The larvae of European eels travel with the Gulf Stream across the Atlantic Ocean, and grow to 75–90 mm within one to three years, before they reach the coasts of Europe. Eels in this so-called “recruitment” developmental stage are known as Glass Eels because of the transparency of their bodies. Glass eels typically refers to an intermediary stage in the eel’s complex life history between the leptocephalus stage and the juvenile (elver) stage. Glass eels are defined as “all developmental stages from completion of leptocephalus metamorphosis until full pigmentation”. The term typically refers to a transparent glass eel of the family Anguillidae.
One well-known location in which the large-scale collection of glass eels takes place (for deli food and stocking) is Epney, on the River Severn, in England. (Glass eels are a food item in Spain.) Once they recruit to coastal areas, they migrate up rivers and streams, overcoming various natural challenges — sometimes by piling up their bodies by the tens of thousands to climb over obstacles — and they reach even the smallest of creeks. These freshwater eels spawn in the ocean, and then enter estuaries as glass eels and swim upstream to live in fresh water during their juvenile growth phase. As the glass eels enter fresh waters, they start to become pigmented and are typically referred to as elvers. The elvers grow larger and are referred to as yellow eels, which are the juvenile stage of eels before their reproductive maturation begins.
They can propel themselves over wet grass and dig through wet sand to reach upstream headwaters and ponds, thus colonising the continent. In fresh water, they develop pigmentation, turn into elvers (young eels), and feed on creatures such as small crustaceans, worms, and insects. After 10 to 14 years, they have matured and grown to a length of 60 to 80 cm. At this stage, they are called yellow eels because of their golden pigmentation. Marine eels of the order Anguilliformes also have a leptocephalus stage, and likely pass through a stage similar to the anguillid glass eels, but they are rarely seen in the ocean.