatic cells within spleen tissue in Rohu, Catla and Mrigal fishes is a peculiar feature.

Mechanism of digestion:

The mechanism of digestion is not clear. The absence of stomach is compensated by the production of pancreatic trypsin and erepsin as well as enterokinase from the intestinal mucosa. Amylase is produced from the pancreatic cells. Lipase and maltase are also reported to be present in the intestinal extracts while their place of secretion has not been recorded.

In this genus of fish which lacks stomach, pepsin and hydrochloric acid are absent. As this fish is herbivorous, the con­centration of carbohydrate-splitting enzymes is highest and the protein-splitting enzymes are lowest in concentration.

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8. Hydrostatic Organ of Rohu:

The swim-bladder is a thin-walled perivis­ceral gas-filled sac. It lies in the cavity of the coelom and situated dorsal to the gut. The swim-bladder is divided into two, an anterior and a posterior chamber. The anterior cham­ber is connected to the oesophagus by a slen­der ductus pneumaticus. The swim-bladder acts as hydrostatic organ.

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9. Respiratory System of Rohu:

There are four pairs of gills contained in the branchial chambers. Each branchial cham­ber is covered by operculum and the branchiostegal membrane which is attached to the posterior margin of the operculum. The wall of the pharynx is perforated by five gill slits on each side and is separated by four gill-arches or inter-branchial septa.

There are four pairs of gills and the gills are holobrach type. Each gill has a double row of gill-filaments (holobranch) and is supported by gill-arch with gill-rakers. The two rows of gill lamellae are separated by the inter-branchial septum which is short and compact.

Each gill arch bears one afferent and two efferent branchial vessels (Fig. 6.24). The pseudo branch of the hyoid arch consists of a comb-like body. Each pseudo branch is com­posed of a single row of giIl-gilaments on the inner surface of operculum.

Physiology of respiration:

Rohu utilizes the oxygen dissolved in water. The physical mecha­nism of respiration can be described under two sequences (Fig. 6.25).

Inspiration:

During inspiration, the outer opening of the gill-chamber remains tightly closed to the body wall by the branchiostegal membrane and the two opercula bulge out to increase the accommodating capacity of the pharyngeal and buccal cavities. As a conse­quence, water from exterior rushes inside through the opened mouth and fill in the buc­copharyngeal cavity.

Expiration:

Immediately with the entry of water, the pharyngeal and the buccal cavities contract and exert pressure to the contained water. As the mouth, by this time, becomes closed by oral valves, the contained water finds the way out through the gill-slits.

The operculum as well as the branchiostegal mem­brane is lifted by this time and the water from the gill-chambers goes out through the open­ing of the gill-chamber. The dilatation and the contraction of the pharyngeal cavity are caused by the alternate retraction and protrac­tion of the hyoid arch supporting the buc­copharyngeal cavity.

Physiology of gaseous exchange:

The gills are highly vascular structures and are supplied by afferent and efferent branchial arteries. The afferent branchial artery carrying the deoxygenated blood is situated very superficially on the outer edge of the gill. The afferent brachial artery breaks up into capillaries in the sub­stance of the gill.

During the transit of water through the gill-slits, the deoxygenated blood in the capillaries of the gill-filaments takes oxy­gen dissolved in water and gives out carbon- dioxide by diffusion. The blood thus aerated, is collected by efferent branchial arteries and is conveyed to the different parts of the body.

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10. Circulatory System of Rohu:

The circulatory system of Rohu is basical­ly built on teleostean plan.

Heart:

The heart is composed of a sinus venosus, an auricle and a ventricle. The conus arteriosus is absent as such and is represented by a pair of valves. A non-contractile bulbus arteriosus is present. The sinus venosus is pro­portionately larger and bears a pair of lateral appendages (Fig. 6.26). It is spongy in nature.

Afferent branchial arteries:

There are four pairs of afferent branchial arteries, supplying deoxygenated blood to the corresponding gills. The ventral aorta divides anteriorly into the first two afferent branchial arteries. The second, third and fourth pairs of the afferent branchial arteries have separate and independent origin from the ventral aorta.

Efferent branchial arteries:

After oxygenation, the blood from the gills is collected by efferent branchial arteries. The venous system, exactly like that of Bhetki, includes the paired anterior and posterior car­dinals, unpaired hepatic and renal portal veins and paired subclavian veins. The renal portal vein is well-represented.

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11. Venous System of Rohu:

The venous system of Rohu consists of the systemic veins and the portal veins. These veins directly or indirectly convey the deoxy­genated blood from the different parts of the body to the heart.

Systemic venous system:

The blood is carried to the sinus venosus by right and left ductus Cuvieri. Each ductus Cuvieri is formed by three principal veins: an anterior cardinal sinus, a jugular sinus and a posterior cardinal sinus.

