Respiration in Fishes

Respiration in Fishes

• Respiration is the process where oxygen is used to oxidize food, releasing energy for vital activities.
• Carbohydrates are primarily responsible for energy release.
• Oxygen is obtained from the surrounding medium, and carbon dioxide is a byproduct of this process.
• There are two types of respiration: aerobic and anaerobic.
• Aerobic respiration requires oxygen and produces carbon dioxide. It occurs in most plants and animals. Organisms that breathe this way are called aerobes.
• Anaerobic respiration does not require oxygen, glucose is metabolized into lactic acid. It occurs in some bacteria and parasitic animals. These organisms are called anaerobes. Living without oxygen is known as anaerobiasis.
• External respiration refers to gas exchange between the blood and water (or air) via respiratory organs.
• Internal respiration is the essential transfer of gases between blood and tissues or cells of the body, resulting in energy release.
• The main respiratory organs in fish are the gills.
• Gill slits are perforated openings along the lateral walls of the pharynx. The first gill slit is called the spiracle (in sharks), located between the mandibular and hyoid arches.
• The hyoidean cleft is between the hyoid arch and the first branchial arch. The remaining gill slits are situated between consecutive branchial arches.
• Gills are vascular filamentous outgrowths formed from the anterior and posterior walls of each gill slit, where gas exchange occurs.
• Other structures, like the skin, air bladder, and accessory organs, also function as respiratory structures in some fishes.

Types of Gills

• Holobranch: A complete gill consisting of a gill arch, gill rakers, and gill filaments. Each gill arch has two sets of hemibranchs (half gills).
• Elasmobranchs (sharks and rays) usually have five pairs of gill slits, but only four pairs are present in bony fishes.
• Teleosts (bony fishes) have a single external branchial aperture on each side of the head due to the operculum covering the gills.
• Pseudobranch: A hyoidean pseudobranch is present in many Actinopterygians (ray-finned fishes) anterior to the first gill. It receives oxygenated blood from the dorsal aorta and is connected to the internal carotid artery. It may contribute to oxygen concentration in the blood flowing to the brain and eye, or be involved in gas bladder filling or intraocular pressure regulation.
• Structure of a teleostean gill: Consists of four pairs of gills. Each gill has:
  • Gill rakers: Project along the inner surface of the gill arch. They prevent food particles from entering the gill slits and can be specialized for filtering water in filter-feeding fishes. They can be soft, thin, thread-like, hard, flat, triangular, or even teeth-like, depending on the fish's feeding habits. They also have taste buds that help the fish detect the chemical nature of the water.
  • Gill arch: A cartilaginous or bony structure supporting the gills. It bears two rows of slender, fleshy projections called gill filaments. Each gill arch encloses afferent and efferent branchial vessels and nerves.
  • Gill filaments (primary gill lamellae): Two pairs per gill arch, rich in capillaries. They contain rows of thin plates or dishes known as secondary lamellae.
  • Secondary lamellae: Thin, folded structures within the gill filaments that increase surface area for gas exchange. Their number and size vary with the fish's activity level.
• The total respiratory area of the gills depends on the number and size of gill lamellae, which varies with the fish's habits.
• The efficiency of the gills is also determined by the diffusion distance – the barrier between blood and water during gas exchange. Water-breathing fishes have larger gill areas and smaller diffusion distances, making them more efficient than air-breathing fishes.

