Fish Gills: Anatomy, Function, and Common Health Issues

Fish gills are the primary organs for gas exchange, osmoregulation, acid-base balance, and nitrogen excretion in fish. For farmers and aquaculture operators, understanding gill structure and function is essential for recognizing early signs of disease, managing water quality, and making informed decisions about treatment and culling. This article explains the anatomy of fish gills, how they work, and the common health issues that affect them, with practical guidance for farm-level observation and record keeping.

At a Glance: Gill Health Reference

Gross Anatomy of Fish Gills

The gill apparatus sits within the branchial chamber on either side of the fish head. In teleost fish, which include most farmed species such as tilapia, trout, salmon, and carp, four pairs of gill arches support the respiratory surfaces. Each arch carries two rows of filaments, and each filament supports rows of plate-like lamellae. The lamellae provide the large surface area needed for gas exchange and are positioned so that blood flows in the opposite direction to water passing over the gills, a counter-current arrangement that maximizes oxygen extraction.

The gross anatomy differs among fish groups. Hagfishes have primitive gill pouches, while lampreys have arch-like gills similar to higher fishes. In lampreys and elasmobranchs such as sharks and rays, the gill filaments are supported by a complete interbranchial septum, and water exits through external branchial slits or pores. In teleosts, the interbranchial septum is much reduced, leaving the ends of the filaments unattached, and the multiple gill openings are replaced by a single caudal opening covered by the operculum. These differences matter for farmers who handle different species, because the visible gill structure and the way gills respond to handling and disease can vary.

The basic functional unit of the gill is the filament, which supports rows of plate-like lamellae. The lamellae are designed for gas exchange with a large surface area and a thin epithelium surrounding a well-vascularized core of pillar cell capillaries. The lamellae are positioned for the blood flow to be counter-current to the water flow over the gills. This counter-current flow is a critical design feature that allows fish to extract a high proportion of dissolved oxygen from water, even when oxygen levels are low.

Microscopic Structure of the Gill Epithelium

Despite marked differences in the gross anatomy of the gill among various fish groups, the cellular constituents of the epithelium are remarkably similar. The lamellar gas-exchange surface is covered by squamous pavement cells, which form a thin, smooth barrier between blood and water. Large mitochondria-rich ionocytes and mucocytes are found in greatest frequency in the filament epithelium. Ionocytes are responsible for active ion transport, which underpins osmoregulation, while mucocytes produce mucus that protects the gill surface and traps particulate matter.

The demands of ionoregulation can upset the balance between gas exchange and ion balance. This trade-off is known as the respiratory-osmoregulatory compromise. The gill must maintain a functional surface area large enough for gas exchange while limiting water and ion movement across the epithelium. Marine teleosts face water loss and must actively excrete ions, while freshwater teleosts face water gain and must actively take up ions. The gill epithelium manages this by adjusting the functional surface area and the activity of ionocytes in response to environmental conditions.

The gill is the most physiologically diversified vertebrate organ, and its vasculature is the most intricate. Three vascular networks can be identified within the gill filament. The arterioarterial respiratory pathway consists of the lamellae and afferent and efferent segments of the branchial and filamental arteries and lamellar arterioles. The body of the filament contains two post-lamellar pathways: the interlamellar and nutrient systems. The interlamellar system is an extensive ladder-like network of thin-walled, highly distensible vessels that traverses the filament between and parallel to the lamellae and continues around the afferent and efferent borders of the filament. Interlamellar vessels are supplied by short, narrow-bore feeder vessels from the medial wall of the efferent filamental artery. A myriad of narrow-bore, tortuous arterioles arise from the basal efferent filamental artery and efferent branchial artery and anastomose to form the nutrient circulation of the arch and filament.

Pillar cells are unique to the fish gill and form the lining of the respiratory vasculature. These cells may have substantial metabolic effects on circulating hormones. The non-respiratory pathways appear to be lined with both typical and unusual endothelial cells, although the fine structure and function of these vessels are largely unknown. Many of the anatomical characteristics of interlamellar vessels are strikingly similar to those of mammalian lymphatic capillaries, with the exception that interlamellar vessels are directly fed by arteriovenous-like anastomoses. It is likely that gill interlamellar vessels and mammalian lymphatics are physiologically, if not embryologically, related.

