Complete Anatomy of Labeo Rohita aka Rohu

Hamid Ali January 5, 2025 2 0

Labeo rohita, commonly known as Rohu is a freshwater fish belonging to the family Cyprinidae, is widely distributed across rivers and lakes in Asia and Africa.

Labeo species have a streamlined body, sub-terminal mouth adapted for bottom-feeding, and cycloid scales. They primarily consume algae, plankton, and detritus. Known for their rapid growth and adaptability to diverse aquatic habitats, Labeo plays a crucial role in inland fisheries.

Taxonomic Classification

Kingdom: Animalia (Multicellular, heterotrophic, lack cell walls)
Phylum: Chordata (notochord, dorsal hollow nerve cord, pharyngeal slits at some stage of development)
Class: Actinopterygii (Ray-finned fishes, bony or horny spines)
Order: Cypriniformes (carp, minnows)
Family: Cyprinidae (freshwater fishes, carps, minnows)
Genus: Labeo (Medium to large-sized freshwater fishes found in rivers and lakes of Asia and Africa)
Species: Labeo rohita (Rohu) (cultivated fish in aquaculture, particularly in South Asia)

Digestive System

The digestive system of Labeo (e.g., Labeo rohita) is adapted for its omnivorous, primarily herbivorous diet, primarily consisting of algae, phytoplankton, and detritus. Its digestive system featuring a long digestive tract and adaptations for consuming food from aquatic environments.

The process begins with the sub-terminal mouth, which is equipped with thick, fleshy lips. This structure allows Labeo to scrape algae and organic matter from surfaces in its habitat. The food enters the buccal cavity, where initial processing occurs. Labeo lacks true teeth, but pharyngeal teeth may be present to help in grinding the food. From the buccal cavity, the food passes through the pharynx into the esophagus, a short muscular tube that transports food to the digestive tract.

One distinctive feature of the digestive system in Labeo is the absence or rudimentary development of a stomach. This is typical of many herbivorous fish, as their diet does not require significant acidic breakdown or storage. Instead, digestion primarily occurs in the intestine, which is long and coiled to maximize nutrient absorption.

The walls of the intestine are lined with mucus-secreting cells and folds to increase the surface area for efficient digestion. The liver plays a crucial role in this process by secreting bile, which emulsifies fats, while the pancreas produces enzymes like amylase, protease, and lipase to break down carbohydrates, proteins, and fats.

After digestion, the nutrients are absorbed through the intestinal walls into the bloodstream for distribution throughout the body. Any undigested food and waste products are expelled through the anus, located near the base of the anal fin. This streamlined process ensures that Labeo can efficiently utilize the plant-based food it consumes.

Respiratory System

As a freshwater fish, Labeo relies on gills as its primary respiratory organs, with a specialized system for efficient gas exchange.

The process begins with the mouth, through which water is drawn into the buccal cavity. Labeo has a sub-terminal mouth, positioned below the snout, which allows it to take in water while feeding. The water then moves into the pharynx, which serves as a passage between the buccal cavity and the gills.

The water is forced through the gill arches by the action of the operculum, a bony flap that covers and protects the gills. The operculum opens and closes to regulate the flow of water over the gills, enabling the fish to extract oxygen efficiently.

The gills themselves consist of four pairs of gill arches located on either side of the pharynx. Each gill arch is covered with gill filaments, which are lined with microscopic structures called lamellae. These lamellae greatly increase the surface area available for oxygen absorption. As water flows over the gills, oxygen from the water diffuses through the thin walls of the lamellae into the bloodstream, while carbon dioxide diffuses out of the blood into the water, a process known as gas exchange. This process is driven by the difference in concentration of oxygen in the water and carbon dioxide in the blood, facilitating efficient respiration.

The countercurrent exchange mechanism further optimizes the respiratory process. In this system, the flow of water over the gills moves in the opposite direction to the flow of blood through the gill filaments. This maximizes the oxygen gradient and allows for greater oxygen uptake by the blood, even when water oxygen levels are lower.

Once oxygen is absorbed into the bloodstream, it is transported via the circulatory system to various parts of the body. Meanwhile, carbon dioxide is carried back to the gills and expelled into the water. This system ensures that Labeo maintains a steady supply of oxygen, even in low-oxygen environments.