The anterior cardinal sinus brings blood from the anterior part of the body (see Fig. 6.27) and the posterior cardinal sinus brings blood from the posterior part of the body. Both the posterior cardinal veins receive segmental veins, renal veins, genital veins, etc.

In addition to the above mentioned three principal veins, the pectoral and pelvic veins form the pectoral and pelvic fins respectively and the slender hepatic vein opens into the ductus Cuvieri.

The blood from the tail region is conveyed by a caudal vein which just entering into the trunk bifurcates into two branches. The right posterior cardinal sinus passes through the substance of the right kidney and opens into the right ductus Cuvieri. The left posterior cardinal vein originates from the capillaries of the renal portal vein.

Portal venous system:

The portal venous system is composed of a special vein which originates in capillaries and end in capillaries and secondly the blood from these veins before going to the heart passes through some inter­mediate organs. When the intermediate organ is the kidney, such a system constitutes the renal portal system and when the organ is liver, the system is called the hepatic portal system.

Renal portal system:

The left branch of the caudal vein after entering into the left kidney breaks up into capillaries and forms the renal portal vein. These capillaries reunite and form the left posterior cardinal vein.

Hepatic portal system:

The capillaries from the alimentary canal and its associated structures unite to form a hepatic portal vein which enters into the substance of liver and breaks up into the capillaries. The capillaries reunite to form the hepatic vein which opens to the ductus Cuvieri.

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12. Nervous System of Rohu:

The brain of Rohu is typically built on the piscine plan. The cerebral hemispheres are small and undivided. The corpora striata are prominent, but the pallium is thin and non-nervous. The olfactory lobes are mode­rately developed.

The reduced diencephalon is provided with a dorsal pineal body and a ventral pituitary body. The optic lobes are large with two large lobi inferiores on the ventral side. The optic chiasma is absent and the optic nerves simply cross one another. The cerebellum is conspicuous and prolongs anteriorly to form the valvula cerebelli.

There are ten pairs of cranial nerves in Rohu. The origin and branching of the cranial nerves are, as in Bhetki, similarly disposed. The organs of special sense are well-developed and are built on typical teleostean plan. The sense of taste is highly developed. Numerous taste-buds are present in the lips, in the epithelium lining the first three gill-slits and on the barbels.

The tactile receptors are abundant all over the body specially on the lips and barbels. The organisation of lateral line system, eyes and ears is strikingly similar to those of other bony fishes already described. The ear is composed of utriculus, sacculus, lagena and three semicircular canals.

The otoliths are remarkably large in size and are three in number:

(a) Sagitta is present inside the sacculus,

(b) Asterisus fills the lagena and

(c) Lapillus is present in the utriculus.

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13. Urinogenital System of Rohu:

The kidneys are extremely elongated bodies extending along the whole length of the visceral cavity. They are situated on the dorsal side of the body wall above the swim-bladder and are distinct anteriorly but become partly fused in the middle region.

The kidneys are of mesonephric (designated to be of opisthonephros type by Greham Kerr) type. The ureters, one from each kidney, open into a thin-walled urinary bladder situated ventral to the cloaca. The urinary bladder opens into the urinogenital sinus (Fig. 6.28).

Sexuality:

During the breeding season, the pectoral fins become greater than or equal to anal fins in males, but in females, the pectoral fins are smaller than the anal fins . Choudhury (1959) has reported that the pectoral fins have a rough dorsal surface in males during breeding season but in females the surface of the pectoral fins is smooth.

Maturity:

Rohu attains maturity at the end of second year  and is polygamous. The sex play lasts about 5-10 seconds.

The gonads become greatly enlarged dur­ing breeding season. In the males, the testes extend the whole length of the abdominal cav­ity. From the posterior end of each testis, a vas deferens arises which finally opens into the urinogenital sinus.

In the female, the ovaries are also paired structures which attain larger size than the testes. The oviducts are lacking. The eggs are released in the body cavity from where the eggs emerge out through a pair of genital pores formed temporarily from the anterior wall of the urinogenital sinus.

In Rohu, enormous number of eggs is laid at a time and the eggs sink to the bottom. Immediately after discharge, the eggs come in contact with the spermatic fluid (milt) and fer­tilization takes place externally. The mecha­nism of cleavage and subsequent develop­mental sequences are not known in Rohu.

It is expected that the development should be like that of other bony fishes. Labeo rohita is a fast- growing carp. The eggs hatch within 2-15 hours.

Freshly hatched youngs (hatchlings) have prominent yolk sac attached to the ven­tral side of the body. Absorption of the yolk sac requires 5-7 days and the young ones (called fry) begin to feed.

The fry attains a size from about 2 mm to 3 mm and is characterised by having fringed lips and a prominent vertical dark spot at the base of the tail which dis­appears in course of growth. When the fry becomes 5 mm long, it is designated as the fingerling which also varies from 5 mm – 15 mm in length. Attainment of sexual maturity requires about two years.