Gill Functions Beyond Respiration

The fish gill is a multifunctional organ responsible for respiration, osmoregulation, acid-base balance, nitrogen excretion, and metabolism of circulating hormones. Two or more microcirculatory systems subserve these activities and form one of the most complex vascular networks found in any vertebrate. For farmers, this means that gill health affects far more than oxygen uptake. A fish with damaged gills may show poor growth, osmoregulatory stress, and increased susceptibility to disease even when dissolved oxygen levels appear adequate.

Gas exchange is the most obvious function. Water flows over the lamellae, and oxygen diffuses across the thin epithelium into the blood, while carbon dioxide diffuses in the opposite direction. The counter-current arrangement ensures that the concentration gradient for oxygen is maintained along the entire length of the lamella. Gas transport in water-breathing fish depends on the efficiency of this arrangement, and any thickening of the gill epithelium, whether from disease, toxins, or nutritional deficiency, reduces gas transfer efficiency.

Osmoregulation is equally important. The gill epithelium actively transports ions to maintain the internal salt and water balance of the fish. In freshwater, fish must take up ions from the dilute environment and excrete large volumes of dilute urine. In seawater, fish must actively excrete ions and conserve water. The mitochondria-rich ionocytes in the filament epithelium are the primary sites of these transport processes. Environmental factors such as water hardness and calcium concentration can influence gill remodeling and ionocyte activity.

Acid-base balance is maintained through the regulated exchange of acid and base equivalents across the gill epithelium. Nitrogen excretion occurs primarily as ammonia, which diffuses across the gill surface. The gills also metabolize circulating hormones, which means that gill health can influence endocrine function throughout the body.

Gill Innervation and Blood Flow Control

Inspection of the dorsal end of fish gills reveals an impressive set of nerve trunks connecting the gills to the brain. These trunks are branches of cranial nerves VII (the facial) and especially IX (the glossopharyngeal) and X (the vagus). The nerve trunks carry a variety of nervous pathways to and from the gills. A substantial fraction of the nerves running in the branchial trunks carry afferent sensory information from receptors within the gills. There are also efferent motor pathways that control muscles within the gills, blood flow patterns, and possibly secretory functions.

The arrangement of the microanatomy, particularly the blood vessels, and its innervation are strikingly complex. The complexity reflects the many functions of the gills and illustrates that the control of blood flow patterns in the gills is of crucial importance in modifying the efficiency of its chief functions: gas transfer and salt balance. The respiratory-osmoregulatory compromise is maintained by minimizing the blood-water exchange functional surface area of the gills to a level where excessive water loss in marine teleosts or gain in freshwater teleosts is kept low while ensuring sufficient gas exchange.

The autonomic nervous system plays a major role in controlling blood flow through the gills. Changes in blood flow distribution can redirect blood to or away from the respiratory lamellae, adjusting the functional surface area in response to oxygen demand, exercise, or environmental conditions. For farmers, this means that stressed or excited fish may have different gill perfusion patterns than calm fish, which can affect how quickly they respond to changes in water oxygen levels.

Gill-Associated Lymphoid Tissue and Immunity

Fish mucosal tissues lack the organized lymphoid structures present in mammals. This is also true for the gills, with the gill-associated lymphoid tissue being mainly composed of B and T cells scattered throughout the epithelium and the lamellae. In some species such as rainbow trout or Atlantic salmon, a more organized structure designated as the interbranchial lymphoid tissue is also identified. The interbranchial lymphoid tissue is one of several mucosal immune structures in teleosts, along with the kidney, thymus, spleen, and nasopharynx-associated lymphoid tissue.

The gill immune response involves both innate and adaptive elements. Studies in rainbow trout have demonstrated that memory responses are locally organized in the gills. Re-infected fish did not up-regulate genes related to inflammation as happens in the primary infection, but preferentially modified genes related to adaptive immunity, mainly to B cell function, experiencing a significant IgM and IgT clonal expansion. Re-infected fish produced increased levels of specific IgM both locally and in serum.