Circulatory System

The circulatory system of Labeo is a closed system that facilitates the transport of oxygen, nutrients, and metabolic waste products throughout the body. It is comprised of a heart, blood vessels, and blood.

Heart

The heart of Labeo is a two-chambered. It consists of an atrium and a ventricle, both of which play distinct roles in circulating blood through the body. The heart is located in the pericardial cavity, just beneath the gills.

Atrium: Receives deoxygenated blood from the body via the sinus venosus and passes it into the ventricle.
Ventricle: Pumps the deoxygenated blood to the gills via the bulbus arteriosus, where gas exchange occurs.

Blood Flow Process

1. Deoxygenated blood returns to the heart via the venous system, primarily through the cardinal veins (posterior and anterior cardinal veins), which drain the body and head, respectively.

2. Blood enters the sinus venosus, a thin-walled cavity that collects venous blood from the body.

3. The blood then flows into the right atrium, which pumps it into the ventricle.

4. From the ventricle, the blood is forced into the bulbus arteriosus, a muscular structure that helps regulate blood flow into the ventral aorta.

5. The ventral aorta carries the deoxygenated blood to the gills through a series of gill arches. In the gills, the blood undergoes gas exchange, where oxygen is absorbed, and carbon dioxide is expelled.

6. Once oxygenated, the blood flows through the dorsal aorta, which distributes oxygen-rich blood to the body via various arteries, reaching organs and tissues, where nutrients and gases are exchanged.

Blood Vessels

Arteries: Carry oxygenated blood from the heart to the rest of the body. The main artery is the dorsal aorta, which branches into smaller arteries that deliver blood to specific regions.

Veins: Collect deoxygenated blood and return it to the heart. Key veins include the cardinal veins, which return blood from the body, and the hepatic portal vein, which collects blood from the digestive organs.

Capillaries: Tiny blood vessels where nutrient and gas exchange occurs between blood and tissues. Capillary beds are found in the organs and muscles, facilitating the diffusion of oxygen, nutrients, and waste products.

Hemoglobin and Blood Composition

The blood of Labeo contains erythrocytes (red blood cells), which are responsible for transporting oxygen due to the presence of hemoglobin. Hemoglobin binds with oxygen in the gills and releases it to the tissues where oxygen is needed.

The blood also contains leukocytes (white blood cells), which are involved in immune responses, and platelets that help in clotting. The plasma carries nutrients, hormones, proteins, and waste products like urea and carbon dioxide to be excreted.

Oxygen Transport and Regulation

As blood passes through the gill lamellae (thin tissue folds in the gills), oxygen diffuses from the water into the blood, while carbon dioxide diffuses from the blood into the water.

The countercurrent exchange system in the gills ensures a high efficiency of oxygen uptake, as the flow of water over the gills is in the opposite direction to the flow of blood, maintaining a concentration gradient that favors the absorption of oxygen.

Once oxygenated, the blood is pumped via the dorsal aorta and distributed throughout the body. Hemoglobin in the blood binds to oxygen in the gills and releases it where it is needed, ensuring that tissues and organs receive adequate oxygen for metabolic processes. Oxygen is released from hemoglobin based on the Bohr effect, where a drop in pH (from increased carbon dioxide) facilitates oxygen release to tissues that require it most.

Excretory System

Labeo, like other fish, faces the challenge of excreting nitrogenous waste and maintaining a stable internal environment in a freshwater habitat, where osmotic regulation is key.

1. Kidneys

The kidneys of Labeo are elongated, paired organs located in the dorsal region of the body cavity, just below the spine. The kidneys of Labeo are mesonephric kidneys, which are characteristic of most teleost fish.
Blood enters the kidneys through the renal arteries, where waste products are filtered out. The kidneys primarily excrete nitrogenous wastes in the form of ammonia, which is highly toxic but water-soluble, making it ideal for excretion in aquatic environments.
Freshwater fish like Labeo have specialized kidneys that help regulate the balance of water and salts. Since Labeo is constantly exposed to hypoosmotic (low salinity) environments, the kidneys help retain essential ions such as sodium and chloride while excreting large amounts of dilute urine to eliminate excess water.

2. Nephrons

The kidney is composed of numerous functional units called nephrons. Each nephron consists of a renal corpuscle (the glomerulus and Bowman’s capsule) and a renal tubule.

