Labeo rohita, Rohu : A Bony Fish

Class Osteichthyes - Class Osteichthyes (Gr., osteon, bone + ichthyes, fish) comprises the true, Bony fishes, both freshwater and marine. They are the familiar and most successful living group of aquatic vertebrates. One half of all vertebrates are Bony fishes belonging to well over 31,000 living species. Their scientific study is known as Ichthyology. They vary greatly in shape and proportions, but they are built on the same basic plan. They have a spindle-shaped, streamlined body covered by dermal scales, also have Bony endoskeleton, swim by fins and breathe by gills. They are superficially similar to dogfish sharks.

                                LABEO ROHITA : ROHU

Systematic Position - 

Phylum - Chordata

Subphylum - Vertebrata

Division - Gnathostomata

Super class - Pisces

Class - Osteichthyes

Subclass - Actinopterygii

Super order - Teleostei

Order - Cypriniformes

Type - Labeo rohita (Rohu)

Several types of Bony fishes (Lates, Mugil, Labeo) have been prescribed by different Indian Universities for study. The following description relates mainly to Labeo rohita which is perhaps the commonest of Indian freshwater Bony fishes and the one most generally esteemed for eating purposes. It is a good example of a Bony fish.

Distribution - Labeo is a large, essentially tropical genus of carps distributed in tropical Africa and East Indies. About two dozens of species are known from India, the most common being Labeo rohita (Rohu) and Labeo calbasu (Kala bans) which occur almost throughout India, Pakisthan and Bangla Desh.

Habit And Habitat - Rohu is commonly found in freshwater ponds, rivers, lakes and estuaries. It prefers clean water and respires by means of gills. It is chiefly herbivorous and a bottom, feeder, eating algae and aquatic plants. But frequently it comes to water surface, to take aur into the air bladder . It is oviparous and breeds in running water in July and August. Fertilization takes place externally.

External Features - Shape, size and colour

Body is spindle-shaped. Colour is greyish or blackish on back and silvery white or pale on the two sides and belly. A full grown individual measures 1 metre in length and 20 to 25 kg in weight. The body is divisible into head, trunk and tail.

1) Head - Head extends from tip of snout to Hindi edge of operculum. Snout is depressed, short and obtuse. Mouth is subterminal. It is a large transverse aperture, bounded by thick and fleshy lips. Usually only two small, thread- like sensory maxillary barbels are present at the corners of mouth, the rostral barbels being generally absent. Teeth are lacking on jaws. Snout bears dorsally a pair of small nostrils, they are peculiar as they do not communicate to the buccal cavity. The lateral eye on head are without eyelids but protected by a transparent protective membrane. Behind the eye, on either side is a large movable Bony gill cover or operculum with free posterior and ventral margins. Beneath each operculum lie four comb- like gills in a branchial chamber.

2) Trunk - It is the thick middle part of body. It is higher than wide and oval in cross section. On either side of trunk extending from the back of operculum upto tail there is a dark line on the mid-ventral portion of the body called, lateral line. Fins are well developed and supported by Bony finrays. On the back of the middle of trunk is a single large somewhat rhomboidal dorsal fin. Just behind operculum are a pair of larger ventro - lateral pectoral fins, followed behind by a pair of smaller ventral pelvic fins. Mid - ventrally at the posterior end of trunk lie in a series three small aperture, anterior anus, middle genital and posterior urinary.

3) Tail - It comprises about one - third posterior part of the body. It is laterally compressed and narrowed behind. At the end of the tail is a median homocercal caudal fin deeply notched into two similar lobes. On the ventral side of tail is a median anal fin lying just posterior to urinary aperture. The tail makes the principal locomotor organ.

Bodywall - Trunk and tail are covered by thin, rounded, overlapping dermal cycloid scales. The concentric or ring - like markings on scales called circulii are used to determine  the age of fish. The skin comprises of two parts outer epidermis and inner epidermis. A thick basement membrane is present between dermis and epidermis. The epidermis consist of several layers but stratum corneum is absent. The dermis is made up of connective tissue, blood vessels, nerves and smooth muscle fibers. Scales are found ambedded in the dermis. The epidermis of fishes contains large mucous cells or Becker's cell and chromatocytes. Chromatocytes are also found in dermis. Two type of sensory cells viz., granular sensory cells and club cells are also present. The dermis is composed of stratum laxum, stratum compactum and subcutaneous layer, sharply demarcated from each other. Stratum laxum is a laminated fibrous layer, in which the scales are found embedded by Sharpey's fibers. Stratum compactum is made of reticulum of delicate fibers. The mucus or slime produced by epidermal mucous glands makes the fish slippery. The principal muscles muscles of trunk and are arranged in zigzag myotomes as in other fishes and vertebrates. Muscles are lined internally by peritoneum which also lines the body cavity or coelom and covers the visceral organs. The colour of fishes is produced by the cells present in the dermis. These cells are of two types  chromatophores and reflecting cells or iridocytes. Chromatophores are branched cells containing pigment granules called melanosomes. Melanosomes may be black (melanin), white, red, orange, yellow (carotenoids or flavis). The iridocytes contain stacks of platelets of guanine.

