Eels are a diverse group of ray-finned fish belonging to the order Anguilliformes, defined by their long, snake-like bodies and the absence of pelvic fins. This group encompasses approximately 820 species inhabiting environments from the deep ocean to freshwater rivers. Size variation is striking, ranging from tiny, thread-like creatures to massive, heavy-bodied predators. All eels begin life as a transparent, leaf-shaped larva called a leptocephalus. Understanding maximum size requires exploring the potential of the largest species and the biological and environmental factors that govern their growth rate.
The Range of Eel Sizes
The maximum size an eel can reach depends entirely on the species, with the largest specimens representing extremes in both length and mass. The greatest length is achieved by the Slender Giant Moray, a marine species found in the Indo-Pacific, which can reach up to 4 meters (over 13 feet). Despite its considerable length, this moray is relatively slender, meaning it is not the heaviest species.
The heaviest eel is the European Conger, a formidable marine predator inhabiting the Northeast Atlantic and Mediterranean Sea. While typically reaching about 3 meters in length, its greater girth allows it to achieve weights up to 110 kilograms, with some reports suggesting weights up to 160 kilograms. The Giant Moray also competes for mass, reaching nearly 3 meters in length and weighing close to 66 pounds.
In contrast, smaller species demonstrate the order’s wide size variation. The smallest known species, such as the one-jawed eel, can measure as little as 5 centimeters in length. The Snyder’s Moray is another diminutive species, with a maximum recorded length of around 11 centimeters. This dramatic range, from a few inches to over 13 feet, underscores that “eel” refers to a body plan shared by hundreds of species, each with a genetically predetermined size limit.
Biological Drivers of Growth
The life cycle of freshwater eels (anguillids) provides a clear example of the biological mechanisms dictating growth and maximum size. These eels undergo several distinct phases, beginning with the leptocephalus larva and progressing through the transparent glass eel and pigmented elver stages. The primary growth period occurs during the “yellow eel” stage, when the eel lives in freshwater or estuarine habitats and feeds actively for years.
The duration of the yellow eel stage is directly proportional to the maximum size attained. Females often spend a significantly longer time in this feeding phase, sometimes up to 20 years, while males mature and migrate sooner. This difference in longevity is a major reason why females of many eel species grow substantially larger than males. Longer lifespans, with some individuals living over 80 years, allow for an extended period of somatic growth.
Growth ceases when the eel undergoes its final metamorphosis into the “silver eel” stage, preparing for reproductive migration. This transformation involves major physiological changes, including the enlargement of eyes and the shutting down of the digestive tract. Once the eel becomes a silver eel, it stops feeding, and energy is redirected entirely toward the long journey to the spawning grounds, effectively ending its growth phase.
Environmental Constraints on Maximum Size
While genetics establish the potential for size, external environmental factors determine how much of that potential is realized. Water temperature is a significant modulator of growth because it directly impacts an eel’s metabolic rate. In warmer waters, metabolism and feeding activity increase, leading to faster growth rates; the optimal range for some juvenile eels is between 28 and 30 degrees Celsius.
However, the relationship is complex; extremely cold temperatures can slow the metabolic rate and extend the yellow eel phase, potentially increasing the total lifespan and ultimate maximum size. Food availability acts as a hard limitation on growth, regardless of temperature. Eels are carnivorous, and a restricted supply of prey means the energy needed for rapid growth is not present, resulting in smaller adult sizes.
Habitat quality, particularly salinity level, also affects growth by influencing the energetic cost of osmoregulation. Many freshwater eels grow better in brackish environments, such as estuaries, compared to pure freshwater. Brackish water (around 15 to 20 parts per thousand of salt) is closer to the eel’s internal body fluid concentration, lowering the energy expenditure required to maintain osmotic balance. This saved energy, combined with the higher productivity and food quality often found in estuarine habitats, contributes to faster growth and larger final sizes.