Eels
Eels are fish with elongated, slender bodies. Around 800 species of eels belong to the group of fish called Anguilliforme and are found all around the world. Eels have very interesting life cycles and complete immense migrations beginning as drifting larvae. A number of species spend distinct periods of their lives in both marine and freshwater habitats.
Different species grow to various sizes but the largest species can weigh more than 20 kg and reach lengths of more than 3.5 m. Eels are carnivorous fish and some species living for over 80 years.
Diversity and taxonomy
There is currently approximately 800 species of eels on Earth and they all belong to an order of fish known as Anguilliforme. The Anguilliformes are teleost fish and are currently split into 15 different families.
Distribution of eels
All eel species begin life in tropical marine habitats and the vast majority remain in salt water for their entire lives. Only 19 species from the genus Anguilla migrate into freshwater habitats where they spend the majority of their lives before migrating back to the sea to spawn. A number of species that were long thought to be strictly marine eels are now known to be able to tolerate freshwater habitats and are sometimes found in estuaries and rivers.
Eels are believed to have evolved in tropical seas of Asia and then dispersed into temperate regions. They are now found living in a wide range of aquatic habitats such as coastal waters, deep ocean, within the oceanic water column and on continental shelves. All species migrate back to their tropical origins to reproduce.
Diet
Eels are primarily carnivores and feed off smaller fish and invertebrates. Typically, small fish are their first choice of food but they will resort to feeding on invertebrates if they are particularly hungry. Their diets change through the course of their life as they grow larger and feed on larger prey.
Metamorphosis and life cycle
Eels have a very interesting life cycle. They go through four stages of metamorphosis from an embryo to an adult. Initially, the embryo hatches into a larvae and the larvae develops into a unique larvae called a ‘leptocephalus’. The leptocephalus larvae can live for over 2 years which is an unusually long period for a fish larvae. The leptocephalus is flattened, transparent and has a leaf-like appearance.
Once the leptocephalus larvae has grown large enough, it metamorphoses into a juvenile glass eel. The glass eel remains transparent but is smaller and cylindrical. As glass eels migrate to their adult habitats and continue to grow they develop into pigmented elver eels and finally into mature adults.
Migration
Most species of eels undergo long distance migrations between their spawning and growing habitats. Migrations begin in tropical spawning areas with larvae that drift in oceanic currents. Juveniles, with long, thin bodies well-designed for migrations, continue to migrate long distances. Different species migrate into different regions – including Europe, America, Asia, Australia or Africa – and different habitats such as coastal, deep oceanic, pelagic or freshwater habitats.
For migration to be worthwhile it must provide significant benefits that improve the eel’s survival and reproduction. The benefits of migrating must outweigh the costs of making the trip. Benefits can come in the form of better and more abundant food or a more idyllic habitat to live in. Migrations can be physically exhausting and the benefits must be substantial to be worth such an investment of energy.
In many cases the spawning habitat remains constant because there are a number of constraints around reproduction that are difficult to change. Eels therefore take advantage of more prosperous habitats for better growth and survival and then return to their birth place solely for reproduction.
Last edited: 17 December 2015