Exoskeleton - Scales and finrays constitue the exoskeleton of Labeo rohita. Scales are cycloid type, thin overlapping Bony plates, partly covered by skin. Exposed part of scales bear pigment cells. Centrally each scales has an area called focus. Around focus their exist concentric rows of ridges called circulii, used to count the age of fishes. Moreover there are several radial lines, staring from focus and extending upto the periphery, called radii. Finrays are fin supporting structures. They are of two types - spines (single ray) and soft rays (segmented rays). Spines in the fins of Labeo are unsegmented, uniserial structures. Soft rays are segmented, often branched and biserial.

Digestive System -  Digestive system comprises the alimentary canal and the associated digestive glands.

Alimentary canal - Alimentary canal starts from the mouth and terminates in the anal opening. The subterminal mouth, bounded by fleshy lips beset with sensory papillae, opens into a broad, dorso-ventrally compressed buccal cavity. Teeth and a district tongue are lacking in rohu. Pharynx is also dorso-ventrally compressed and differentiated into a broad anterior respiratory part and a narrow posterior masticatory part. Anterior part is perforated laterally by four pairs of internal gill-slits leading into branchial chambers. From branchial arches project into pharyngeal cavity small spiny gill rakers, to prevent passage of food through gill-slits. Either ventro - lateral wall of posterior pharyngeal chamber bears 3 rows of homodont teeth or masticating plates with truncated crowns for crushing and grinding food.

Teeth are borne by the inferior pharyngeal bones (reduced 5th branchial arches). Floor of pharynx is folded transversely. Pharynx leads posteriorly into a short, narrow tubular esophagus. It's mucous lining forms prominent longitudinal folds.

A pneumatic duct from air bladder of fish opens dorsally into oesophagus. Labeo and many other teleost fishes do not have stomach which is difficult to understand. Oesophagus opens behind straight into an elongated, swollen, thick - walled intestinal bulb. Opening between the two is guarded by an oesophageal valve to prevent regurgitation of food. The intestinal bulb has an anterior broader cardiac part into which open dorsally the pancreatic and bile ducts, and a posterior narrower pylori  part without pyloric caeca so common in other teleost fishes. The mucous lining of cardiac part shows honey - comb - like folds, and that of pyloric part contains bold longitudinal folds. Gastric glands are lacking. Intestinal bulb is followed by a thin - walled, narrow and extremely elongated (about 8 metres long) intestine.

It is a much coiled tube of practically uniform diameter. The mucous lining forms oblique transverse folds in its anterior region, and distinct longitudinal folds in the posterior region. Pyloric caeca, intestinal villi and scroll valve are absent. Intestine is longer in Labeo because of its herbivorous habit but it tends to be shorter in carnivorous fishes. The rectum which follows is nearly 1 metre long, slightly wider and thin - walled. Rectal gland is lacking. Internal lining forms inconspicuous transverse folds. Rectum opens to the exterior by anus mid - ventrally just in front of the anal fin. It functions for the muscular expulsion of the facial material.

Digestive Glands - Digestive glands of Labeo are liver and pancreas. Liver is a large, dark brown gland. It consists of a narrower right love and a broader left love interconnected anteriorly, by connecting median lobes. A thin - walled elongated sac - like gall bladder, 8 cm long and 2.5 cm broad , lies dorsally between right liver lobe and intestinal bulb. A cystic duct arises from the anterior end of gall bladder, receives three hepatic ducts from the liver lobes and forms a bile duct which opens dorsally into the roof of cardiac part of intestinal bulb.

Pancreas is rather diffused found scattered in liver , spleen and intestinal mesentery. A pancreatic duct, which is enclosed in a common sheath with bile duct, opens separately in the intestinal bulb.