Respiratory and Non-Respiratory Pathways

• Respiratory pathway: Associated with respiration. Oxygenation of blood in teleosts is achieved through rhythmic water inhalation and exhalation through the bucco-pharyngeal cavity. This is accomplished by suctioning water into the cavity and expelling it through the gill slits.
• Sharks allow water in by opening their mouths, close them, and then raise the floor of their mouths to pump water through the gills. Deoxygenated water exits through separate gill slits. The spiracle also allows water to enter.
• Bony fishes are more efficient due to the presence of the operculum. The operculum closes as the pharynx expands, drawing in water. The mouth closes, and the opening of the opercula pushes water through the gills.
• Fast-swimming species may keep their mouths and opercula open, providing a continuous current of water for gill bathing.
• Afferent branchial vessels bring deoxygenated blood from the body into the gill arch. It breaks down into capillaries, where gas exchange occurs through countercurrent gas exchange.
• Oxygen from the water irrigated through the gills is transferred to deoxygenated blood.
• Oxygenated blood is collected by the efferent lamellar vessel of the primary gill lamella, then the efferent branchial vessel of the gill arch, and finally returned to the body.
• Countercurrent gas exchange: Water flow across the lamellae is in the opposite direction of blood flow, increasing oxygen diffusion. The oxygen concentration in the water is always higher than in the blood.
• Non-respiratory pathway: Consists of a complex network of sinuses and veins that directly carry blood to the heart, bypassing systemic circulation.
• The non-respiratory pathway provides nutrients and oxygen to the filament tissue and might be involved in hormone circulation.
• The pathway includes the efferent filament artery, nutritive blood channel, central venous sinus, venules, and branchial veins.

Other Organs for Respiration

• Bimodal breathers: Fishes that leave the water to breathe air.
• Air-breathing fishes in aquatic habitats often breathe synchronously to reduce predation risk.
• Following mechanisms are adaptations for aquatic surface respiration:
  • Modification of the gills:
    • Labyrinth organ: A suprabranchial accessory breathing organ that allows fish to take oxygen directly from the air (e.g., Clarias batrachus – walking catfish). It has thickened, widely spaced lamellae on the dorsal side of the filaments and a branched bulbus dendritic structure emanating from the 2nd and 4th gill arches.
  • Use of the skin:
    • Cutaneous respiration: Gas exchange through capillaries across the skin (e.g., eels). They use atmospheric air through well-vascularized skin and to a lesser extent, their gills.
  • Mouth:
    • Electric eels (Electrophorous electricus) are obligate air breathers with a well-vascularized area in the buccal cavity for oxygen uptake. The buccal cavity has a large surface area for gas exchange due to convolutions and papillae. Anabas has also adapted its mouth parts for aerial respiration.
    • Obligate air breathers: Must surface to inhale every 10 minutes or so (e.g., electric eels). About 80% of their oxygen is obtained this way.
    • Facultative air breathers: Optionally utilize air breathing to supplement aquatic respiration.
  • Gut:
    • Some catfish (e.g., Haplosternum, Anicistrus, and Plecostomus) have specialized areas of their gut for oxygen uptake by swallowing air. Oxygen is taken in via the gut, and carbon dioxide is released through the gills.
  • True lungs:
    • Lungfish (Dipnoi) are obligate air breathers. They have well-developed and vascularized lungs subdivided into air sacs for efficient gas exchange. They survive dry periods by aestivation and take in air through a small vent made in the mud. Carbon dioxide is released through vestigial gills.
    • Eastivation: Similar to hibernation, but in response to warm temperatures, the fish becomes inactive and stops feeding.
  • Swim bladders:
    • Bichir (Polyoterus), bowfin (Amia), and gars (Lepisosteus) use their swim bladders for gas exchange.

Factors Affecting Oxygen Consumption Rate

• Life stage:
  • Fish eggs use minimal oxygen until hatching.
• Body weight:
  • Juvenile and adult fishes use more oxygen per hour than smaller ones due to greater metabolic demands of their larger tissue mass.
• Level of activity:
  • Swimming fish use more oxygen than resting ones due to higher metabolic demands of exercising red swimming muscles.
• Environmental temperature:
  • Fish in warmer waters, with adequate dissolved oxygen, generally have higher oxygen consumption rates because warmer temperatures increase the metabolic demands of ectotherm organisms.
• Feeding:
  • Oxygen consumption rates increase after feeding, as extra energy is required for digestion and growth. This extra energy is termed Specific Dynamic Action (SDA).

Explore the fascinating processes of respiration in fishes, including aerobic and anaerobic methods. Understand how oxygen is utilized for energy and the role of various respiratory organs. This quiz will assess your knowledge of external and internal respiration, as well as the overall significance of gas exchange in aquatic life.