Gill transcriptomes are enriched in phagosome, focal adhesion, extracellular matrix-receptor interaction, and cytokine-cytokine receptor interaction pathways, accompanied by increased expression of major histocompatibility complex class I and II, mannose receptor, integrin, and calnexin. These patterns indicate innate activation, enhanced phagocytic capacity, epithelial barrier reinforcement, and adaptive immune coordination. For farmers, this means that gill health is closely tied to disease resistance. Fish with damaged gill epithelium are more vulnerable to bacterial, viral, and parasitic infections.

Gill Remodeling and Environmental Adaptation

Some fish species undergo gill remodeling via the proliferation or regression of an interlamellar cell mass, resulting in the modification of gill surface area in response to environmental hypoxia or ion levels. This remodeling is a phenotypically plastic response that allows fish to adjust their gill surface area to match environmental conditions. In low oxygen conditions, fish may increase gill surface area by reducing the interlamellar cell mass. In high oxygen or high ion conditions, the interlamellar cell mass may proliferate, reducing the functional surface area and limiting ion and water exchange.

Environmental calcium regulates gill remodeling in euryhaline teleost fish. Studies on mangrove rivulus have shown that fish exposed to hard water had a significantly lower interlamellar cell mass height than fish exposed to low calcium water. The addition of calcium to low calcium water restored gill surface area. This provides evidence of an ionic trigger for gill remodeling in teleost fishes. For farmers, water hardness and calcium levels in the culture system can influence gill structure and function, which may affect fish performance and disease susceptibility.

Amphibious fishes that move onto land face additional challenges. Delicate structures such as gills are especially vulnerable to collapse and loss of surface area out of water. Some amphibious fish species solve this problem using phenotypically plastic changes that provide mechanical support and increase stiffness at the level of the gill lamellae, the filaments, and the whole arches. After a period in terrestrial conditions, enlargement of an inter-lamellar cell mass filled the water channels between gill lamellae, possibly to provide structural support or reduce evaporative water loss. Similar gill remodeling has been described in several other fish species, suggesting this may be an ancestral trait.

Common Gill Diseases and Health Issues

Gill health issues in farmed fish can arise from infectious agents, water quality problems, nutritional deficiencies, and environmental stressors. Recognizing the signs of gill disease early is critical for effective management.

Bacterial Gill Infections

Bacterial pathogens represent major infectious threats to farmed fish. In Nile tilapia, pathogens including Flavobacterium oreochromis, Aeromonas veronii, Streptococcus agalactiae, and Edwardsiella tarda are significant causes of disease. Flavobacterium columnare, the causative agent of columnaris disease, is a particular concern for gill health. Infection with Flavobacterium columnare can cause severe gill necrosis, and the gill is a primary route of entry for this pathogen.

Dietary nutrition influences gill resistance to bacterial infection. Studies in grass carp have shown that dietary phosphorus deficiency resulted in a decline in gill antimicrobial compound production and reduced expression of antimicrobial peptides. Phosphorus deficiency also increased post-infection gill inflammation and could impair immune barrier function. The study demonstrated that phosphorus deficiency enhanced apoptosis signaling, inhibited antioxidant response pathways, decreased antioxidant enzyme transcript levels and activities, and increased reactive oxygen species, malondialdehyde, and protein carbonyl levels. These changes suggest that protection against oxidative damage was compromised with phosphorus deficiency. Phosphorus deficiency also up-regulated myosin light chain kinase transcript levels, a response associated with the down-regulation of tight junction proteins including zonula occludens 1, occludin, and claudin.

Dietary iron status also affects gill immune function and structural integrity. Iron deficiency or excess can impair immune function and structural integrity in the gill after infection with Flavobacterium columnare, with regulation involving NF-kB, TOR, JNK, p38MAPK, Nrf2, and MLCK signaling pathways. For farmers, this means that feed formulation and mineral supplementation directly influence gill health and disease resistance.