The glomerulus filters the blood, allowing waste products such as ammonia, urea, and salts to pass into the Bowman’s capsule.
The filtered fluid moves into the proximal convoluted tubule, where reabsorption of essential nutrients, ions, and water occurs.
In the distal convoluted tubule and collecting ducts, additional ions like sodium and potassium are regulated, and the final concentration of urine is determined, ensuring water and ion balance.

3. Urinary Bladder
Labeo has a rudimentary urinary bladder, a sac-like structure that stores urine before excretion through the urethra. The urine is released through the anus or urogenital opening.

Nervous System

The brain and nervous system of Labeo (*Labeo rohita*) are essential for coordinating various physiological processes, controlling movement, and responding to environmental stimuli. Like all fish, Labeo has a central nervous system (CNS) and a peripheral nervous system (PNS), which work together to control and regulate its body functions, behavior, and interactions with the environment. Central Nervous System (CNS)

The central nervous system of Labeo consists of the brain and spinal cord, which are responsible for processing information and controlling the body’s actions.

Brain

Labeo’s brain is divided into several distinct regions, each responsible for specific functions such as sensory processing, motor control, and autonomic regulation. The brain of Labeo is relatively small but highly efficient, with adaptations to support life in an aquatic environment.

1. Forebrain (Prosencephalon):

The forebrain consists of the olfactory lobes, telencephalon, and diencephalon.
A. Olfactory lobes: These are involved in the sense of smell. As a fish, Labeo relies heavily on olfactory information to locate food and navigate its environment. The size and development of the olfactory lobes in Labeo are adapted for enhanced olfaction.
B. Telencephalon: This part of the brain is involved in higher-order functions such as learning and memory. Although it is not as developed as in higher vertebrates, it plays an important role in processing sensory input.
C. Diencephalon: The diencephalon includes the thalamus and hypothalamus, which are responsible for regulating basic bodily functions such as temperature regulation, hormonal control, and homeostasis. The hypothalamus also plays a role in regulating feeding behavior and circadian rhythms.

2. Midbrain (Mesencephalon):

The midbrain is involved in processing visual and auditory information. It contains the optic lobes, which receive input from the eyes and help with visual processing, allowing Labeo to respond to changes in light and detect prey. In Labeo, the midbrain is particularly important for coordinating responses to visual stimuli in the aquatic environment.

Additionally, the torus semicircularis, located in the midbrain, is responsible for processing auditory and balance information, ensuring Labeo can detect sound and maintain equilibrium while swimming.

3. Hindbrain (Rhombencephalon):

The hindbrain consists of the cerebellum and medulla oblongata.

A. Cerebellum: The cerebellum is involved in motor coordination and balance. It helps Labeo maintain smooth and precise movements, allowing it to navigate efficiently through the water and perform complex swimming maneuvers.
B. Medulla oblongata: This region regulates vital autonomic functions such as breathing, heart rate, and digestion. It is responsible for controlling the basic functions necessary for survival and maintaining homeostasis.

Spinal Cord

The spinal cord is the main communication pathway between the brain and the rest of the body. It runs along the length of the body, enclosed within the vertebral column. The spinal cord coordinates reflexes, such as escape responses to predation, and transmits sensory information from the body to the brain and vice versa. It also contains motor neurons that control voluntary movements, such as swimming.

Peripheral Nervous System (PNS)

The peripheral nervous system consists of nerves and ganglia that extend throughout the body, connecting the central nervous system to the organs and muscles. The PNS is responsible for transmitting sensory information to the CNS and carrying motor commands from the CNS to the body.

A. Cranial Nerves: Labeo has 10 pairs of cranial nerves that arise from the brain and serve specific sensory and motor functions. For example, the olfactory nerve transmits smell information from the olfactory receptors to the brain, while the optic nerve carries visual information from the eyes.

B. Spinal Nerves: 30-32 nerves emerge from the spinal cord and carry information to and from the body. They are involved in controlling movements and transmitting sensory signals from the skin, muscles, and internal organs.

C. Autonomic Nervous System (ANS): The ANS regulates involuntary functions such as heart rate, blood pressure, digestion, and respiration. It consists of the sympathetic and parasympathetic divisions, which work in balance to ensure the body’s internal environment is maintained under different conditions. The sympathetic division prepares the body for fight or flight, while the parasympathetic division promotes rest and digestion.

