Gills are specialized organs that allow many aquatic animals to extract dissolved oxygen from water and release carbon dioxide. They serve as the primary means of respiration for a vast array of organisms. This function is essential, as water contains significantly less oxygen than air, enabling diverse aquatic species to inhabit various watery environments across the globe.
How Gills Work
The efficiency of gills stems from their design, which maximizes the surface area available for gas exchange. In many aquatic animals, particularly fish, gills are supported by bony or cartilaginous structures known as gill arches. Extending from these arches are numerous delicate filaments, which are further covered with tiny, leaf-like folds called lamellae.
As water flows over these lamellae, oxygen dissolved in the water diffuses across the thin walls of the gill tissue and into the bloodstream. Simultaneously, carbon dioxide diffuses from the fish’s blood into the water. This exchange is driven by concentration gradients, where oxygen moves from an area of higher concentration (water) to lower concentration (blood), and carbon dioxide moves in the opposite direction.
An efficient mechanism found in many aquatic vertebrates, including fish, is countercurrent exchange. In this process, blood flows through the gill capillaries in the opposite direction to the water passing over the gills. This opposing flow maintains a steep oxygen concentration gradient along the entire respiratory surface, allowing the blood to continuously pick up oxygen from the water. This system enables gills to extract a substantial amount (often over 80% to 90%) of the available oxygen from the water.
Different Types of Gills
Gills exhibit diversity in form and location across the aquatic animal kingdom, broadly categorized into external and internal types. External gills protrude directly from an animal’s body and are exposed to the surrounding water. These feathery or filamentous structures are richly supplied with blood vessels and are characteristic of certain larval amphibians, such as axolotls and larval salamanders, and some fish larvae like juvenile bichirs. Nudibranchs (sea slugs) also possess external gills. While effective for respiration, their exposed nature makes them vulnerable to damage.
Internal gills, in contrast, are housed within a protective body cavity. This type is prevalent in most fish, where the gills are located on either side of the head, typically covered by a bony plate called an operculum. Water is actively pumped over these internal gills, ensuring a continuous flow for gas exchange. Many crustaceans, such as crabs and shrimp, also possess internal gills located under their carapace within a gill chamber. Mollusks, like clams and snails, similarly utilize internal gills for respiration and, in some cases, for filter feeding.
Gills Beyond Breathing
Beyond their primary role in respiration, gills perform several other functions that support an aquatic animal’s overall physiological balance. One such function is osmoregulation, maintaining water and salt balance within the body. Freshwater fish, which are saltier than their environment, actively absorb ions like sodium and chloride from the water through specialized cells in their gills to prevent excessive salt loss. Conversely, saltwater fish, living in an environment saltier than their internal fluids, use their gills to actively excrete excess salts, preventing dehydration.
Gills also play a significant role in ion exchange, regulating ions like hydrogen (H+), bicarbonate (HCO3-), and ammonia. This capability allows fish to maintain acid-base balance within their bodies and excrete nitrogenous waste products, primarily ammonia, into the water. These processes are facilitated by specialized cells, often called ionocytes or chloride cells, located within the gill epithelium.
In some semi-aquatic animals, like land crabs, gills have undergone modifications to enable survival both in and out of water. While still present, the gills of terrestrial crabs have a reduced surface area for gas exchange compared to their aquatic counterparts. Their primary role has shifted to water storage and ion regulation, with accessory breathing organs, such as lung-like structures in the gill chamber, handling much of the air breathing when on land. These adaptations highlight the versatility of gill structures in response to diverse environmental pressures.
Environmental Factors Affecting Gills
Gills are sensitive to changes in their surrounding aquatic environment. Water temperature significantly impacts gill function; as water warms, its capacity to hold dissolved oxygen decreases, while the metabolic demand for oxygen in cold-blooded fish increases. This can lead to oxygen limitation, potentially restricting fish growth and survival in warmer waters. Extreme temperatures can also induce structural changes in gill tissue.
Low dissolved oxygen levels impair gill function, leading to reduced oxygen uptake and physiological stress in aquatic animals. Fish may become lethargic, experience reduced growth rates, or die if oxygen concentrations fall below critical thresholds, often around 2-4 mg/L for many species. Different species and life stages exhibit varying tolerances to low oxygen.
Water pH (acidity or alkalinity) also affects gill health. High acidity or alkalinity can cause physical damage to gill tissues, leading to increased mucus production and thickening of the gill epithelium, which hinders gas exchange. Changes in pH can also disrupt the gill’s ability to transport ions, affecting acid-base balance and overall physiological stability.
The presence of pollutants and sedimentation can compromise gill function. Suspended sediments, for example, can physically abrade and clog gill filaments, increasing mucus secretion and making fish more susceptible to disease. Various chemical pollutants, such as heavy metals, can also damage gill epithelium, impairing ion regulation and gas exchange. These environmental stressors underscore the interconnectedness of aquatic health and the well-being of gill-breathing organisms.