Parasitic Gill Infections

Parasitic infections of the gills are common in farmed fish. Parasites such as monogenean flukes, copepods, and protozoans can cause significant gill damage. Clinical signs include flashing, piping at the water surface, reduced feeding, and increased mucus production on the gills. Severe infestations can cause anemia, respiratory distress, and secondary bacterial infections.

Gill parasites are often visible on gross examination. The gills may appear pale, have excess mucus, or show visible parasites on the filament tips. Microscopic examination of gill scrapings is needed to identify the specific parasite species. Treatment options depend on the parasite and the fish species, and withdrawal periods must be observed for food fish.

Fungal and Mycobacterial Infections

Systemic granulomatous mycobacteriosis has been reported in aquarium fish, with nodular lesions in multiple internal organs including the gills, spleen, and kidney. Histopathological analysis demonstrated severe chronic systemic granulomatous inflammation, and Ziehl-Neelsen staining highlighted abundant intralesional acid-fast bacilli. Molecular analysis showed that the detected organism was most closely related to Mycobacterium marinum. This case emphasizes the need for continuous disease surveillance and improved diagnostic awareness of non-tuberculous mycobacterial infections in fish.

Mycobacterial infections are slow-growing and difficult to treat. Infected fish may show chronic weight loss, skin lesions, and nonspecific signs of illness. Because mycobacteria can infect humans, farm workers should use gloves when handling fish with suspected mycobacterial infections and practice good hygiene.

Gas Bubble Trauma

Gas bubble trauma occurs when fish are exposed to water supersaturated with dissolved gases, usually nitrogen. Supersaturation can occur when water is heated, when air enters pressurized water systems, or when photosynthetic activity is high. Gas bubble trauma affects gill function and can cause significant mortality. The condition is a known cause of gill dysfunction in fish.

Clinical signs of gas bubble trauma include bubbles in the gill filaments, eyes, and skin. Fish may show abnormal swimming behavior, flashing, and gasping at the surface. Prevention focuses on maintaining proper gas levels in the water supply, particularly in recirculating systems and hatcheries where water is pumped and heated.

Gill Health in Recirculating Aquaculture Systems

Recirculating aquaculture systems have been proposed as the future of aquaculture because they can be used anywhere regardless of access to water, offer a high level of control over the farming environment including biosecurity, and are considered sustainable. However, despite continuous development, there can still be issues with water quality affecting gill health of fish farmed in these systems.

Several experimental studies have inadequately reported conditions, particularly water quality, making it difficult to determine if the observed effects were due to water quality issues or the RAS system itself. It is crucial for studies investigating the impact of RAS on fish to report water quality during the study. Assessments of RAS effects on gill health should include sufficient independent replicates and flow-through controls using a common water source.

Various methods have been used to assess gill health in RAS, including gill histology, presence of pathogens, gene expression in the gills, and gill microbiome analysis. Differences in gill health in fish from RAS and a flow-through system have been shown for a number of freshwater and marine fish species. However, these results have been inconsistent across studies, and some results have been challenging to interpret as indicators of gill health. Holistic studies including a number of different methods to assess fish gills would give more conclusive results. More research is needed, in particular on brackish and marine RAS, to fully understand their impacts on gill health.

For farmers operating RAS, regular monitoring of water quality parameters including ammonia, nitrite, nitrate, pH, dissolved oxygen, carbon dioxide, and total gas pressure is essential. Gill health assessments should be part of routine health checks, and any changes in gill appearance or fish behavior should trigger a review of water quality and system performance.

Practical Gill Health Assessment on the Farm

Regular gill examination is a core skill for fish farmers. Gill health assessment should be part of routine health monitoring, and findings should be recorded to track changes over time.

Step 1: Observe Fish Behavior

Behavioral signs of gill problems include piping at the water surface, flashing or rubbing against surfaces, reduced feeding, lethargy, and gathering at water inlets. Fish with gill damage may show rapid opercular movements or breathe with the mouth open. These signs are nonspecific and can indicate water quality problems, infectious disease, or both.