Sea Lamprey (Petromyzon marinus) Brief Introduction- Click here

Sensory Systems

Labeo’s nervous system is well-developed to sense and respond to environmental cues in its aquatic habitat. Its sensory organs are specialized for detecting stimuli in water.

Vision

Labeo has well-developed eyes with good vision, adapted for detecting changes in light and movement in the aquatic environment. The retina contains specialized photoreceptor cells that enable the fish to see in low-light conditions, an important adaptation for survival in various water depths.

Olfaction (Smell)

The olfactory system is highly developed in Labeo and plays a critical role in food detection, mate selection, and environmental navigation. The fish has olfactory lobes in the brain that process scent signals received from the water.

Lateral Line System

The lateral line is a sensory organ that detects vibrations and water currents. It helps Labeo sense the presence of predators, prey, and obstacles, facilitating schooling behavior and navigation in dark or murky water.

Hearing

Labeo can detect sound vibrations through its inner ear. The otoliths (small bones) in the inner ear detect pressure changes in the water, allowing the fish to perceive sound and balance.

Endoskeleton

The endoskeleton of Labeo consists of bones and cartilage and is divided into two main parts: the axial skeleton and the appendicular skeleton.

Axial Skeleton

The axial skeleton includes the skull, vertebral column, and ribs. It forms the central structure of the body, supporting the fish’s body, protecting vital organs, and providing a point of attachment for muscles involved in movement.

Skull: Labeo’s skull is bony and protects the brain, eyes, and other sensory organs. The structure of the skull is streamlined, helping the fish reduce drag while swimming. It is made up of several fused bones, including the cranium (which encases the brain) and the jaw bones, which are involved in feeding and are adapted for the fish’s specific dietary needs.

Vertebral Column: The vertebral column is composed of vertebrae that extend from the skull to the tail. It provides structural support and houses the spinal cord, which is protected within the vertebral canal. Labeo’s vertebral column is highly flexible, allowing for the smooth, undulating movement of the body that is characteristic of fish swimming.

Ribs: Labeo has rib bones that extend from the vertebrae and provide additional protection for internal organs like the heart, lungs, and digestive system. The ribs also help support the body, especially when the fish is swimming.

Appendicular Skeleton

The appendicular skeleton is made up of the limb girdles and fins. In Labeo, this includes the pectoral girdles, pelvic girdles, pectoral fins, pelvic fins, and caudal fin. These structures help the fish with locomotion, balance, and maneuverability in water.

Pectoral Girdle and Fins: The pectoral girdle is connected to the skull and serves as the attachment site for the pectoral fins, which are responsible for stabilizing the fish while swimming and providing some propulsion.

Pelvic Girdle and Fins: The pelvic girdle supports the pelvic fins, which are located on the underside of the fish and play a role in controlling the fish’s direction and stability.

Caudal Fin: The caudal fin (tail fin) is the primary organ of propulsion in Labeo. It is attached to the caudal peduncle (the narrow, muscular portion at the base of the tail) and provides the force necessary for swimming. The structure of the caudal fin is adapted to provide efficient propulsion through the water.

Functions of the Endoskeleton

Structural Support: The endoskeleton provides a rigid framework that supports the body’s shape and prevents the body from collapsing under the pressure of the surrounding water. It also acts as a scaffolding for the attachment of muscles, enabling movement.

Protection: The skull protects the brain, while the vertebral column protects the spinal cord. The ribs protect the heart and internal organs from physical damage, especially during rapid swimming or collisions with objects in the environment.

Locomotion: The appendicular skeleton, particularly the fins, plays a crucial role in Labeo’s swimming and maneuvering. The pectoral fins assist in stabilizing the fish and allow it to make precise movements, while the pelvic fins help with balance. The caudal fin is the primary fin used for propulsion, providing the force needed for forward movement through the water.

Mineral Storage: Labeo’s bones act as a storage site for minerals, especially calcium and phosphorus, which are important for maintaining physiological processes, including muscle contraction and bone remodeling.

Buoyancy and Balance: Labeo’s skeleton, along with the swim bladder (a gas-filled sac), helps the fish maintain its position in the water column by providing balance and neutral buoyancy. This allows Labeo to conserve energy while swimming.

Reproduction in Labeo

Rohu fish exhibits external fertilization, and reproduction is closely linked to environmental factors, particularly water temperature and monsoon season. Understanding the reproductive system of Labeo involves examining the mechanisms of gamete production, mating behavior, and fertilization.

