Freshwater eels belong to the genus Anguilla and are distinct from other species often called eels, such as moray or electric eels. Their biology involves one of the most remarkable migratory cycles in the animal kingdom. Freshwater eels spend the majority of their lives growing and maturing in rivers, lakes, and estuaries. The defining characteristic of these species is their unique habit of leaving their freshwater homes to reproduce in deep ocean waters, a migratory pattern that sets them apart from most other fish.
Catadromous Life Cycle
The life journey of the freshwater eel is defined by its catadromous nature: they migrate from fresh water down to the sea to spawn. This cycle begins in remote oceanic areas, such as the Sargasso Sea in the western Atlantic Ocean, where adult eels gather to reproduce before they die. After the eggs hatch, the first stage is the leptocephalus, a transparent, leaf-shaped larva. These larvae drift on ocean currents, sometimes for up to two years, carried thousands of miles toward the coasts where they will eventually live.
As they approach the continental shelf and enter brackish waters, the larvae undergo metamorphosis into the glass eel stage, becoming cylindrical yet remaining mostly transparent. This transparency helps them avoid predators during the transition from the open ocean to coastal estuaries and river mouths. They then develop pigment and are referred to as elvers, which begin the strenuous upstream journey into freshwater systems. Elvers possess a strong drive to ascend rivers, sometimes in massive, collective movements.
The longest phase of their life is spent in freshwater, where they are known as yellow eels due to their greenish-yellow coloring. This phase can last five to twenty years, depending on the species and environmental conditions, during which they mature and accumulate the fat reserves necessary for their final migration. Once they reach sexual maturity, they undergo a profound metamorphosis into silver eels. This transformation involves the digestive tract shutting down, a significant increase in muscle mass, and the enlargement of their eyes to better suit the deep-sea environment. The silver eels then undertake the long, non-feeding migration back to the oceanic spawning grounds to complete the cycle.
Major Eel Species Worldwide
The genus Anguilla includes about 15 species distributed across the globe. Among the most recognized are the European Eel (Anguilla anguilla) and the American Eel (Anguilla rostrata), which both share the Sargasso Sea as their common Atlantic spawning site. Despite sharing a single breeding area, ocean currents carry their larvae to opposite sides of the ocean, leading them to colonize rivers from Greenland down to Morocco and from the Gulf of Mexico up to Canada.
Another commercially significant species is the Japanese Eel (Anguilla japonica), which inhabits river systems across East Asia, including Japan, Korea, China, and Taiwan. Unlike their Atlantic cousins, the Japanese Eel migrates to spawning grounds located in the western Pacific Ocean, near the West Mariana Ridge. The focus on these species highlights the immense geographical scale of their life cycle.
Remarkable Physical Adaptations
The long, serpentine body shape of the freshwater eel is a major adaptation that facilitates its unique lifestyle. Unlike most fish, their specialized skin allows them to absorb a significant amount of oxygen directly from the water or the air. This ability, known as cutaneous respiration, can account for up to 35% of their total oxygen uptake while submerged and even more when they are out of the water. This respiratory flexibility allows them to survive in low-oxygen environments, such as stagnant mud or during periods of drought.
Their powerful musculature and ability to breathe through their skin enable them to traverse short distances over damp ground. This overland movement helps them bypass obstacles like waterfalls or small dry patches between water bodies. Furthermore, their elongated dorsal and anal fins are continuous with the tail fin, creating a single, ribbon-like structure that propels them efficiently through water in a sinuous motion. As opportunistic predators, yellow eels use their streamlined bodies to navigate complex riverine habitats, consuming insects, crustaceans, and smaller fish.
Conservation Status
The complex, multi-habitat life cycle of freshwater eels has made them highly vulnerable to global environmental changes and human activity. The European Eel is listed as Critically Endangered by the International Union for Conservation of Nature (IUCN), reflecting a dramatic decline in recruitment. The Japanese Eel is also listed as Endangered, indicating severe population stress across its range.
The construction of dams and hydroelectric power plants is a significant threat, acting as impassable barriers for both upstream elvers and downstream silver eels. Eels attempting to migrate downstream are often killed passing through the turbines. Other major factors contributing to population decline include the overfishing of glass eels for the international aquaculture trade, which substantially reduces the number of young eels entering freshwater systems. Pollution, such as polychlorinated biphenyls (PCBs), and the potential impact of climate change on the ocean currents that transport their larvae are also major concerns.