Step 2: Examine the Gills

To examine gills, sedate or euthanize a representative sample of fish following approved protocols. Lift the operculum and examine the gill arches. Healthy gills are bright red, have distinct filaments and lamellae, and are free of excess mucus, parasites, or lesions. Abnormal findings include pale or discolored gills, excess mucus, clubbed or fused lamellae, frayed filament tips, visible parasites, nodules, or hemorrhages.

Step 3: Collect Samples for Microscopy

Gill scrapings and gill clips can be examined under a microscope to identify parasites, bacteria, and cellular changes. Wet mounts of gill tissue can reveal protozoan parasites, monogeneans, and bacterial mats. Histopathology requires sending samples to a diagnostic laboratory and is useful for identifying chronic or subtle changes.

Step 4: Review Water Quality

Gill health is closely tied to water quality. Review records for dissolved oxygen, temperature, pH, ammonia, nitrite, nitrate, alkalinity, hardness, and total gas pressure. Compare current values to historical baselines and to the tolerance ranges for the species being farmed. Sudden changes in water quality can cause acute gill damage, while chronic suboptimal conditions can predispose fish to disease.

Step 5: Record Findings

Maintain a gill health log that includes the date, tank or pond identification, species, number of fish examined, behavioral observations, gill appearance, water quality parameters, and any treatments applied. Consistent records allow you to identify trends and respond early to emerging problems.

Records and Measurements for Gill Health Monitoring

Common Failure Patterns in Gill Health Management

Several recurring problems undermine gill health on farms. Recognizing these patterns can help farmers avoid costly losses.

Ignoring Early Behavioral Signs

Fish often show subtle behavioral changes before gill damage becomes visible. Reduced feeding, slight increases in opercular movement, or fish holding near the water surface may be the first signs of gill irritation. Farmers who wait for visible gill lesions or elevated mortality before acting lose valuable time. Early investigation of behavioral changes, including water quality testing and gill examination, is the most effective way to prevent gill disease outbreaks.

Inadequate Water Quality Monitoring

Gill tissue is delicate and responds quickly to changes in water quality. Ammonia and nitrite are directly toxic to gill tissue, and low dissolved oxygen causes respiratory stress. Farmers who monitor water quality infrequently or only after problems appear miss the early warnings that gill health is at risk. Regular monitoring with calibrated equipment and proper record keeping is essential.

Poor Nutrition and Feed Management

Dietary deficiencies in minerals such as phosphorus and iron can impair gill immune function and structural integrity, making fish more susceptible to bacterial infection. Feeding poor-quality feed, underfeeding, or using feed with imbalanced mineral content compromises gill health even when water quality is good. Feed should be sourced from reputable manufacturers and stored properly to prevent nutrient degradation.

Overlooking Biosecurity

Gill pathogens can enter a farm through infected fish, contaminated equipment, or water sources. Farmers who do not quarantine new fish, disinfect equipment, or control visitor access increase the risk of introducing gill diseases. Biosecurity protocols should be written, implemented, and reviewed regularly.

Delaying Professional Diagnosis

Some gill conditions require professional diagnosis. Farmers who attempt to treat gill disease without a confirmed diagnosis may use ineffective treatments, waste money, and allow the disease to progress. When gill problems do not respond to initial management changes, or when mortality is elevated, contact a fish health professional or diagnostic laboratory.

Welfare and Safety Considerations

Gill health is a direct welfare concern. The gill is a sensitive organ, and damage to it causes respiratory distress and pain. Fish with severe gill damage may be unable to maintain adequate oxygen uptake, leading to lethargy, loss of appetite, and increased susceptibility to predation and disease. Farmers have a responsibility to maintain water quality and manage disease to prevent gill damage.

Handling fish for gill examination causes stress and can damage delicate gill tissue. Fish should be sedated or euthanized humanely before gill examination, and handling time should be minimized. Workers should be trained in proper fish handling techniques and humane euthanasia methods.

Some gill pathogens can infect humans. Mycobacterium marinum, which can cause gill lesions in fish, is a zoonotic pathogen that can cause skin infections in people who handle infected fish or contaminated water. Farm workers should wear gloves when handling fish, wash hands thoroughly after working with fish, and cover any cuts or abrasions. Workers with compromised immune systems should avoid handling fish with suspected mycobacterial infections.

