How do fish breathe underwater?
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How Do Fish Breathe Underwater?
Fish have evolved a variety of mechanisms to breathe underwater, primarily through the use of gills. However, some species have also developed the ability to breathe air, a trait that has significant evolutionary and physiological implications.
Gills: The Primary Respiratory Organ
Most fish rely on gills for underwater respiration. Gills are specialized organs that extract oxygen from water and expel carbon dioxide. Water flows over the gill filaments, where oxygen is absorbed into the bloodstream and carbon dioxide is released. This process is highly efficient in oxygen-rich environments but can become challenging in hypoxic (low oxygen) conditions.
Air Breathing in Fish: Evolution and Adaptations
Evolutionary Background
Air breathing in fish has evolved multiple times throughout their evolutionary history. This adaptation has allowed certain species to exploit environments where water oxygen levels are insufficient for survival2. Air-breathing fish are found in various habitats, including freshwater, brackish, and marine environments, although they are less common in well-oxygenated marine waters1 2.
Structural Adaptations
Air-breathing fish exhibit a range of structural adaptations to facilitate aerial respiration. These adaptations can include modified gills, specialized air-breathing organs (ABOs) such as lungs or swim bladders, and even the use of the stomach or skin for gas exchange1 5. For example, the armoured catfish (Ancistrus chagresi) uses its stomach as an ABO, allowing it to survive in hypoxic conditions by increasing its oxygen utilization both through gills and the ABO5.
Sensory Control of Respiration
The transition from water to air breathing in bimodal breathers (fish that can breathe both water and air) is regulated by sensory information from peripheral receptors. Oxygen-sensitive chemoreceptors play a dominant role, stimulating both aquatic and aerial respiration based on oxygen availability. Mechanoreceptors in the air-breathing organs also help regulate the switch between water and air breathing3.
Environmental Triggers for Air Breathing
Hypoxia and Carbon Dioxide Levels
Hypoxia is a primary trigger for air breathing in fish. When water oxygen levels drop, fish may switch to air breathing to meet their metabolic needs. Additionally, high levels of carbon dioxide in the water can stimulate air breathing. For instance, the climbing perch (Anabas testudineus) increases its air-breathing frequency in response to elevated CO2 levels, completely switching to aerial respiration in extremely high CO2 conditions6.
Behavioral and Ecological Factors
Air breathing can also be influenced by behavioral and ecological factors. Some fish, like the swamp eel (Synbranchus marmoratus), breathe air during terrestrial excursions or when dwelling in hypoxic water. The threshold for initiating air breathing can vary with body size and environmental conditions7. Additionally, air-breathing behavior can be linked to activities such as feeding, escaping predators, or reproductive migrations8.
Conclusion
Fish have developed a range of adaptations to breathe underwater, primarily through the use of gills. However, in response to environmental challenges such as hypoxia, some species have evolved the ability to breathe air. This adaptation involves various structural changes and is regulated by sensory mechanisms that respond to oxygen and carbon dioxide levels. Understanding these mechanisms provides insight into the evolutionary and ecological diversity of fish respiration.
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Most relevant research papers on this topic
9 Air Breathing in Fishes
Fishes with air-breathing adaptations are primarily teleosts, with some exceptions, and their use is primarily driven by oxygen shortages in their environment.
Highly CitedControl of the respiratory mode in air-breathing fishes
Control of respiratory mode in air-breathing fishes relies on sensory information from three major peripheral receptors, with external chemoreceptors shifting the ventilatory emphasis from water to air breathing and internal chemoreceptors increasing both aquatic and aerial respiration.
Learning to Air-Breathe: The First Steps.
Air-breathing in vertebrates has evolved for the most obligate of fish, allowing for combined control of ventilation and acid-base status, offering promising research avenues.
The Transition to Air Breathing in Fishes: II. Effects of Hypoxia Acclimation on the Bimodal Gas Exchange of Ancistrus Chagresi (Loricariidae)
Hypoxia acclimation in Ancistrus chagresi fish improves its bimodal gas exchange capacity, increasing oxygen utilization by both gills and stomach, and allowing longer air breaths and reduced oxygen partial pressure.
Respiration in an Air-Breathing Fish, the Climbing Perch, Anabas Testudineus : II. Respiratory Patterns and the Control of Breathing
Climbing perch fish's respiratory patterns are influenced by carbon dioxide content, pH, and oxygen content, with air-breathing increasing in high CO2 water and aquatic respiration stopping when more than 20% CO2 is bubbled in.
THE TRANSITION TO AIR BREATHING IN FISHES III. EFFECTS OF BODY SIZE AND AQUATIC HYPOXIA ON THE AERIAL GAS EXCHANGE OF THE SWAMP EEL SYNBRANCHUS MARMORATUS
The swamp eel Synbranchus marmoratus can breathe air during terrestrial excursions and in hypoxic water, but no respiratory CO2 is released to air, requiring aquatic CO2 extrusion.
Swimming in air-breathing fishes.
Air-breathing fishes exhibit diverse swimming physiology, with some relying on air breathing for aerobic metabolism, while others prefer aquatic respiration for recovery and predator avoidance.
Non-RCT
Animal StudyAir- breathing in fish: Air- breathing organs and control of respiration: Nerves and neurotransmitters in the air-breathing organs and the skin.
Fishes' air-breathing control is influenced by peripheral and central nervous system receptors, with neuroepithelial cells in gills and lungs playing a role in respiratory reflexes.
Comparative transcriptome analysis between aquatic and aerial breathing organs of Channa argus to reveal the genetic basis underlying bimodal respiration.
The genetic basis for bimodal breathing in fish, such as Channa argus, involves angiogenesis, elastic fiber formation, ion uptake, and acid-base balance, with the aerial breathing organ being more efficient for gas exchange.
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