Eels are among the most enigmatic and specialized creatures in the aquatic world, possessing a serpentine form. These elongated animals navigate a vast range of habitats, from the deep ocean to inland rivers, performing some of the longest and most mysterious migrations known in nature. Their lives are characterized by dramatic physical transformations and unique adaptations that enable them to thrive across diverse environments. Understanding what eels do requires examining their complex anatomy, life cycle, and specialized survival techniques.
Defining the True Eel Classification and Anatomy
The term “eel” is often used loosely for any long, slender fish, but the true eels belong exclusively to the Order Anguilliformes. This order encompasses over 800 species, including familiar groups like moray eels and conger eels, and is distinct from other look-alikes. The freshwater eels of the Family Anguillidae are the only ones that regularly inhabit freshwater for much of their lives, making their behavior particularly notable.
True eels possess an elongated body with a continuous, fused fin structure where the dorsal, caudal (tail), and anal fins merge into one ribbon-like fringe. A defining characteristic is the complete absence of pelvic fins, and their pectoral fins are often either very small or entirely missing. Most species lack large, visible scales, instead having smooth skin or tiny, embedded scales.
The Phenomenon of Catadromous Migration
The life cycle of the freshwater eel, such as the American or European eel, is a remarkable example of catadromous migration, meaning they live primarily in freshwater but travel to the sea to reproduce. Both species begin their lives in the Sargasso Sea, a vast, current-defined region of the North Atlantic Ocean where spawning takes place at unknown depths. The adults spawn once and then die, completing their life’s journey far from the rivers they once inhabited.
The newly hatched larvae are known as leptocephali, which are transparent, leaf-shaped creatures drifting on ocean currents like the Gulf Stream for up to two years. These larvae undergo a transformation as they approach the continental shelf, changing into the transparent, cylindrical glass eel. The glass eels then utilize tides and currents to enter coastal estuaries and begin their upstream journey into freshwater river systems.
Once they enter the rivers and begin to develop pigmentation, they are referred to as elvers, which actively migrate hundreds of miles inland. This stage transitions into the yellow eel phase, the longest period of the eel’s life, lasting anywhere from 5 to 20 years in rivers, lakes, and streams. The yellow eel is the immature adult, spending its time growing and building up the necessary fat reserves for the final migration.
When the time for reproduction arrives, the yellow eel undergoes metamorphosis into the silver eel, preparing for its final journey back to the Sargasso Sea. Their skin color changes to a silvery-black, their eyes enlarge significantly to aid deep-sea vision, and their digestive tract degenerates, as they will not feed again. This final, massive migration involves a swim of several thousand miles, guided by a navigation system that scientists still do not fully understand.
Daily Behavior Hunting and Habitat
The yellow eel stage is characterized by site fidelity and a specific daily routine. These benthic eels exhibit a strong tendency to burrow into the soft mud, sand, or gravel of their freshwater or estuarine habitats. They often remain hidden in these shelters during the day, using holes and crevices as their refuge.
Eels are primarily nocturnal predators, emerging from their daytime retreats after sunset to forage. Their diet is varied, consisting of small fish, insects, crustaceans, and other invertebrates, making them opportunistic carnivores and scavengers. Their acute sense of smell is crucial for locating prey and carrion in the often murky waters where they reside.
The way eels move is known as anguilliform locomotion. They propel themselves using serpentine, wave-like movements of their entire flexible body, allowing them to navigate tight spaces, rocks, and dense vegetation with remarkable agility. This fluid swimming style, coupled with their ability to seek out and return to specific resting places, demonstrates their adaptation to complex riverine environments.
Unique Adaptations Electric Shock and Slime
While eels are famous for their shock-delivering capability, only the electric eel, a South American knifefish (Electrophorus electricus), possesses this power, and it is not a true eel (Anguilliformes). The electric eel generates powerful shocks of up to 860 volts using specialized, modified muscle cells called electrocytes, which are stacked like batteries in three electric organs. These organs are used for both stunning prey during hunting and for defense against predators.
Conversely, true eels rely on other physical adaptations for survival, most notably a thick, protective layer of slime. This slimy coating serves multiple purposes, reducing friction for efficient swimming and acting as a physical barrier against pathogens and parasites in the water. The mucus also helps to protect the skin from injury as the eels burrow into abrasive substrates.
In addition to their slime, true eels possess cutaneous respiration, absorbing oxygen directly through their skin. Although their gills are the primary means of breathing in the water, this secondary respiratory mechanism is important when eels are exposed to air or low-oxygen water. This adaptation permits some species to travel short distances over damp land, allowing them to bypass obstacles or move between bodies of water.