Food safety is also relevant. Treatments for gill diseases must be approved for use in food fish, and withdrawal periods must be observed. Farmers should keep accurate treatment records and follow label instructions for all medications and chemicals.

Professional Escalation Criteria

Contact a fish health professional or diagnostic laboratory when any of the following conditions are present:

• Mortality exceeds the normal baseline for the farm or system
• Gill lesions are severe, widespread, or worsening despite management changes
• Multiple species or multiple tanks are affected
• Fish show neurological signs such as spiraling or loss of equilibrium
• A zoonotic pathogen such as Mycobacterium marinum is suspected
• Water quality problems cannot be corrected with available resources
• The cause of gill disease is unclear after initial investigation

When submitting samples to a diagnostic laboratory, include live or freshly euthanized fish, water quality records, treatment records, and a description of the clinical signs and farm history. Good samples and complete information improve the accuracy of diagnosis and the usefulness of treatment recommendations.

Limitations of On-Farm Gill Assessment

On-farm gill examination has limitations. Gross examination can reveal obvious lesions, parasites, and color changes, but it cannot identify subtle cellular changes, early infections, or the specific cause of gill damage. Microscopy of gill scrapings requires skill and equipment, and some pathogens are difficult to identify without specialized staining or molecular testing. Histopathology, which is the gold standard for diagnosing many gill conditions, requires sending samples to a laboratory and waiting for results.

Water quality testing also has limitations. Test kits can give inaccurate results if they are expired, stored improperly, or used incorrectly. Probes require regular calibration and maintenance. Farmers should use quality equipment, follow manufacturer instructions, and verify results with independent methods when accuracy is critical.

Nutritional influences on gill health are complex and not fully understood. While studies have shown that dietary phosphorus and iron affect gill immune function, the optimal levels vary by species, life stage, and environmental conditions. Feed manufacturers provide guidance on mineral levels, but farmers should monitor fish performance and adjust feeding practices based on observed outcomes.

Frequently Asked Questions

What do healthy fish gills look like?

Healthy fish gills are bright red, which indicates good blood flow and oxygenation. The filaments should be distinct, evenly spaced, and free of excess mucus, parasites, or lesions. The operculum should close fully and smoothly. Pale, brown, or discolored gills may indicate anemia, poor water quality, or disease.

Why are my fish gasping at the water surface?

Gasping at the water surface, also called piping, usually indicates low dissolved oxygen or gill damage that impairs oxygen uptake. Check dissolved oxygen levels first. If oxygen is adequate, examine the gills for parasites, excess mucus, or lesions. Also check ammonia, nitrite, pH, and temperature, as these can affect oxygen availability and gill function.

How often should I examine fish gills?

Examine gills as part of routine health checks, which should occur at least weekly in intensive systems and more frequently during periods of stress such as grading, transport, or disease outbreaks. Examine gills whenever fish show behavioral changes, reduced feeding, or elevated mortality. Keep records of all examinations.

Can poor nutrition cause gill disease?

Yes. Dietary deficiencies in minerals such as phosphorus and iron can impair gill immune function and structural integrity, making fish more susceptible to bacterial infection. Feed a complete, balanced diet from a reputable manufacturer and store feed properly to prevent nutrient degradation.

What water quality parameters are most important for gill health?

Dissolved oxygen, ammonia, nitrite, pH, temperature, and total gas pressure are the most important water quality parameters for gill health. Carbon dioxide and alkalinity also matter. Maintain these parameters within the tolerance ranges for your species and monitor them regularly with calibrated equipment.

How do I know if a gill problem is infectious or environmental?

Infectious and environmental causes of gill disease can produce similar signs. Review water quality records for recent changes or excursions. Examine gills for visible parasites or lesions. If water quality is within normal ranges and gill lesions are present, an infectious cause is more likely. Submit samples to a diagnostic laboratory for confirmation.

Can gill diseases spread between tanks or ponds?

