Eels are elongated, ray-finned fish belonging to the order Anguilliformes, comprising approximately 1,000 species. Eels are fish that possess gills for respiration. However, some species also exhibit abilities to survive for periods outside of water, which often prompts questions about their breathing mechanisms. This adaptability allows them to navigate diverse environments, including freshwater, saltwater, and even terrestrial landscapes temporarily.
Underwater Respiration in Eels
Eels primarily rely on gills to extract oxygen from water for their survival. Gills are specialized organs found on both sides of a fish’s throat, equipped with capillaries. These capillaries are within gill filaments or lamellae, providing a large surface area for gas exchange.
Underwater breathing begins when an eel draws water into its mouth. Unlike many bony fish that use a protective gill cover (operculum) to pump water, some eels, like moray eels, manually force water over their gills by rhythmically opening and closing their mouths. Water is pushed over gill filaments, where dissolved oxygen diffuses into the eel’s bloodstream. Carbon dioxide, a waste product, moves from the blood into the water and is expelled through gill openings. This countercurrent exchange system, where blood flows opposite to water, allows gills to extract a high percentage of oxygen.
Eel Adaptations for Air Breathing
While gills are the primary respiratory organs for eels underwater, certain species possess adaptations that allow them to survive temporarily out of water. This is due to cutaneous respiration, the absorption of oxygen through the skin. Eels have a dense network of capillaries beneath their skin, facilitating gas exchange when they are on land or in low-oxygen water.
To maintain skin breathing, eels secrete mucus that keeps their bodies moist and prevents desiccation. This slime is important because cutaneous respiration is most effective when the skin remains damp. Eels might utilize this ability to migrate across wet land to find new water sources, bypass obstacles like dams, or escape stagnant water bodies with depleted oxygen levels. For instance, the European eel can absorb over one-third of its total oxygen intake through its skin when in water, and this contribution can increase significantly, up to 50% or more, when out of water. This flexibility enables eels to endure challenging conditions, though it remains a temporary survival mechanism rather than a substitute for sustained gill function.