Sexual Dimorphism and Gonads

Labeo exhibits sexual dimorphism, meaning males and females can be differentiated based on their physical characteristics, especially during the breeding season.

Male Reproductive System

The male reproductive system consists of paired testes, located in the abdominal cavity. The testes produce sperm (male gametes), which are released into the sperm ducts and then pass into the urethra.
During the breeding season, the testes enlarge as they fill with sperm, and the male fish exhibits increased activity and a more pronounced courtship behavior.

Female Reproductive System:

The female reproductive system consists of paired ovaries located in the posterior part of the body cavity. The ovaries produce ova (eggs), which mature and are released into the oviducts.

Labeo females typically have larger abdomens during the breeding season due to the presence of mature eggs. The female releases her eggs in response to external stimuli from the male, particularly during spawning events.

Spawning Behavior and Environmental Triggers

Labeo is a spawning fish, which means it lays its eggs in water, and fertilization occurs externally. The spawning behavior of Labeo is closely tied to the monsoon season, which provides optimal environmental conditions for reproduction.

Timing of Spawning

Spawning in Labeo typically occurs during the rainy season (monsoon), when water temperatures rise and there is an abundance of food in the environment. The increased water flow during the monsoon season provides an ideal environment for the fish to spawn, as it increases the availability of oxygen and ensures a more favorable environment for the development of eggs and larvae.

Female Labeo are stimulated to spawn when the water temperature reaches a certain threshold (typically between 25-30°C), and the male’s presence and courtship behavior also play a significant role.

Courtship Behavior

Courtship rituals are highly prominent in Labeo. Males often chase and nudge the female, which signals the female to prepare for egg release. The male typically guards the female, displaying aggressive behavior towards other males, ensuring that he has the opportunity to fertilize the eggs.

Spawning Process and External Fertilization

A. Egg Release and Fertilization:

Once the female is ready to spawn, she releases her eggs into the water, typically near the riverbed or other suitable substrates such as rocks, plants, or submerged objects. The female may release several thousand eggs during a single spawning event, depending on her size and health.

The male then releases sperm (milt) over the eggs in a process called external fertilization. The sperm fertilizes the eggs as they are released into the water, usually within seconds of egg release. This method of fertilization is common among fish and ensures genetic diversity.

B. Development of Eggs:

The fertilized eggs develop in the water and are initially transparent. Over time, they become opaque as the embryo inside develops. The eggs are generally adhesive and stick to the substrate or aquatic vegetation, providing protection from current flow and potential predation.

The development of the eggs takes place over a period of 2-4 days, depending on water temperature and environmental conditions. Once the embryos have fully developed, they hatch into larvae.

C. Larval Development and Hatching

The newly hatched larvae are initially yolk-sac larvae, meaning they rely on the yolk stored in their bodies for nutrition. This yolk provides energy for the first few days of life until the larvae are capable of feeding on external food sources.
After about 2-3 days, the larvae begin to exhibit free-swimming behavior, and they start to search for small food particles, such as plankton and micro-organisms. At this stage, the larvae are fully functional and begin to grow rapidly, feeding on their own.

D. Parental Care

Unlike some other species, Labeo does not exhibit significant parental care post-fertilization. The eggs and larvae are left to develop in the water, and the parents do not guard the eggs or the young. The larvae face threats from predation, and only a small percentage survive to adulthood.

E. Maturation and Growth

The juveniles grow rapidly in the early stages of development. They go through several stages, including fry, fingerlings, and eventually become adult fish.
Labeo reaches sexual maturity between the ages of 1-2 years, depending on factors like water temperature, food availability, and environmental conditions. Once they mature, they begin the reproductive cycle again.

Environmental Factors Influencing Reproduction

Several environmental factors influence Labeo’s reproductive success:

Water Temperature: Labeo is more likely to spawn when the water temperature is between 25-30°C, typical of the monsoon season.
Water Flow: The increased water flow during the rainy season provides more oxygen and an abundance of nutrients, which are beneficial for the development of eggs and larvae.
Food Availability: An abundant supply of food in the form of plankton and other small organisms in the water encourages healthy growth of larvae and juveniles.
Water Quality: Clean, well-oxygenated water is essential for successful reproduction. Pollution or poor water quality can negatively affect the development of eggs and larvae.