Yes. Gill pathogens can spread through shared water, contaminated equipment, infected fish, and personnel. Implement biosecurity protocols including quarantine of new fish, disinfection of equipment, and control of water flow between units. Isolate affected tanks or ponds when disease is suspected.

Are fish gill diseases dangerous to humans?

Most gill pathogens are not zoonotic, but some can infect humans. Mycobacterium marinum, which can cause gill lesions, can cause skin infections in people who handle infected fish or contaminated water. Wear gloves when handling fish, wash hands thoroughly, and cover cuts and abrasions. Seek medical attention if you develop skin lesions after handling fish.

Related Farming Guides

• Alpaca and Llama Health Management: Common Diseases and Preventive Care
• Fish Health Observation and Mortality Investigation
• Aquaponics Fish Health and System Management
• Biosecurity for Fish Farms
• Feeding Farmed Fish Efficiently

References and Further Reading

• FAO Animal Production and Health. Food and Agriculture Organization of the United Nations.
• Animal Health and Welfare. USDA National Agricultural Library.
• Animal and Veterinary Resources. U.S. Food and Drug Administration.
• Animal Health and Welfare. World Organisation for Animal Health.
• Animal Production and Protection. USDA Agricultural Research Service.
• Vascular anatomy of the fish gill.. The Journal of experimental zoology, 2002.
• Fish gill morphology: inside out.. The Journal of experimental zoology, 2002.
• Anatomy of teleost fish immune structures and organs.. Immunogenetics, 2021.
• Branchial innervation.. The Journal of experimental zoology, 2002.
• Vasculature of the fish gill: anatomical correlates of physiological functions.. Journal of electron microscopy technique, 1991.
• Gill Health in Fish Farmed in Recirculating Aquaculture Systems (RAS): A Review.. Journal of fish diseases, 2025.
• Gill remodelling during terrestrial acclimation in the amphibious fish Polypterus senegalus.. Journal of morphology, 2019.
• Environmental calcium regulates gill remodeling in a euryhaline teleost fish.. Journal of experimental zoology. Part A, Ecological and integrative physiology, 2017.
• Transcriptomic profiling and IgM and IgT repertoire dynamics in rainbow trout gills following primary and secondary challenge with &lt,i&gt,Lactococcus petauri&lt,/i&gt,.. 2026.
• Systemic Granulomatous Mycobacteriosis in Orbiculate Batfish (&lt,i&gt,Platax orbicularis&lt,/i&gt,) Associated with &lt,i&gt,Mycobacterium marinum&lt,/i&gt,-like Organism in an Aquarium in South Korea.. 2026.
• Transcriptomic responses of gill and intestinal tissues in Nile tilapia (Oreochromis niloticus) to bacterial infection following sequential nanoimmersion and hydrogel-based multivalent vaccination.. 2026.
• Causes and consequences of gas bubble trauma on fish gill function. Journal of Comparative Physiology □ B, 2024.
• The origins of gas exchange and ion regulation in fish gills: evidence from structure and function. Journal of Comparative Physiology □ B, 2024.
• Gas Transport and Gill Function in Water-Breathing Fish. 2009.
• Research advances in the structure, function, and regulation of the gill barrier in teleost fish. Water Biology and Security, 2023.
• Dietary phosphorus deficiency caused alteration of gill immune and physical barrier function in the grass carp (Ctenopharyngodon idella) after infection with Flavobacterium columnare. Aquaculture, 2019.
• The impaired immune function and structural integrity by dietary iron deficiency or excess in gill of fish after infection with Flavobacterium columnare: Regulation of NF-&kgr,B, TOR, JNK, p38MAPK, Nrf2 and MLCK signalling. Fish and Shellfish Immunology, 2018.
• 1 General Anatomy of the Gills. Fish Physiology, 1984.
• The fish gill: Where fish physiology begins. Fish Physiology, 2023.
• Anatomy, histology, and morphology of fish gills in relation to feeding habits: a comparative review of marine and freshwater species. BMC Zoology, 2025.

This article is educational and is not a substitute for veterinary diagnosis, treatment, public-health guidance, or regulatory reporting.